elisavet d. michailidi thesis

EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY

FACULTY OF ENGINEERING

DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY

MSc in OIL AND GAS TECHNOLOGY

MASTER THESIS

EFFECT OF NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER

ELISAVET MICHAILIDI B.Sc. Petroleum Engineer

SUPERVISOR:PROF. ATHANASIOS MITROPOULOS

KAVALA2016

EFFECT OF NANONOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF

WATER

by

Elisavet D. Michailidi

Submitted to the Department of Petroleum and Natural Gas Technology,

Faculty of Engineering

in Partial Fulfillment of the Requirements for the Degree of

Masters of Sciences in the Oil and Gas Technology

at the

Eastern Macedonia and Thrace Institute of Technology

APPROVED BY:

Thesis Supervisor: Athanasios Mitropoulos

Committee member:

Committee member:

Date defended: xx.xx.2016

EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY

FACULTY OF ENGINEERING

DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY

MSc in OIL AND GAS TECHNOLOGY

MASTER THESIS

EFFECT ON NANOBUBBLES ON THE PHYSICOCHEMICAL PROPERTIES OF WATER

ELISAVET D. MICHAILIDI B.Sc. Petroleum Engineer

SUPERVISOR: PROF. ATHANASIOS MITROPOULOS

KAVALA 2016

EASTERN MACEDONIA AND THRACE INSTITUTE OF TECHNOLOGY FACULTY OF ENGINEERING DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY©2016 This Master Thesis and its conclusions in whatsoever form are the property of the author and of the

Department of Petroleum and Natural Gas Technology. The aforementioned reserve the right to

independently use and reproduce (partial or total) of the substantial content of this thesis for teaching and

research purposes. In each case, the title of the thesis, the author, the supervisor and the department

must be cited.

The approval of this Master Thesis by the Department of Petroleum and Natural Gas Technology does

not necessarily imply the acceptance of the author’s views on behalf of the department.

--------------------------------------------------------------

The undersigned hereby declares that this thesis is entirely my own work and it has been submitted to the

Department of Petroleum and Natural Gas Technology in partial fulfillment of the requirements for the

degree of Masters of Sciences in the Oil and Gas Technology. I declare that I respected the Academic

Integrity and Research Ethics and I avoided any action that constitutes plagiarism. I know that plagiarism

can be punished with revocation of my master degree.

Signature

Elisavet D. Michailidi

ABSTRACT

Nanobubbles are nanoscopic gaseous cavities in aqueous solutions which demonstrate

an extended lifetime. Furthermore, water that is enriched with nanobubbles has

completely different fundamental physicochemical and physicomechanical

characteristics compared with water which not contains nanobubbles. The most notable

characteristic of nanobubbles is their apparent extraordinary longevity, being able to last

for weeks and months. To date, experimental evidence concerning the existence of

nanobubbles is sound; however a theoretical understanding is still lacking. The first step

was to review the literature, trying to present formation theories and explain the

pertinent nanobubble stability; the most notable applications of nanobubbles are also

presented and discussed. The purpose of the dissertation was to elucidate the effects of

nanobubble suspensions, produced with nanobubbles generators, and study the

nanobubble formation, size distribution, coalescence, stability and dynamic behavior.

Consequently, gain insight into the properties of nanobubbles. This study discussed the

effects of bulk nanobubbles on the physicochemical properties of water based on

research results from a variety of experiments. Two different types of NB generators

were used. The “porous plug generator” is an innovative device which was designed in

EMaTTech and is under EPO patent. Hence, it was of vital importance to thoroughly

examine both generators and compare their performance. As it derives from Dynamic

Light Scattering and zeta potential measurements, nanobubbles produced from the

porous plug generator are smaller (≃580 nm) and more stable ( -20 mV for 40 mins of

operation) compared to those produced from the nozzle generator, the mean size of

which is ≃580 nm and their zeta potential is -6 mV. It seems that the porous plug

generator produces ≃750×103 NB/cm2 or 750×106 NB/ml. On the other hand, the

nozzle generator only produces ≃125×103 NB/cm2 or 125×106 NB/ml. Furthermore,

nanobubbles have been found to increase electrical conductivity of the water. Vapor

pressure have been found to increase by 116%.

SUBJECT AREA: Nanotechnology

KEYWORDS: nanobubbles, porous plug generator, stability, bulk, DLS

To my beloved ones…

ACKNOWLEDGEMENTS

After an intensive period of seven months, today is the day: writing this note of thanks is

the finishing touch on my thesis. It has been a period of intense learning for me, not only

in the scientific arena, but also on a personal level. Writing this thesis has had a big

impact on me. I would like to reflect on the people who have supported and helped me

so much throughout this period.

First and foremost, I would like to express my sincere gratitude to my advisor Prof.

Athanasios Mitropoulos for the continuous support of my M.Sc. study and research, for

his patience, motivation, enthusiasm, and immense knowledge. His guidance helped

me in all the time of research and writing of this thesis.

Besides my advisor, I would like to thank Dr. Evaggelos Favvas, for his encouragement

and insightful comments. My sincere thanks also goes to Mr. Georgios Bomis for his

precious help over mechanical engineering issues. He designed and manufactured the

NB generators; so his participation in this project was vital. Of course, I especially thank

my labmate Mrs. Ramonna Kosheleva; for her participation and help during the whole

study.

I thank my fellow classmates and friends in Eastern Macedonia Institute of Technology:

Mr. Fotis Zachopoulos, Mr. Stephanos Kyriakidis, Mr. Aris Mitsis and Mrs. Eleni –

Plousia Kosteroglou, for the stimulating discussions, for the sleepless nights we were

working together before deadlines, and for all the fun we have had during our M.Sc.

I take this opportunity to express gratitude to all of the Department of Petroleum

Engineering faculty members for their help, support and valuable lessons.

Moreover, I would like to thank Dr. Eleni Efthimiadou from N.C.S.R. “Demokritos” for her

valuable help with the Dynamic Light Scattering experiments.

Finally, I must express my very profound gratitude to my parents: Mr. Dimitrios

Michailidis and Mrs. Anastasia Stefanidou and to my beloved aunt Mrs. Anastasia

Kaiafa for providing me with unfailing support and continuous encouragement

throughout my years of study and through the process of researching and writing this

thesis. This accomplishment would not have been possible without them. Thank you.

…

TABLE OF CONTENTS

PREFACE ..................................................................................................................... 18

1. CHAPTER1 INTRODUCTION ............................................................................... 19

1.1 INTRODUCTORY PARAGRAPH .................................................................... 19

1.2 PURPOSE OF THE STUDY ............................................................................ 19

1.3 STRUCTURE OF THE DISSERTATION ......................................................... 19

1.4 DEFINITION OF TERMS ................................................................................. 20

1.4.1 BASIC PRINCIPLESOF GAS CAVITIES .................................................. 20

1.4.2 PROPERTIES OF WATER ....................................................................... 22

1.4.3 WATER PHASE DIAGRAM ...................................................................... 25

2. CHAPTER 2 THEORETICAL BACKGROUND ..................................................... 27

2.1 INTRODUCTION ............................................................................................... 27

2.2 STABILITY OF NANOBUBBLES ..................................................................... 27

2.3 NANOBUBBLE FORMATION .......................................................................... 31

2.4 APPLICATIONS OF NANOBUBBLES ............................................................. 32

2.4.1 FLOTATION .............................................................................................. 32

2.4.2 WATER TREATMENT .............................................................................. 33

2.4.3 ELECTROCHEMICAL ANTIFOULING ...................................................... 34

2.4.4 MEDICAL APPLICATIONS ....................................................................... 34

2.4.5 OIL RECOVERY ....................................................................................... 35

2.4.6 AGRICULTURAL & BIOLOGICAL APPLICATIONS ................................. 36

3. CHAPTER 3 EXPERIMENTAL PROCEDURE ..................................................... 37

3.1 INTRODUCTION ............................................................................................. 37

3.2 NANOBUBBLE GENERATION........................................................................ 37

3.2.1 POROUS PLUG GENERATOR ................................................................ 37

3.3 POROUS PLUG CHARACTERIZATION ......................................................... 40

3.3.1 PRINCIPLES OF SCANNING ELECTRON MICROSCOPY ...................... 40

3.4 OPTICAL AND CONFOCAL MICROSCOPY ................................................... 42

3.4.1 CONFOCAL MICROSCOPE ...................................................................... 42

3.5 DYNAMIC LIGHT SCATTERING MEASUREMENTS ...................................... 43

3.5.1 PRINCIPLES OF DYNAMIC LIGHT SCATTERING ................................... 43

3.5.2 DYNAMIC LIGHT SCATTERING EXPERIMENTAL PROCEDURE .......... 45

3.6 ZETA POTENTIAL ........................................................................................... 46

3.6.1 ELECTROPHORETIC LIGHT SCATTERING INSTRUMENTATION ......... 47

3.6.2 Ζ POTENTIAL EXPERIMENTAL PROCEDURE ....................................... 49

3.7 VAPOR PRESSURE ........................................................................................ 49

3.7.1 EXPERIMENTAL PROCEDURE ............................................................... 49

4. CHAPTER 4 RESULTS AND DISCUSSION ........................................................ 51

4.1 INTRODUCTION................................................................................................ 51

4.2 TYNDALL EFFECT .......................................................................................... 51

4.3 SIZE DISTRIBUTION ....................................................................................... 53

4.3.1 SIZE OF MNB AS A FUNCTION OF TIME ................................................ 53

4.3.2 SIZE OF NB AS A FUNCTION OF TEMPERATURE ................................ 58

4.4 ZETA POTENTIAL ........................................................................................... 59

4.5 OPTICAL & CONFOCAL MICROSCOPY ........................................................ 61

4.6 POROUS PLUG CHARACTERIZATION .......................................................... 63

4.7 VAPOR PRESSURE MEASUREMENTS ......................................................... 64

4.8 CONDUCTIVITY MEASUREMENTS ............................................................... 66

4.9 EFFECT ON BIOLOGICAL MATTER; THE CASE OF PLANTS ...................... 67

5. CHAPTER 5 CONCLUSIONS ............................................................................... 71

5.1 CONCLUSIONS .................................................................................................. 71

5.2 FURTHER RESEARCH ................................................................................... 73

6. ABBREVIATIONS – INITIALS ............................................................................ 75

7. REFERENCES ....................................................................................................... 77

LIST OF FIGURES

Figure 1.1 (A) the tetrahedral structure of a single water molecule with the oxygen atom

in the center and the two hydrogen atoms in two corners of the tetrahedron.(B) Ball and

stick model of a water molecule[11]. ................................................................................ 22

Figure 1.2 Water phase diagram ................................................................................... 26

Figure 2.1Basic schematic representation of the Ostwald ripening process .................. 28

Figure 3.1 Schematic representation of the main components of the Nanobubble

Generating Device, and their interconnection. ............................................................... 38

Figure 3.2 Schematic representation of G1, consisting of two rotary pumps connected in

series ............................................................................................................................. 39

Figure 3.3 Pre-chamber, collecting liquid from pumps 1 and 2 ...................................... 39

Figure 3.4 Generator's sample collection tank ............................................................... 40

Figure 3.5 Schematic of a SEM ..................................................................................... 41

Figure 3.6 Principle of confocal microscopy .................................................................. 42

Figure 3.7 Brownian motion, relation of particle size to speed of movement ................. 44

Figure 3.8 Typical Dynamic Light Scattering setup ........................................................ 45

Figure 3.9 Figure depicting the EDL on a negatively charged particle. .......................... 47

Figure 3.10 Schematic showing the instrumentation of ZP measurement by

electrophoretic light scattering ....................................................................................... 48

Figure 3.11 Vapor Pressure experimental configuration ................................................ 50

Figure 4.1 Presence of Tyndall scattering in a sample containing micro-nanobubbles . 51

Figure 4.2 Tyndall scattering is observed in the first two samples which contain

nanobubbles. The phenomenon cannot be observed to the last sample (right) which is

simple water .................................................................................................................. 52

Figure 4.3 Tyndall effect. in colloidal solution light beam is visible. This is due to the

particles (in this case bubbles) absorb light energy and then emit it .............................. 52

Figure 4.4 Size - Time Diagram for Porous Plug 10 min Sample .................................. 53




Figure 4.8 Size- Time Diagram for Vibrating Generator Samples ................................. 55

Figure 4.9 Size distribution of NB produced by porous head (blue) and nozzle (green)

generators. At the left: The auto-correlation coefficient (ACF) diagram in the same

colors. ........................................................................................................................... 57

Figure 4.10 Size-Production Time Diagram for Vibrating and Porous Plug Generator . 57

Figure 4.11 Nanobubble size as a function of temperature for the porous plug (P40) and

nozzle (V40) generators after 40 minuntes of operations ............................................ 58

Figure 4.12 Zeta potential as a function of time for the porous plug generator samples 59

Figure 4.13 Zeta potential as a function of time for the vibrating generator samples .... 60

Figure 4.14 Effect of production time on zeta potential for the porous plug (blue) and

nozzle generator (red)................................................................................................... 61

Figure 4.15 Optical microscopy images for the nozzle (left) and porous plug generators

(right). Both of the samples were taken after 40 mins of operation. .............................. 62

Figure 4.16 Confocal Microscopy Image of a Nanobubble Sample, with fluoresceine .. 63

Figure 4.17 Scanning Electron Microscope images from the sintered porous plug at

X140 (up) and X40 (down) ............................................................................................ 64

Figure 4.18 Vapour Pressure of NB samples, produced from the porous plug generator

at different temperatures; 20 oC (blue), 30 oC (red) and 40 oC (green) ......................... 65

Figure 4.19 Electrical Conductivity as a function of production time ............................. 66

Figure 4.20 Distribution of ions at and near the gas-water interface in an aqueous

solution of electrolyte. The electrolyte ions are attracted to the interface and create the

electrical double layer. .................................................................................................. 67

Figure 4.21 Left: Oat seeds watered with oxygen nanobubbles; Middle: Oat seeds

watered with atmospheric air nanobubbles; Right: Oat seeds watered with normal water

...................................................................................................................................... 68

Figure 4.22 Left: Soya seeds watered with oxygen nanobubbles; Middle: Soya seeds

watered with atmospheric air nanobubbles; Right: Soya seeds watered with normal

water ............................................................................................................................. 68

Figure 4.23 Wheat plant dry weight as a function of time. ............................................ 69

LIST OF TABLES

Table 1.1The electronegativity of hydrogen and the Group 16 elements, and the

molecular weight (MW), melting points (MP) and boiling points (BP) of their hydrides[14]

...................................................................................................................................... 23

Table 1.2 Surface tension of water as a function of temperature[15] ............................... 24

Table 3.1 Samples for DLS measurements ................................................................... 45

Table 3.2 Samples for ζ potential measurements .......................................................... 49

Table 4.1 NB size as a function of temperature ............................................................. 58

PREFACE

The hereby presented dissertation entitled “The effect of nano-bubbles on the

physicochemical properties of water” was submitted to the Department of Petroleum

and Natural Gas Technology, Faculty of Engineering, Eastern Macedonia & Thrace

Institute of Technology in Partial Fulfillment of the Requirements for the Degree of

“Master of Philosophy in Petroleum Engineering”. This dissertation is based on research

upon the physicochemical properties of nanobubbles in aqueous solutions. The

research took place at the Department of Petroleum Engineering, EMaTTech using the

facilities of Hephaestus Advanced Laboratory. Part of the research, and more particular

the Dynamic Light Scattering and confocal microscopy experiments, were conducted at

the National Center for Scientific Research, NCSR Demokritos. The work includes an

extended literature review, examining the topic in detail. The research was conducted

during the period of June 2015-November 2016.

CHAPTER 1: INTRODUCTION

Elisavet D. Michailidi - 19 - 2016

1. CHAPTER1

INTRODUCTION

1.1 INTRODUCTORY PARAGRAPH

Nanobubbles are nanoscopic gaseous cavities in aqueous solutions which demonstrate

an extended lifetime. Furthermore, water that is enriched with nanobubbles has

completely different fundamental physicochemical and physicomechanical

characteristics compared with water which not contains nanobubbles. The most notable

characteristic of nanobubbles is their apparent extraordinary longevity, being able to last

for weeks and months. Existing theories, however, predict that they should dissolve

extremely quickly. Thus, to fully exploit their potential benefits, major developments are

needed in the science underpinning their existence and behavior. In this direction, over

the last few years, the emerging field of microbubble and nanobubble (MNB)

technologies have drawn great attention due to their physicochemical properties and,

as a matter of fact, their applications in many fields of science and technology.

1.2 PURPOSE OF THE STUDY

The overall aim of this dissertation is to study both experimentally and theoretically the

underlying mechanisms by which these nanobubble dispersions come to exist and

persist, and explain some of their unusual properties. The purpose is to elucidate the

effects of nanobubble suspensions, produced with nanobubbles generators, and study

the nanobubble formation, size distribution, coalescence, stability and dynamic

behavior. Consequently, gain insight into the properties of nanobubbles. This

dissertation discusses the effects of bulk nanobubbles on the physicochemical

properties of water based on research results from a variety of experiments.

1.3 STRUCTURE OF THE DISSERTATION

The work is structured as follows: Chapter 1 is the introductory chapter, defining the

research problem and the purpose of the study. Moreover, it presents some basic

principles and definitions. The second chapter establishes the theoretical background

of the study. Chapter 3 thoroughly described the experimental procedures that were

followed. The outcomes of the research are presented and discussed in Chapter 4,

entitled “Results & Discussion”. Finally, the conclusions are presented in Chapter 5.

MASTER THESIS DEPARTMENT OF PETROLEUM AND NATURAL GAS TECHNOLOGY


1.4 DEFINITION OF TERMS

1.4.1 BASIC PRINCIPLESOF GAS CAVITIES

In order to have a thorough understanding of the topic, it is considered important to

define the basic principles pertaining multiphase systems and more precisely, gas

bubbles.

Bubbles are defined as gas filled cavities with internal equilibrium pressures at least that

of the external environment. Each bubble is surrounded by an interface with diversified

properties than the bulk solution[1]. Bubbles are formed when a pure homogeneous

liquid undergoes a phase change. Gas bubbles are formed when the amount of

dissolved air in a solution exceeds the saturated solubility. Saturated solubility is the

amount of gas that eventually dissolves in a solution in equilibrium state[2].

Νotwithstanding, this quantity varies depending on the type of solution, type of gas,

temperature, and pressure.

Nucleation is typically defined[3] as:“A process by which a cluster of molecules of one

phase forms in the presence of a bulk phase which has been moved from phase

equilibrium to a metastable region by a change in temperature, pressure, or composition

(in the case of multicomponent systems)”.

Nucleation is the onset of a phase transition in a small region of a medium. The phase

transition can be the formation of a bubble in a liquid or of a droplet in saturated vapour.

There are two main types of nucleation models: homogeneous nucleation and

heterogeneous nucleation.Homogeneous nucleation will be discussed in this section, as

it is the type of nucleation that takes place during bubbles formation in bulk.

Homogeneous nucleation takes place in a liquid phase without the prior presence of

additional phases[4, 5]. It is a consequence of the distribution of thermal energy among

the molecules comprising a volume of liquid. Because some molecules will be more

energetic than others, random processes will occasionally produce groupings of higher

energy molecules. If the average energy is high enough, such a grouping of molecules

represents an inclusion consisting of gas and vapour in the bulk of the liquid. A gas

bubble will dissolve in an undersaturated solution and the effect of surface tension will

cause it to dissolve in a saturated solution. In supersaturated solutions, a bubble can be

in equilibrium because the tendency for the bubble to dissolve due to surface tension is

opposed by the tendency for the bubble to grow by diffusion of gas into it. This



equilibrium is unstable; the bubble will grow or dissolve depending on whether the

perturbation increases or decreases the bubble’s radius relative to its equilibrium

radius[6]. Therefore, a liquid would be free of bubbles after a short period of time. This

does not imply that gas bubbles could not serve as cavitation nuclei. It does imply,

however, that in order for gas bubbles to serve as cavitation nuclei, they must be

stabilized at a size small enough to prevent their rising to the surface of the liquid, yet

large enough so that they will grow when exposed to negative pressure as low as a few

bars. In other words, a stabilization mechanism must exist for a gas bubble before it can

act as a cavitation nucleus[7].

The formation of gas bubbles and their subsequent rise due to buoyancy are very

important fundamental phenomena that contribute significantly to the hydrodynamics in

gas-liquid reactors. The rise of a bubble in dispersion can be associated with possible

coalescence and dispersion followed by its disengagement from the system[2]. The

phenomenon of bubble formation decides the primitive bubble size in the system (which

latter attains an equilibrium size), whereas the rise velocity decides the characteristic

contact time between the phases which governs the interfacial transport phenomena as

well as mixing.

The degree of saturation next to a bubble depends on the gas pressure within the

bubble. Smaller bubbles have higher internal pressure and release gas into under-

saturated solution whereas larger bubbles grow by taking up gas from supersaturated

solution.

According to the widely-accepted Young-Laplace equation, the pressure in the interior

of gas bubbles is inversely proportional to their diameter, with excess pressure. The

Young–Laplace equation describes the equilibrium pressure difference sustained

across the interface between two static fluids, such as water and air, due to the

phenomenon of surface tension.

The Young-Laplace equation can be expressed in its simplified form as follows (Eq.

1.1.):

2P

R

Eq. 1.1



Where:

ΔΡ=Pvap-Pliq are the pressure inside (vapor phase) and outside (liquid phase) of the

bubble, respectively, and γ is the surface tension.

The relative terminal rising velocity of a single gas bubble, moving into a liquid phase, is

determined by its size, by the interfacial tension, by the density and viscosity of the

surrounding liquid.

1.4.2 PROPERTIES OF WATER

As the main purpose of this dissertation is to examine the effect of bulk nanobubbles on

the physicochemical properties of water, the most important properties will be analysed

in this chapter.

The structure of isolated water molecules is well-known. The oxygen atom has six

valence electrons and each hydrogen atom has one, such that the two hydrogen atoms

form covalent bonds with the oxygen leaving two lone pairs of electrons on the oxygen

(Fig. 1.1). The length of the O–H bond is 1 A and the angle between the bonds is

104.5o, or very close to the angle between the vertices of a regular tetrahedron (109o).

Although the structure appears trivial, the physicochemical properties of water are far

from simple[8-10].

Figure 1.1 (A) the tetrahedral structure of a single water molecule with the oxygen atom in the

center and the two hydrogen atoms in two corners of the tetrahedron.(B) Ball and stick model of a

water molecule[11]

.

Based on electronegativity, the electrostatic surface of water is associated with a dipole

with a partially negative oxygen atom and partially positive hydrogen atoms. (Fig. 1).

The polarity of each water molecule results in an attraction between it and other water

molecules, resulting in formation of a hydrogen bond. Hydrogen bonds are relatively



strong (∼5–40 kJ/mol) compared to van der Waals interactions (∼1–10 kJ/mol) but

much weaker than covalent bonds (∼200–1000 kJ/mol). Intermolecular

hydrogenbonding of water leads to enhanced molecular cohesion.

Due to this molecular cohesion, water behaves differently than the hydrides in its group.

Water is primarily a liquid under standard conditions, which is not predicted from its

relationship to other analogous hydrides of the oxygen family (Group 16) in the periodic

table (Table 1.1), which are gases such as hydrogen sulfide. The elements surrounding

oxygen in the periodic table, nitrogen, fluorine, phosphorus, sulfur and chlorine, all

combine with hydrogen to produce gases under standard conditions.

It has been estimated that if water did not possess this extensive cohesion such that it

behaved more like other group 16 hydrides, then its boiling point (BP) would be about

−90 ◦C or almost 200 ◦C lower than the actual value. The electronegativity of the

heavier Group 16 elements, i.e., sulfur, selenium and tellurium, is much lower than that

of oxygen, and close to that of hydrogen. Thus, their hydrides are unable to form

hydrogen bonds[12]and consequently both their melting point (MP) and BP are much

lower than that of water (Table 1.1). Hydrogen bonds also affect other physicochemical

properties of liquid water, such as its dielectric constant (ε 78.5 at 25 ◦C), density (1.000

g/ml at 3.98 ◦C), surface tension and heat of vaporization (40.65 kJ/mol), making them

all higher than expected[13].

Table 1.1The electronegativity of hydrogen and the Group 16 elements, and the molecular weight

(MW), melting points (MP) and boiling points (BP) of their hydrides[14]

Name Symbol Electronegativity Group

16 hydrides

MW MP (oC)

BP (oC)

Oxygen O 3.5 H2O 18 0 100

Sulfur S 2.5 H2S 34 -85 -60

Selenium Se 2.4 H2Se 81 -66 -41

Tellurium Te 2.1 H2Te 130 -49 -2

1.4.2.1 SURFACE TENSION

The cohesive forces among liquid molecules are responsible for the phenomenon of

surface tension. In the bulk of the liquid, each molecule is pulled equally in every

direction by neighboring liquid molecules, resulting in a net force of zero. The molecules

at the surface do not have the same molecules on all sides of them and therefore are



pulled inwards. This creates some internal pressure and forces liquid surfaces to

contract to the minimal area. Surface tension can be defined in terms of force or energy.

Another way to view surface tension is in terms of energy. A molecule in contact with a

neighbor is in a lower state of energy than if it were alone (not in contact with a

neighbor). The interior molecules have as many neighbors as they can possibly have,

but the boundary molecules are missing neighbors (compared to interior molecules) and

therefore have a higher energy. For the liquid to minimize its energy state, the number

of higher energy boundary molecules must be minimized. The minimized quantity of

boundary molecules results in a minimal surface area.

Surface tension, usually represented by the symbol γ, is measured in force per unit

length. Its SI unit is newton per meter.

Water has a high surface tension of 71.99 mN/m at 25 °C, caused by the strong

cohesion between water molecules, the highest of the common non-ionic, non-metallic

liquids.A table, with the surface tension of water at various temperatures is given below

(Table 1.2):

Table 1.2 Surface tension of water as a function of temperature[15]



1.4.3 WATER PHASE DIAGRAM

The liquid-vapour phase boundary in the diagram summarises the variation in the

vapour pressure of liquid water with temperature.

The solid-liquid boundary represents the variation of the melting point with pressure. Its

very steep slope indicates that the pressure changes required to noticeably affect the

melting point are enormous. The line also has a negative gradient up to about 2000

atm, which is highly unusual, indicating as it does that an increase in pressure lowers

the melting point. The reason behind this behaviour can be traced to the fact that ice

has a larger molar volume than liquid water close to its freezing point. (Due to the

hydrogen bonding between water molecules in the solid which enforces a fairly open

cage-like structure.) Raising the pressure thus makes it more favourable for the solid to

transform into the liquid, as it can reduce its volume (and the pressure acting upon it) by

doing so.



Figure 1.2 Water phase diagram

CHAPTER 2: THEORETICAL BACKGROUND


2. CHAPTER 2

THEORETICAL BACKGROUND

2.1 INTRODUCTION

To date, experimental evidence concerning the existence of nanobubbles is sound;

however a theoretical understanding is still lacking. The purpose of this chapter is to

review the literature, trying to present formation theories and explain the pertinent

nanobubble stability. Furthermore, the most notable applications of nanobubbles are

presented and discussed.

2.2 STABILITY OF NANOBUBBLES

As it was mentioned in the introduction, it is observed that nanobubbles demonstrate an

extremely long lifetime, ranging to several weeks[16]or even months[17]. This fact has

triggered the interest of the researchers and a large number of studies discuss the

reasons and the phenomena behind this extraordinary property.

Taking into consideration the classical thermodynamics, a paradox seems to appear in

systems containing nanobubbles[18, 19]due to the fact that the longevity of nanobubbles

is not in accordance with the the Young-Laplace Law.

Obviously, since the pressure inside gas bubbles is inversely proportional to their

diameter, microscopic bubbles have large internal pressure. This implies that the air

inside the nanobubble cannot be in equilibrium with the atmosphere.

2P

R

Eq. 2.1

Where:

ΔΡ=Pvap-Pliq are the pressure inside (vapor phase) and outside (liquid phase) of the

bubble, respectively, and γ is the surface tension.

Thus, it would be expected that bubbles which their diameter is in the nanoscale (<1

μm), dissolve immediately, within a few microseconds, in favor of larger ones according

to the phenomenon of Ostwald ripening[20] (Figure 2.1.).



Figure 2.1Basic schematic representation of the Ostwald ripening process

However, nanobubbles demonstrate an extended lifetime compared to larger diameter

bubbles and can be observed in aqueous solutions even after several weeks.

For small diameter bubbles, which are approximately perfect spheres due to surface

tension dominant effect of on their shape, Stokes equation[21] provides a reasonably

accurate description. (Eq.2.2.):

2

18

g dR

Eq. 2.2

Where: R = rise rate (m∙s-1), ρ = density (kg∙m-3), g = gravity (m∙s-2), d = bubble

diameter (m) and μ = dynamic viscosity (Pa∙s).

Hence, micro and nano bubbles have a very slow rise rate.

One of the most dominant theories, suggest that the stability of the nanobubbles is

caused by the fact that their surface is charged. The nanobubble gas/liquid interface is

charged, introducing an opposing force to the surface tension, so slowing or preventing

their dissipation. Each nanobubble is surrounded by a double layer[22, 23]. It is believed[24]

that the developed double layer plays a critical role in the formation and stability of

nanobubbles in aqueous solutions by providing a fairly high repulsive force, which

prevents inter-bubble aggregation and coalescence of the stable bubbles.

Many authors claim that the gas bubbles in aqueous media always develop negative

charges on their surfaces, which suggests that cations (protons) are more likely

hydrated and therefore have a tendency to stay in the bulk aqueous medium, whereas

the smaller, less hydrated and more polarised anions tend to adsorb on the bubbles’

surfaces. However, this specific adsorption has not been fully explained, and its

existence has not been universally accepted



It is also claimed that zeta potential possesses a key role on nanobubble stability. The

zeta potential of a bubble is an important factor in many engineering applications, as it

determines the interaction of the bubble with other materials such as oil droplets and

solid particles[25-27]. Zeta potential assists in predicting long-term stability in colloidal

system[28]. If all the particles in suspension have a high zeta potential (negative or

positive), then they confer stability (the suspension or dispersion will resist

agglomeration)[29]. On the other hand, if the particles have a low potential then they tend

to come closer and will flocculate[30]. Zeta potential can be determined using simple

Smoluchowski equation. The zeta potential is generally negative and varied depending

on the kind of gas introduced[31]. The negative zeta potential value could be explained

by the excess of hydroxyl ions (OH−) relative to hydrogen ions (H+) at the gas–water

interface in pure water[32, 33]. The charging mechanism of nanobubbles was also

believed to be due to preferential adsorption of OH− in electrolytic solution[34]. Ushikubo

et al.[24, 35] found that bubbles with zeta potential >30 mV were much more stable than

those with <30 mV. They suggested that the stability of nanobubbles is mainly due to

the magnification of electrostatic repulsive forces caused by overlapping electrical

double layers of the neighboring bubbles.

Apart from the zeta potential, Ohgaki et al.[36] suggested that the stability of a

nanobubble is strongly related to hydrogen bonding at water–gas interface. They

reported that the surface of the nanobubble contains hard hydrogen bonds that may

reduce the diffusivity of gases through the interfacial film. More recently, Wang, Liu, and

Dong[37] (2013) reported that the surface of a nanobubble is kinetically stable and the

water–gas interface is gas impermeable. A reduction in surface tension of the vapor–

liquid interface reduces the Laplace pressure (Eq. 1.1.) and increases nanobubble

stability. Tolman and others predict a decrease of the surface tension for large curvature

on small scales[38-41]. Specifically, Tolman calculated theoretically that the surface

tension in drops should decrease significantly at small sizes.

In agreement with the hypothesis of a lower internal pressure of nano-bubbles[42] based

on molecular simulation data, concluded that there are too few vapor atoms inside

nano-bubbles, so the interior gas pressure would not be high enough to support the

force balance of a nano-bubble. The authors stated that the internal pressure should be

much lower than predicted by Y-L equation, so it should not be valid for nano-bubbles.



Instead, the liquid–gas interface should play an important role in the stabilization of

nano-bubbles.

Another theory from Lohse and Weijs[43]claims that the anomalous stability of

nanobubbles comes from the slow rate of dissolution of gas into a surrounding liquid

already saturated with it.

However, physical chemist Vincent Craig[44] at the Australian National University in

Canberra disagrees, saying that “this theory wouldn’t explain the stability of

nanobubbles produced by electrolysis, where it is unlikely that significant

supersaturation occurs. Also, they predict the bubbles should be steadily shrinking over

time. But as far as I know nobody has seen that – although with pinning it might not be

obvious. An obvious experiment is to measure nanobubble volume over regular

intervals and compare this to the theory.”

A pertinent article by Ducker suggests that a film of water insoluble contaminant at the

vapor–liquid interface decreases the surface tension and increase contact angle of

nanobubbles. Additionally, the layer of water insoluble contaminates acts as a barrier to

diffusion of gases from the bubble, further increasing nanobubble lifetime.Despite the

sufficient theoretical and experimental validation of the contamination mechanism, a

number of areas need to be investigated before this stabilization approach is taken as

standard.

The experiments of Oshikubo et al[35]. indicated that the gas supersaturation condition

of the water can reduce the gas transfer rate from the bubble to the liquid. However, this

factor should be not the only one that can explain the nano-bubble stability, since nano-

bubbles should exist even near or after reaching the saturation equilibrium. Other

contribution to the nano-bubble should be related to the electrical charge at the bubble

surface, as indicated by the ζ -potential measurements. The high absolute value of ζ -

potential could avoid the bubble coalescence by the creation of repulsion forces. At a

high absolute ζ -potential, the electrical charged particles tend to repel each other,

avoiding aggregation of particles in a colloidal dispersion. In the case of a bubble

dispersion, the high ζ-potential could create repulsion forces that would avoid the

coalescence of bubbles and contribute to the stabilization of the bubbles.

Seddon et al.[45, 46] (2011) provided a model for this remarkable nanobubbles stability to

bulk dissolution. Their argument is that the gas in a nanobubble is of Knudsen type.



This leads to the generation of a bulk liquid flow which effectively forces the diffusive

gas to remain local. The model appears to be in good agreement with experimental

atomic force microscopy. Thus, nanobubbles last for so long because gas molecules

inside them do not escape into the main liquid, but instead hitch a ride on a circular.

Thus, because nanobubbles are so small, a gas molecule would be able to travel from

one side of a bubble to the other, without colliding with any other gas molecule. The

other is that a gas molecule sticking to the surface inside the bubble is most likely to

leave the surface in the perpendicular direction.

2.3 NANOBUBBLE FORMATION

While many theories have been developed concerning the formation of nanobubbles on

hydrophobic surfaces, the discussion about the formation of NB in bulk is still limited.

The formation of nanobubbles is often achieved when the homogenous liquid phase

undergoes a phase change caused by sudden pressure reduction below a critical value,

which is known as cavitation[47]. The cavitation is commonly induced by the passage of

ultrasonic wave (acoustic cavitation), or by high pressure variations in the flowing liquid

(hydrodynamic cavitation)[48-50]. Nanobubbles can be induced by ultrasonication[51, 52].

Nanobubbles can also be generated by means of chemical reactions such as

electrolysis[53]. Venturi-type generator has been widely used for the generation of

nanobubbles based on hydrodynamic cavitation mechanism[48, 50, 54]. The Venturi

system is composed of three main parts, i.e. inflow, tubule, and tapered outflow[55].

Reduction of pressure in Venturi tube can be achieved by increasing liquid velocity in

the conical convergent zone of the tube due to narrow diameter[50]. In the Venturi-type

generator system, both gas and liquid are passed simultaneously via the Venturi tube to

generate the bubble. When pressurized fluid is introduced in the tubular part, the liquid

flow velocity in the cylindrical throat becomes higher whereas pressure becomes lower

compared to the inlet section, thus resulting in cavitation[50]. Ahmadi and Khodadadi

Darban (2013)[54] generated the nanobubbles with a mean diameter of 130-545 nm via

Venturi tube based on hydrodynamic cavitation mechanism. Fan et al. were able to

generate nanobubbles of a mean diameter with less than 50 nm using Venturi tube. Kim

et al. generated nanobubbles with a mean diameter of 300–500 nm via ultrasonication

using palladium-coated electrode. Oeffinger and Wheatley (2004) generated

nanobubbles with a mean diameter of 450–700 nm via ultrasonication of a mixed

surfactant solution with regular purging using octafluoropropane gas. Cho et al. (2005)



produced nanobubbles with a mean diameter of 750 and 450 nm by ultrasonification in

pure water and adding surfactant, respectively. More recently, Wu et al. (2012)

successfully generated nanobubbles with a mean diameter less than 500 nm using a

batch high-intensity agitation based on hydrodynamic cavitation. At present, many

nanobubble generators are commercially available in the market for both laboratory and

pilot-scale experiments.

In the current study, a novel nanobubble generator was developed and described at the

next chapter. The generator utilizes a porous plug through which gas-liquid mixture is

forced, leading to the formation of MNB.

2.4 APPLICATIONS OF NANOBUBBLES

The growing significance of nanotechnology as well as the special properties of

nanobubbles has drawn huge attention in many sectors due to their wide range of

applications, including mine industry, medical applications, food processing and

wastewater treatment. Some of the most prominent applications will be analyzed in this

section.

2.4.1 FLOTATION

Solid–liquid separation is the primary step in any wastewater treatment system that can

be achieved by various techniques[56, 57]. Flotation is widely accepted as the most

reliable and practical separation method used for removing suspensions that contain

fats, oil, and grease mixed with low-density organic suspended solids and colloids[58].

The separating mechanism is based on adsorption of gas bubbles (while rising) upon

the surface of finely suspended particles, which reduces the effective specific gravity of

the particles and makes the contaminants rise up to the surface (increase rising velocity

of contaminants)[59]. This technique is often used to separate extremely fine particles or

globules from the solution, which do not possess a significant settling rate[58].

Tsai et al.[60] investigated the nanobubble flotation technology (NBFT) with coagulation

process for the cost-effective treatment of chemical mechanical polishing (CMP)

wastewater in both laboratory and pilot scale flotation reactors. They reported that the

application of NBFT with coagulation increased the wastewater clarification efficiency by

40%, compared with the conventional coagulation/flocculation process. The authors



also suggested that the operating as well as chemical costs required for NBFT with

coagulation process was much lesser than those in conventional coagulation process.

Nanobubbles form on the surface of hydrophobic particles with higher contact angle (θd)

and ultimately increase Fp and Fe , creating suitable conditions for bubble–article

attachment ). In addition, presence of fine bubbles, particularly nanobubbles and

microbubbles, can reduce Fd and considerably decreases the detachment force[61, 62].

Thus, application of nanobubbles on surfaces of coarse hydrophobic particle increases

the adhesion forces of bubble–particle[63], i.e. Fp and Fe , and decreases the detachment

force; Fd.. The probability of bubble–particle detachment can be explained by the

following equation:

1

1d

at

de

PF

F

………………………………………………………………………………………Eq. 2.3

where Pd is the bubble–particle probability, Fat and Fde are the total attachment force

and detachment force, respectively.

2.4.2 WATER TREATMENT

In the past few years, more and more attention has been given to the potential

applications of the MBs and NBs for water treatment due to their ability to generate

highly reactive free radicals. Recently, MBs/NBs have been used for detoxification of

water[64], while it has been reported that air and nitrogen MBs/NBs can enhance the

activity of aerobic and anaerobic microorganisms in submerged membrane bioreactor.

Evidence shows that nitrogen MBs/NBs cannot only be applied for water and

wastewater treatment, but also for fermentation, brewing as well for human waste

treatment. MBs/NBs have been found to catalyze chemical reactions, and enhance the

detoxification efficiency, thereby improving the efficiency of chemical treatment of water.

The main purpose of water pretreatment is to reduce biological, chemical and physical

loads in order to reduce the running costs and increase the treated water quality. In this

context, air MBs/ NBs as a pretreatment means has been shown to be highly beneficial

for downsizing the water/wastewater treatment plants and improving the quality of

product water.



2.4.2.1 DEGRADATION OF ORGANIC POLLUTANS

MBs generated through hydrodynamic cavitation have been employed for degradation

of various organic compounds. Nanobubble technology has been applied for the

degradation of surfactants coupled with UV irradiations[65]. In this line, Tasaki et al.[66]

investigated the effect of 8-W low-pressure mercury lamp in the presence of

nanobubbles (diameter: 720 nm) for the decomposition of sodium

dodecylbenzenesulfonate (SDBS), as a model compound in aqueous solution. The

degradation experiments were conducted with and without nanobubbles using ozone

lamp (185–254 nm). Their experimental results showed that both oxidation and

mineralization rate of SDBS were significantly enhanced under 185-254-nm irradiation

when coupled with nanobubbles. They reported the SDBS removal is effective in the

integrated nanobubbles/vacuum UV system, and observed 99.8% oxidation of SDBS

and 76.8% total organic compound removal after 24 h of irradiation.

2.4.3 ELECTROCHEMICAL ANTIFOULING

The electrochemical formation of nanobubbles at conductive surfaces has recently been

applied to the prevention of surface fouling and de-fouling surfaces. The studies by Wu

et al.[67] show that proteins can be removed from a surface by electrochemically forming

nanobubbles at the surface. Additionally, pre-existing nanobubbles reduce the

absorption of proteins at a surface.

2.4.4 MEDICAL APPLICATIONS

Micro/nanobubbles have wide applications in both disease diagnosis and therapy. By

comparison with microbubbles (1–999 μm), nanobubbles have smaller carrying

capacity. Where the fine bubble quite often leaves the system having transferred only a

small fraction of its “cargo”, the microbubble, due to its small carrying capacity and high

transfer rates (surface area per unit volume increases with decreasing diameter),

frequently becomes capacity-limited[68, 69] and, inevitably, transfer with microbubbles is a

transient process, whereas the historic models of gas–liquid transfer processes assume

pseudo-steady state[70]. With nanobubbles, transfer rates are enormous, yet in fact, the

delivery of bulk materials with nanobubbles is expensive due to the low volumetric flow

rates and gas phase holdups, as well as the energy cost of their production. Yet, even

nanobubbles expensively produced with high energy density techniques have high

transfer efficiencies and performance. Nevertheless, quite a lot of the biomedical uses



of nanobubbles stem from their ability to deliver materials in a controlled fashion. The

migration of nanobubbles can be directed by ultrasonic fields, and their interfaces can

be loaded with surfactant materials that are held by high interfacial affinity. Watanabe et

al. [71] illustrate this paradigm beautifully by delivering genetic materials with ultrasound

— a non-viral “vector” for genetic engineering. Controlled release of materials can be

achieved optically with plasmonically induced nanobubble rupture[72]. Nanobubbles are

also very useful for ultrasound imaging, providing the compressible surface for reflection

of the irradiated waves. Imaging of tumors by a mixture of nanoparticles and

nanobubbles, stabilized by block co-polymers, has proven a successful technique for

medical diagnostics. For biomolecular separations, very small bubbles, termed aphrons,

have shown particularly useful extraction properties. Lye and Stuckey[73], for instance,

report large mass transfer coefficients in the extraction of erythromycin using colloidal

liquid aphrons, an emerging technique for the recovery of microbial secondary

metabolites, such as antibiotics in pre-dispersed solvent extraction (PDSE) processes.

Protein separations are typically by interfacial affinity, rather than phase transfer. For

instance, Noble et al.[74] showed that protein affinity to foams was largely chemical

interactions, but to gas aphrons, electrostatic and hydrophobic interactions dominated.

Most current applications for nanobubbles are high value added activities – such as

drug delivery – for which the cost of production is a secondary issue.

2.4.5 OIL RECOVERY

Oil recovery could benefit from gas-lift technology if sufficiently small microbubbles can

be generated in situ downhole[75], driving such innovations as new pump cavitation

mechanisms[76].

Microbubble CO2 can be generated by injecting CO2 through special porous filters

attached to borehole casing or gas tubing. When injecting microbubble CO2 into saline

aquifers, dissolution of injected CO2 into formation water can be accelerated up more

than 20%, compared to conventional CO2 injection. As a result, microbubble CO2

injection will minimize the free CO2 fraction in the subsurface and consequently

contribute to the long term safety of large-scale CO2 storage. Microbubble CO2 injection

will also lead to effective use of pore space within the reservoirs. P-wave velocity and

resistivity changes obtained when injecting CO2 in microbubble and normal bubble sizes

into artificial brine-saturated porous sandstones indicate more pore water displaced by

the injected microbubble CO2 in terms of sweep efficiency. Combined effects of



enhanced dissolution and sweep efficiency in microbubble CO2 injection can reduce the

potential risk of CO2 leakage from the subsurface and enable us to store more CO2 in in

same reservoirs compared to normal CO2 injection as well as economic benefit of

achieving higher oil recovery.

2.4.6 AGRICULTURAL & BIOLOGICAL APPLICATIONS

Recently, the application of MNB technology in biological processes has been focused

upon considerably. Water that contains MNBs has been reported to accelerate the

growth of plants and shellfish and has also been used in the aerobic cultivation of yeast.

The air micro-bubble supply resulted in a better cultivation of oysters (Heterocapsa

circularisquama) in terms of size and taste[31].. Kurata et al.[77], who applied oxygen

micro-bubbles in an osteoblast cell-culture system, reported greater alkaline

phosphatase activity, which was related to increased osteoblastic cell activity. Park and

Kurata[78] found that fresh weights of micro-bubble treated lettuces were 2.1 times

greater than those of the macro-bubble treated lettuces, when grown under a similar

dissolved oxygen (DO) concentration. Ushikubo et al.[79] showed that when barley

coleoptile cells were floated in water after the generation of oxygen MNBs, cytoplasmic

streaming rates inside the cells were accelerated. Moreover, nanobubbles (NBs) may

provide a transport mechanism for gas delivery to a membrane or cell and thus affect

trans-membrane proteins or the membrane structure. Both effects considerably alter the

cell function[45, 80].

Liu et al[31]., concluded that the germination rates of barley seeds dipped in water

containing NBs were 15–25 percentage points greater than those of seeds dipped in

distilled water, which verified the clear effect of NBs on the physiological activity.

According to NMR results, the number of NBs had a positive correlation with the T2

value of the water, which indicated that NBs could increase the mobility of the water

molecules in bulk. These results suggested that NBs in water could influence its

physical properties, which provides an explanation for the effect of NB promotion on the

physiological activity of living organisms. The other explanation is that negatively

charged NBs may influence the bioelectric field of plants, which is strongly related to

their elongation growth. After completely understanding NBs' ability to promote plant

growth is achieved, the manipulation of NBs will provide an efficient and cost-effective

approach for the cultivation of hydroponic vegetables and allow the development of a

new technology in agriculture applications.

CHAPTER 3: EXPERIMENTAL PROCEDURE


3. CHAPTER 3

EXPERIMENTAL PROCEDURE

3.1 INTRODUCTION

3.2 NANOBUBBLE GENERATION

Two MNB generators, each one based on different principle, were used in the present

study in order to produce nanobubbles in water. The first generator uses a porous plug

head through which nanobubbles are produced, while the second generator utilizes a

nozzle through which the gas-liquid mixture is ejected.

Both generators were developed at EMaTTech and will be described below.

3.2.1 POROUS PLUG GENERATOR

Figure 3.1 is a schematic illustrating the main components of the Nanobubble

Generating Device, and their interconnection. The system consists of three generators

connected in series.

Gas and a liquid are introduced to G1 [100] to produce a MB-containing liquid. The

liquid is fed to G2 [200] where it passes through a porous material [201], generating

MNB. These are stored in G2-Tank [400] and can be collected for various applications,

into G2-Tank [400]. The MNB-containing liquid from this tank can be circulated back to

G1 or pumped to G3 [300]. In G3 the liquid is compressed at 150bar and oscillated back

and forth through a fractal porous material [301] to generate an UNB-containing liquid.

This can be collected in Tank 3 [500], or deposited on a hydrophobic surface such as

Highly Ordered Pyrolytic Graphite1 (HOPG) [501].

1 Highly oriented pyrolytic graphite (HOPG) is a highly pure and ordered form of synthetic graphite. It is

characterised by a low mosaic spread angle, meaning that the individual graphite crystallites are well

aligned with each other. The best HOPG samples have mosaic spreads of less than 1 degree. The

method used to produce HOPG is based on the process used to make pyrolytic graphite, but with

additional tensile stress in the basal plane direction



In specific, G1 [100] consists of rotary pump 1 [101] and rotary pump 2 [102] connected

in series (Fig. 3.2). Gas and a liquid enter a mixer [103] through a capillary tube [104]

from a gas tank. Two check valves [105] ensure the flow is one directional. The MB-

containing liquid passes through pump 1 and then pump 2, then into G2 [200] pre

chamber [202] shown in Fig.3.3 Said generator contains a diaphragm assembly [203]

for compressing the MB-containing liquid and a fan [204] for stirring the MB-containing

liquid. In this step, the pre chamber [202] is pressurized at 30 to 40bar to ensure

permeability of the MB-containing liquid through the porous material [201]. Said porous

material can be rotated by switching on the DC motor [205], which in turn rotates the

two rollers [207], [208], a belt [206] and the porous material. The MNB are generated as

the liquid passes through the porous material and are collected in G2-Tank [400] shown

in detail in Fig. 3.4. A bypass system can be used to return the MNB-containing liquid

back to G1 [100].

Figure 3.1 Schematic representation of the main components of the Nanobubble Generating

Device, and their interconnection.



Figure 3.2 Schematic representation of G1, consisting of two rotary pumps connected in series

Figure 3.3 Pre-chamber, collecting liquid from pumps 1 and 2



Figure 3.4 Generator's sample collection tank

3.3 POROUS PLUG CHARACTERIZATION

The above mentioned porous plug, part of the MNB generator, was characterized by

Scanning Electron Microscopy (SEM). The porous plug consists of sintered brass

particles.

3.3.1 PRINCIPLES OF SCANNING ELECTRON MICROSCOPY

A scanning electron microscope (SEM) is a type of electron microscope that produces

images of a sample by scanning it with a focused beam of electrons. The electrons

interact with atoms in the sample, producing various signals that contain information

about the sample's surface topography and composition. The electron beam is

generally scanned in a raster scan pattern, and the beam's position is combined with

the detected signal to produce an image. SEM can achieve resolution better than 1

nanometer. Specimens can be observed in high vacuum, in low vacuum, in wet



conditions (in environmental SEM), and at a wide range of cryogenic or elevated

temperatures.

The most common SEM mode is detection of secondary electrons emitted by atoms

excited by the electron beam. The number of secondary electrons that can be detected

depends, among other things, on specimen topography. By scanning the sample and

collecting the secondary electrons that are emitted using a special detector, an image

displaying the topography of the surface is created.

Figure 3.5 Schematic of a SEM

As a first step, the porous plug was rigorously cleaned according to the following

procedure:

1. The plug was placed in acetone and left in the sonicator for 15 minutes.

2. The plug was placed in Methanol and left in the sonicator for 15 minutes.

3. The plug was placed in isopropanol and left in the sonicator for 15 minutes.

After each step the plug was thoroughly cleaned with cotton swabs soaked in

acetone, methanol and isopropanol respectively. Finally, the plug was soaked in

acetone and left aside to dry completely.

SEM analyses were carried out on a Jeol JSM-6390LV instrument. Superficial

observations were performed on the specimens. For cross-section analyses, the sample

was cut.



3.4 OPTICAL AND CONFOCAL MICROSCOPY

Produced nanobubbles were observed both by optical and by confocal microscopes.

3.4.1 CONFOCAL MICROSCOPE

3.4.1.1 CONFOCAL MICROSCOPY PRINCIPLES

Confocal microscopy, most frequently confocal laser scanning microscopy (CLSM), is

an optical imaging technique for increasing optical resolution and contrast of a

micrograph by means of adding a spatial pinhole placed at the confocal plane of the

lens to eliminate out-of-focus light. The principle of confocal imaging was patented in

1957 by Marvin Minskyand aims to overcome some limitations of traditional wide-field

fluorescence microscopes. In a conventional (i.e., wide-field) fluorescence microscope,

the entire specimen is flooded evenly in light from a light source. All parts of the

specimen in the optical path are excited at the same time and the resulting fluorescence

is detected by the microscope's photodetector or camera including a large unfocused

background part. In contrast, a confocal microscope uses point illumination (see Point

Spread Function) and a pinhole in an optically conjugate plane in front of the detector to

eliminate out-of-focus signal - the name "confocal" stems from this configuration. As

only light produced by fluorescence very close to the focal plane can be detected, the

image's optical resolution, particularly in the sample depth direction, is much better than

that of wide-field microscopes. Figure 3.1is a schematic representation of a confocal

microscope’s principle.

Figure 3.6 Principle of confocal microscopy



3.4.1.2 EXPERIMENTAL PROCEDURE

A sample of nanobubbles produced with the cavitation generator after 10mins of

operation was colored with Fluorescein isothiocyanate (FITC) and examined with the

confocal microscope.

3.5 DYNAMIC LIGHT SCATTERING MEASUREMENTS

Dynamic Light Scattering (DLS) measurements were conducted in order to determine

the size distribution of the nanobubbles produced with the aforementioned generators.

3.5.1 PRINCIPLES OF DYNAMIC LIGHT SCATTERING

Dynamic light scattering (DLS) is a technique in physics that can be used to determine

the size distribution profile of small particles in suspension or polymers in solution. The

basic principle is simple: In a typical DLS experiment, a solution/suspension of analyte

is irradiated with monochromatic laser light and fluctuations in the intensity of the

diffracted light are measured as a function of time. Intensity data is then collected using

an autocorrelator to determine the size distribution of particles or molecules in a sample.

Figure 3.2 depicts a typical Dynamic Light Scattering setup.

In general, when a sample of particles with diameter much smaller than the wavelength

of light is irradiated with light, each particle will diffract the incident light in all directions.

This is called Rayleigh scattering. In practice, particle samples are typically not

stationary because they are suspended in a solution and as a result they are moving

randomly due to collisions with solvent molecules. This type of motion is called

Brownian motion. Brownian motion is defined as:

“The random movement of particles in a liquid due to the bombardment by the

molecules that surround them”.



Figure 3.7 Brownian motion, relation of particle size to speed of movement

The particles in a liquid move about randomly and their speed of movement is used to

determine the size of the particle. Brownian motion is vital for DLS analysis because it

allows the use of the Stokes-Einstein equation (Eq. 3.1.) to relate the velocity of a

particle in solution to its hydrodynamic radius:

6

ktD

Eq. 3.1

In the Stokes-Einstein equation, D is the diffusion velocity of the particle, k is the

Boltzmann constant, T is the temperature, η is the viscosity of the solution and a is the

hydrodynamic radius of the particle. The diffusion velocity (D) in the Stokes-Einstein

relation is inversely proportional to the radius of the particle (a) and this shows that for a

system undergoing Brownian motion, small particles should diffuse faster than large

ones.



Figure 3.8 Typical Dynamic Light Scattering setup

3.5.2 DYNAMIC LIGHT SCATTERING EXPERIMENTAL PROCEDURE

A Malvern Zetasizer Nano ZS instrument was used to determine the NB size distribution

is a series of samples that were prepared (Table 3.1). The Zetasizer Nano system

measures the rate of the intensity fluctuation and then uses this to calculate the size of

the particles.

Table 3.1 Samples for DLS measurements

Porous Plug Vibration

P10 10min

P20 20min

P30 30min

P40 40min

V10 10min

V20 20min

V30 30min

V40 40min

Cavitation Counter Flow

C10 10min

C20 20min

C30 30min

C40 40min

CF10 10min

CF20 20min

CF30 30min

CF40 40min

The first measurement for the samples from the porous plug generator (P) and the

vibration generator (V) were conducted 24 hours after their production, while for

Cavitation (C) and Counter Flow (CF) generators the first measurements were

conducted immediately after production.



Deionized water is used as a reference for P and V samples while tap water is used as

a reference for C and CF samples.

Samples P40 and V40 were measured every 24 hours for a period of 7 days and then

every few days for a month.

3.6 ZETA POTENTIAL

Zeta potential is a scientific term for electrokinetic potential[1] in colloidal dispersions. In

the colloidal chemistry literature, it is usually denoted using the Greek letter zeta (ζ),

hence ζ-potential. From a theoretical viewpoint, the zeta potential is the electric potential

in the interfacial double layer (DL) at the location of the slipping plane relative to a point

in the bulk fluid away from the interface. In other words, zeta potential is the potential

difference between the dispersion medium and the stationary layer of fluid attached to

the dispersed particle.

The zeta potential is caused by the net electrical charge contained within the region

bounded by the slipping plane, and also depends on the location of that plane. Thus it is

widely used for quantification of the magnitude of the charge.

The zeta potential is a key indicator of the stability of colloidal dispersions. The

magnitude of the zeta potential indicates the degree of electrostatic repulsion between

adjacent, similarly charged particles in a dispersion. For molecules and particles that

are small enough, a high zeta potential will confer stability, i.e., the solution or

dispersion will resist aggregation. When the potential is small, attractive forces may

exceed this repulsion and the dispersion may break and flocculate. So, colloids with

high zeta potential (negative or positive) are electrically stabilized while colloids with low

zeta potentials tend to coagulate.

Zeta potential is not measurable directly but it can be calculated using theoretical

models and an experimentally-determined electrophoretic mobility or dynamic

electrophoretic mobility.



Figure 3.9 Figure depicting the EDL on a negatively charged particle.

Figure 3.3 shows the EDL on a negatively charged particle. Immediately on top of the

particle surface there is a strongly adhered layer (Stern layer) comprising of ions of

opposite charge i.e. positive ions in this case. Beyond Stern layer a diffuse layer

develops consisting of both negative and positive charges. During electrophoresis the

particle moves towards the electrodes with the slipping plane becoming the interface

between the mobile particles and dispersant. The ζ potential is the electrokinetic

potential at this slipping plane.

Electrophoretic light scattering is commonly used to determine the zeta potential.

3.6.1 ELECTROPHORETIC LIGHT SCATTERING INSTRUMENTATION

The electrophoretic mobility (μe) of the particles is first calculated as (Eq.3.2.):

e

V

E Eq. 3.2

Where V = particle velocity (μm/s), E = electric field strength (Volt/cm) – both known

quantities. The ZP is then calculated from the obtained μe by the Henry's equation

(Eq.3.3.):



02 ( )

3

re

e e f K

Eq. 3.3

Whereεr = relative permittivity/dielectric constant, ε0 = permittivity of vacuum, ζ = ZP,

f(Ka) = Henry's function and η = viscosity at experimental temperature.

The mobile particles during electrophoresis scatter an incident laser. As the particles

are mobile the scattered light has different frequency than the original laser and the

frequency shift is proportional to the speed of the particles (Doppler shift). The

instrumentation used for this technique is shown in Fig. 3.4.

The laser beam is split into two and while one beam is directed towards the sample the

other one is used as reference beam. The scattered light from the sample is combined

or optically mixed with the reference beam to determine the Doppler shift. The

magnitude of particle velocity (V) is deduced from the Doppler shift and then the ZP is

measured through the series of mathematical equations enlisted as Eqs. 3.2 and 3.3.

This technique is often used in conjunction with DLS.

Figure 3.10 Schematic showing the instrumentation of ZP measurement by electrophoretic light

scattering



3.6.2 Ζ POTENTIAL EXPERIMENTAL PROCEDURE

A Malvern Zetasizer Nano ZS instrument was used to determine the NB size distribution

is a series of samples that were prepared (Table 3.2).

Table 3.2 Samples for ζ potential measurements

Porous Plug Vibration

P10 10min

P20 20min

P30 30min

P40 40min

V10 10min

V20 20min

V30 30min

V40 40min

Cavitation Counter Flow

C10 10min

C20 20min

C30 30min

C40 40min

CF10 10min

CF20 20min

CF30 30min

CF40 40min

The first measurement for the samples from the porous plug generator (P) and the

vibration generator (V) were conducted 24 hours after their production, while for

Cavitation (C) and Counter Flow (CF) generators the first measurements were

conducted immediately after production.

Deionized water is used as a reference for P and V samples while tap water is used as

a reference for C and CF samples.

3.7 VAPOR PRESSURE

Vapor pressure (VP) is defined as the pressure exerted by a vapor in thermodynamic

equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed

system.

The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to

the tendency of particles to escape from the liquid (or a solid). A substance with a high

vapor pressure at normal temperatures is often referred to as volatile. The pressure

exhibited by vapor present above a liquid surface is known as vapor pressure. As the

temperature of a liquid increases, the kinetic energy of its molecules also increases. As

the kinetic energy of the molecules increases, the number of molecules transitioning

into a vapor also increases, thereby increasing the vapor pressure.

3.7.1 EXPERIMENTAL PROCEDURE

Experimental measurement of vapor pressure is a simple procedure for common

pressures between 1 and 200 kPa.[1] Procedure consists of isolating the sample in a

container, evacuating the gas, then measuring the equilibrium pressure of the gaseous

phase of the sample in the container at different temperatures. Better accuracy is



achieved when care is taken to ensure that the entire substance and its vapor are at

the prescribed temperature. This is done by submerging the containment area in a liquid

bath.

For the measurement of VP, a self-made experimental configuration was used (Figure.

The configuration consists of a vacuum pump, a liquid container, a pressure transmitter

and a water bath. The samples were measured at 20 oC, 30 oC and 40 oC.

A Wika D-10-P pressure transmitter is used in conjunction with a computer, running the

EasyCom 2011 V2.1.2 software, to record the measurements. In order to keep a

constant temperature a Julabo F25 water bath is used.

Figure 3.11 Vapor Pressure experimental configuration

CHAPTER 4: RESULTS AND DISCUSSION


4. CHAPTER 4

RESULTS AND DISCUSSION

4.1 INTRODUCTION

As stated in Chapter 1, this study aimed at giving evidence for the existence of

nanobubbles as well as studying their effect on the properties of water. As part of the

research, a number of experiments were conducted; the experimental procedures are

extensively described at Chapter 3. The purpose of this chapter is to summarize the

collected data, present them in a comprehensive way. Finally, the results are

extensively discussed and associated the extracted results with the theory.

4.2 TYNDALL EFFECT

The first experimental evidence of the existence of micro-nanobubbles in water was the

observation of the Tyndall effect in MNB-enriched water.

The sample was hit with monochromatic green light beam, and the Tyndall effect was

observed as it can be seen in Figure 4.1 and Figure 4.2.

Figure 4.1 Presence of Tyndall scattering in a sample containing micro-nanobubbles



Figure 4.2 Tyndall scattering is observed in the first two samples which contain nanobubbles. The

phenomenon cannot be observed to the last sample (right) which is simple water

The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a

colloid or else particles in a very fine suspension (Figure 4.3). In this case, the Tyndall

effect indicates the presence of gaseous phase in the form of nanobubbles in the water.

Due to the particles (in this case bubbles), which absorb light energy and then emit it

the beam can be seen in the sample.

Figure 4.3 Tyndall effect. in colloidal solution light beam is visible. This is due to the particles (in

this case bubbles) absorb light energy and then emit it



4.3 SIZE DISTRIBUTION

The size distribution of O2nanobubbles in water was determined by Dynamic Light

Scattering. The results were extracted using the ZetaSizer software and processed

using Microsoft Excel.

4.3.1 SIZE OF MNB AS A FUNCTION OF TIME

The distribution of size of micro-nanobubbles, is varied as a function of time. The

samples were measured for 7 weeks after their production, and the results are

presented in the following diagrams.

Figure 4.4 Size - Time Diagram for Porous Plug 10 min Sample

Obviously, during the first day after production, microbubbles still occur in the water and

the average size of the MNB is 1071 nm. In the second day after production, the mean

size drops to 649 nm. This fact indicates that bigger bubbles had burst while smaller

size bubbles still occur. However, three days after production the mean size rises again

up to 865 nm and reaches 991 nm at day 8. This result, indicates that some of the

bubbles a coalescing and form bigger bubbles, following the Ostwald rippening

phenomenon.

1071

649,9

865,1 859

974,3 991,7

0

200

400

600

800

1000

1200

Size

(n

m)




The same phenomenon was also observed for the 20 min porous plug sample. Here,

the decrease of size is much more steep, dropping from 2370 nm the first day, to 769

nm two days after production and then remaining quite stable.


However, as the production time is increased to 30 mins, the size, one day after

production is significantly lower (796.8 nm). This means that the generator produces

less microbubbles.

2370,00

769,60 756,90 794,70 776,30 777,90

0,00

500,00

1000,00

1500,00

2000,00

2500,00

1st day 2nd day 3rd day 4th day 5th day 8th day

Size

(n

m)

796,8

847,9 860,9

764,7

722,4

650

700

750

800

850

900

1st day 2nd day 3rd day 4th day 8th day

Size

(n

m)




The 40 min sample seems to have a great stability over time. It can be observed that for

a period of 7 weeks, the mean size of bubbles dropped from 578 nm the first day after

production to 516 nm the last day. Due to the long production time, no micro-bubbles

occur.

Figure 4.8 Size- Time Diagram for Vibrating Generator Samples

The samples generated from the vibrating device were measured every 24 hours for

seven days. Here, it is also observed that the NB size tends to increase over time, while

the production time has an important effect on the size. The 40 min sample, appears to

578,8 545,4 534,3

610,6 597,8

546,7 563,1

588,5 554,3

588,8

435,2

516

0

100

200

300

400

500

600

700

1st day 3rd day 4th day 5th day 8th day 9th day 10th day 11th day 15th day 17th day 24th day 7 weeks

Size

(n

m)

0

200

400

600

800

1000

1200

1400

1600

1800

1st day 7th day

10 min

20 min

30 min

40 min



not only have smaller sized bubbles but also is more stable. As the time passes, the

water is re-circulates into the tank, so bigger bubbles burst driving to the formation of

smaller size bubbles.

The 40 min sample was measured again after seven weeks. The average size is at this

time 200 nm. The hypothesis is again that bigger bubbles had burst but the

concentration is much lower.

Figure 4.6 depicts the size - intensity distribution for porous plug generator and vibration

generator for the 40 min sample. Also, the Auto Correlation Function diagram is cited.

Comparing the results of DLS for both methods (i.e. porous plug and nozzle) it can be

seen that the size of NB from porous plug generator is smaller than those produced by

the nozzle (824 nm), fact also shown from auto-correlation function (ACF) decay time,

as it can be seen that the curve for the porous plug is much steeper. However, the

nozzle performs more uniform distribution compared to the porous plug; where two

peaks are observed at 580 nm and 120 nm. Figure 4.6 shows the correlation curve of

the typical size of particles. Since the Brownian motion of large particles is slow and the

fluctuation of scattering light intensity changes slowly, the correlation will persist for a

long period of time. Moreover, since the Brownian motion of small particles is fast and

the fluctuation of scattering light intensity changes quickly, the correlation will reduce for

a short period of time. The diffusion coefficient of particles



Figure 4.9 Size distribution of NB produced by porous head (blue) and nozzle (green) generators.

At the left: The auto-correlation coefficient (ACF) diagram in the same colors.

Figure 4.10 Size-Production Time Diagram for Vibrating and Porous Plug Generator

y = -9,2173x + 979,14

y = -16,813x + 1427,4

300

500

700

900

1100

1300

1500

0 5 10 15 20 25 30 35 40 45

Size

(n

m)

Production Time (min)

Porous Generator Vibrating Generator Linear (Vibrating Generator)



Figure 4.7, presents the effect of production time to the average size of the MNB for

both generators. It is observed that for both samples the tendency is negative, and the

average size decreases as a function of time. This can be explained by the fact that as

the process continues, the O2 saturation of the water increases as it re-circulates into

the tank, leading to the formation of smaller bubbles. In any case, the porous plug

generator produces smaller sized bubbles than the vibrating generator.

4.3.2 SIZE OF NB AS A FUNCTION OF TEMPERATURE

Temperature is an important factor affecting the size distribution of nanobubbles. The

samples were measured at different temperatures and the results are shown below.

Table 4.1 NB size as a function of temperature

Day 1

p40 size (nm)

37 469,1

25 529,9

5 640

Day 1

v40 Size (nm)

37 538,9

25 404,2

5 664,5

Figure 4.11 Nanobubble size as a function of temperature for the porous plug (P40) and nozzle

(V40) generators after 40 minuntes of operations

The samples were measured at 37 oC, 25 oC and 5 oC. Interestingly, the mean size

slightly decreases as the temperature is increased. This could be caused by the fact

0

100

200

300

400

500

600

700

800

900

1000

37 25 5

P40

V40



that in lower temperatures the hydrogen bonds are stronger, then preventing the gas

escaping the bubble and expanding.

4.4 ZETA POTENTIAL

As mentioned earlier, zeta potential is indicative of the stability of nanobubbles. The

measurements were run along with DLS measurements and the results are shown

below.

Figure 4.12 Zeta potential as a function of time for the porous plug generator samples

The absolute value of zeta potential increases to a maximum of -18.3 mV for the 20 min

sample and then constantly decreases. However, seven days after the production, zeta

potential increases as the production time increases. In conjunction with the DLS

results, absolute zeta potential value increases as the bubble size decreases. This is

explained by the fact that smaller bubbles offer more surface for ions to be absorbed

onto the surface, thus repelling the bubbles.

The high value of ζ-potential can be related to the stability of bubbles, explained by the

repulsion forces generated by the electrically charged surfaces of bubbles, which avoid

the bubble coalescence.

The negative value is explained by Kelsall et al. (1996) as attributed to the

predominance of hydroxide ions in the first molecular layers of water at the gas-liquid

interface. It is also described by Najafi et al. (2007) that the negative charge on the

bubble surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It

is also described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1st day 7th day

10 min

20 min

30 min

40 min



1104 and -446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous

phase, leaving space at the gas-water interface for OH- . Similar understanding is that

an increase in OH concentration near the bubble surface suppresses the dissolution of

gas from bubbles into water and serves as “shells” for the bubbles, thus improving

stability (Takahashi, 2005). Apart from the zeta potential, an explanation for the NBs

stability is reported as the interface of NBs consists of hard hydrogen bonds that are

similar to the hydrogen bonds found in ice and gas hydrates (Ohgaki et al., 2010).

At a high absolute zeta potential, the electrical charged particles tend to repel each

other, avoiding aggregation of particles in a colloidal dispersion. In the case of NB

dispersion, the high absolute values of zeta potential could create repulsion forces that

would avoid the coalescence of NBs and contribute to the stabilization of the NBs.

In conclusion, the higher initial concentration of dissolved gas in water could explain the

extension of the NB stability because a higher dissolved gas concentration is expected

to suppress the dissolution of gas from NB into water.

However, for the vibrating generator, the results are quite different.

Figure 4.13 Zeta potential as a function of time for the vibrating generator samples

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

1st day 7th day

10 min

20 min

30 min

40 min



The combined results for the average zeta potential for both generators, is presented

below.

Figure 4.14 Effect of production time on zeta potential for the porous plug (blue) and nozzle

generator (red)

4.5 OPTICAL & CONFOCAL MICROSCOPY

Images of nanobubbles were recorded using laser scanning confocal microscope. The

sample was dyed with fluorescein. Fluorescein is a synthetic organic compound

available as a dark orange/red powder slightly soluble in water and alcohol. It is widely

used as a fluorescent tracer. In low concentrations, the color in aqueous solutions is

green. The images are a strong evidence of the existence of nanobubbles in aqueous

solutions. The sample used was produced from the low-pressure cavitation nanobubble

generator. It is observed that the size of bubbles is approximately 1000 nm.

For the optical microscopy images, the sample used was produced from the nozzle (left)

and porous plug generator (right), after 40 minutes of operation. It can be optically

observed that the porous plug generator, gives a larger concentration of nanobubbles

compared to the nozzle generator.

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 5 10 15 20 25 30 35 40 45

zeta

po

ten

tian

(m

V)

Production time (min) Porous Plug Vibrating



More specifically, it was calculated that the NB concentration for the porous plug

generator is approximately 750×103 NB/cm2 or 750×106 NB/ml. On the other hand, the

nozzle generator only produces 125×103 NB/cm2 or 125×106 NB/ml.

Figure 4.15 Optical microscopy images for the nozzle (left) and porous plug generators (right).

Both of the samples were taken after 40 mins of operation.

Furthermore, it can be seen that in the nozzle sample the bubbles are aggregated. The

aggregation happens due to the fact the size of the bubbles is bigger compared to the

porous plug generator. As a result, the bubbles are less stable and this leads to form

aggregates. According to Hunter, this happens due to the fact that bigger bubbles have

low potential and fluctuate[30].

Figure 4.16, is a photograph taken with confocal microscope. The nanobubbles, colored

with fluorescein can be clearly seen as bright green spots. Indicatively, some of them

are marked with red arrows. The mean size is 1000 nm.



Figure 4.16 Confocal Microscopy Image of a Nanobubble Sample, with fluoresceine

4.6 POROUS PLUG CHARACTERIZATION

The porous plug was characterized using a Scanning Electron Microscope, and the

following images were captured.

The mean size of the sinterened metal spheres is about 150 μm. If it is assumed that

the angle formed between three tangents of the circles is 60o then the radius of the

inscribed circle is:

r=75(1-cos30o)/cos30o=11.5μm.

The porous plug structure plays a critical role to the size of the produced nanobubbles

as the mixture of water and gas is forced through the plug under great pressure and

nanobubbles are formed during this procedure. Thus, the smaller size of the metal

spheres leads to smaller bubbles. Moreover, the NB size distribution is depended on the

size distribution of the spheres; a more uniform sintered spheres distribution leads to

more uniform NB size distribution. Hence, the porous plug is an expedient to control the

properties of the produced samples.



Figure 4.17 Scanning Electron Microscope images from the sintered porous plug at X140 (up) and

X40 (down)

4.7 VAPOR PRESSURE MEASUREMENTS

The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to

the tendency of particles to escape from the liquid (or a solid). A substance with a high

vapor pressure at normal temperatures is often referred to as volatile. The pressure

exhibited by vapor present above a liquid surface is known as vapor pressure. As the

temperature of a liquid increases, the kinetic energy of its molecules also increases. As



the kinetic energy of the molecules increases, the number of molecules transitioning

into a vapor also increases, thereby increasing the vapor pressure.

Figure 4.18 Vapour Pressure of NB samples, produced from the porous plug generator at different

temperatures; 20 oC (blue), 30

oC (red) and 40

oC (green)

It is observed that VP value for the NB45 sample shows an increase of approx.116%. It

is well known that increased vapor pressure indicates weaker intermolecular forces.

This is related to the surface tension of water, changes with the introduction of

nanobubbles according to many researchers. Ohgaki et al.[36] suggested that this is

strongly related to hydrogen bonding at water–gas interface. They reported that the

surface of the nanobubble contains hard hydrogen bonds. More recently, Wang, Liu,

and Dong[37] (2013) reported that the surface of a nanobubble is kinetically stable and

the water–gas interface is gas impermeable. A reduction in surface tension is observed.

Tolman and others predict a decrease of the surface tension for large curvature on

small scales[38-41]. Specifically, Tolman calculated theoretically that the surface tension

in drops should decrease significantly at small sizes.



4.8 CONDUCTIVITY MEASUREMENTS

Figure 4.19 Electrical Conductivity as a function of production time

Figure 4.16 presents the effect of production time on the electrical conductivity of the

samples as a function of time. It is evident that electrical conductivity highly increases

with the introduction of NB in water and is strongly depended on production time.

The results are in agreement with the theory, which suggests the existence of –OH and

–H ions. . It is described by Najafi et al[34]. (2007) that the negative charge on the bubble

surface is believed to be due to preferential adsorption of hydroxyl ions (OH- ). It is also

described that as the enthalpy of hydration of hydrogen ion (H+ ) and OH- is -1104 and

-446.8 kJ·mol-1, respectively, H+ preferentially remain in the bulk aqueous phase,

leaving space at the gas-water interface for OH-. Therefore, hydrogen ions (H+) are

remaining in the bulk and their presence increases the electrical conductivity.

0

5

10

15

20

25

30

35

-10 10 30 50 70 90 110 130 150

Co

nd

uct

ivit

y (μ

S)

Production Time (min)

Conductivity-Production Time



Figure 4.20 Distribution of ions at and near the gas-water interface in an aqueous solution of

electrolyte. The electrolyte ions are attracted to the interface and create the electrical double

layer.

4.9 EFFECT ON BIOLOGICAL MATTER; THE CASE OF PLANTS

The application of MNB technology in biological processes has been examined. Water

that contains MNBs has been reported to accelerate the growth of plants. Experiments

were conducted using oxygen and atmospheric air nanobubbles on soya and oat plants.

Micro-nanobubble enriched water was used on the aforementioned plants and their

growth rate was examined in comparison with normal water. All the plants were

exposed to the same environmental conditions and watered with the same volume.

In Figure 4.21, the results for oat plants are pictured after 8 days, while figure 4.22

shows the soya plants growth.



Figure 4.21 Left: Oat seeds watered with oxygen nanobubbles; Middle: Oat seeds watered with

atmospheric air nanobubbles; Right: Oat seeds watered with normal water

Figure 4.22 Left: Soya seeds watered with oxygen nanobubbles; Middle: Soya seeds watered with

atmospheric air nanobubbles; Right: Soya seeds watered with normal water

More experiments were conducted on wheat; and total plant weight was measured.



Figure 4.23 Wheat plant dry weight as a function of time.

These results suggested that NBs in water could influence its physical properties, which

provides an explanation for the effect of NB promotion on the physiological activity of

living organisms. Negatively charged NBs may influence the bioelectric field of plants,

which is strongly related to their elongation growth. Previous studies[81] have

demonstrated that hyperoxia promotes the growth of plants; air and oxygen-

nanobubbles may affect the growth of life by changing oxygen condition. Furthermore, it

is speculated that larger specific surface area of the microbubbles as well as negative

electronic charges on their surface may promote the growth of plants because

microbubbles can attract positively charged ions that are dissolved in the nutrient

solution.

It is suggested that hyperoxia may induce hypermetabolic state to maintain higher rate

of food digestion and absorption. These reports are in accordance with the results of our

study, suggesting that air and oxygen-nanobubble water solution may contribute to

elevated metabolism and promoted growth.

0

50

100

150

200

250

1 2 3 4 5

We

igh

t (g

r)

Days

NANOBUBUBBLES Ο2

Normal Water



As we have not used other gas-nonobubble water, whether promoting effect on growth

is due to nanobubbles themselves or elevated oxygen concentration in the water is still

unresolved. Further examination using other gas nanobubbles is required to determine

the effect of nanobubbles themselves on growth of lives. Although there are several

limitations, air and oxygen-nanobubble water significantly promoted the growth of plants

After completely understanding NBs' ability to promote plant growth is achieved, the

manipulation of NBs will provide an efficient and cost-effective approach for the

cultivation of hydroponic vegetables and allow the development of a new technology in

agriculture applications.

CHAPTER 5: CONCLUSIONS


5. CHAPTER 5

CONCLUSIONS

5.1 CONCLUSIONS

The results of this study are strong experimental evidences for the existence of

nanububbles in the bulk as well as their effect on important water properties such as

vapor pressure and conductivity.

As it is is mentioned, two types of nanobubble generators were designed and

manufactured; the “porous plug generator” and the “nozzle generator”. The nozzle

generator is based on the Venturi effect. In the Venturi-type generator system, both gas

and liquid are passed simultaneously via the Venturi tube to generate the bubble. When

pressurized fluid is introduced in the tubular part, the liquid flow velocity in the cylindrical

throat becomes higher whereas pressure becomes lower compared to the inlet section,

thus resulting in cavitation. According to the literature, similar generators already exist

and are studied by many researchers. However, the porous plug generator is an

innovative device which was designed in EMaTTech and is under EPO patent. Hence,

it was of vital importance to thoroughly examine both generators and compare their

performance.

The first experimental evidence of the existence of micro-nanobubbles in water was the

observation of the Tyndall effect in MNB-enriched water. In this case, the Tyndall effect

indicates the presence of gaseous phase in the form of nanobubbles in the water. Due

to bubbles, which absorb light energy and then emit it, the beam can be seen in the

sample.

As it derives from Dynamic Light Scattering and zeta potential measurements,

nanobubbles produced from the porous plug generator are smaller (≃580 nm) and more

stable ( -20 mV for 40 mins of operation) compared to those produced from the nozzle

generator, the mean size of which is ≃580 nm and their zeta potential is -6 mV. This

fact is also shown from auto-correlation function (ACF) decay time, as it can be seen

that the curve for the porous plug is much steeper. However, the nozzle performs more

uniform distribution compared to the porous plug; where two peaks are observed at 580

nm and 120 nm.



The high value of ζ-potential can be related to the stability of bubbles, explained by the

repulsion forces generated by the electrically charged surfaces of bubbles, which avoid

the bubble coalescence. The negative value is explained by the predominance of

hydroxide ions in the first molecular layers of water at the gas-liquid interface.

The zeta-potential measurement shows that the nanobubbles are negatively charged

with an electric double layer, presumably due to adsorption of negative OH- ions at the

gas/water interface. It is this double layer that plays a critical dual role in the formation

of stable nanobubbles in aqueous solutions. It not only provides a repulsive force to

prevent interbubble aggregation and coalescence but also reduces the surface tension

at the gas/water interface to decrease the internal pressure inside each bubble.

Another important factor to examine was the concentration of nanobubbles in the liquid.

The concentration was calculated based on microscopy images. It seems that the

porous plug generator produces ≃750×103 NB/cm2 or 750×106 NB/ml. On the other

hand, the nozzle generator only produces ≃125×103 NB/cm2 or 125×106 NB/ml.

It was observed that production time is an important factor for both generators; large

production time leads to the formation of smaller and more stable bubbles. . It is

observed that for both samples the tendency is negative, and the average size

decreases as a function of time. This can be explained by the fact that as the process

continues, the O2 saturation of the water increases as it re-circulates into the tank,

leading to the formation of smaller bubbles.

All of the samples were examined for several weeks. According to Dynamic Light

Scattering Measurements, the average size of the bubbles tends to decrease a few

days after production due to the fact that larger bubbles burst and smaller ones remain.

However, as times passes, the mean size tends to increase again. This can be

explained by the phenomenon of Ostwald ripening.

Again, porous plug nanobubbles seem to have an excellent stability over time. After 8

weeks their size dropped from 578 nm to 516 nm.

Temperature also has an effect on size, which decreases with reduction of temperature.

Electrical conductivity and vapor pressure which are some of the most important

properties of water were studied. It turns out that both of them were affected from the

introduction of nanobubbles. Nanobubble-enriched water has a significantly higher

electrical conductivity than normal water, due to the excess of free ions in the bulk.

CHAPTER 5: CONCLUSIONS


Moreover, the vapor pressure is higher due to the fact of weaker bonds between the

molecules.

It is observed that vapor pressure value shows an increase of approx.116%. It is well

known that increased vapor pressure indicates weaker intermolecular forces. This is

related to the surface tension of water, changes with the introduction of nanobubbles

according to many researchers. It is suggested that this is strongly related to hydrogen

bonding at water–gas interface. They reported that the surface of the nanobubble

contains hard hydrogen bonds.

The growing significance of nanotechnology as well as the special properties of

nanobubbles has drawn huge attention in many sectors due to their wide range of

applications, including mine industry, medical applications, food processing and

wastewater treatment.

The effect on biological matter has been studied; water that contains MNBs has been

reported to accelerate the growth of plants. Micro-nanobubble enriched water was used

on soya, oat and wheat plants and their growth rate was examined in comparison with

normal water. All the plants were exposed to the same environmental conditions and

watered with the same volume. These results suggested that NBs in water could

influence its physical properties, which provides an explanation for the effect of NB

promotion on the physiological activity of living organisms. Negatively charged NBs may

influence the bioelectric field of plants, which is strongly related to their elongation

growth. Hyperoxia promotes the growth of plants; air and oxygen-nanobubbles may

affect the growth of life by changing oxygen condition. Furthermore, it is speculated that

larger specific surface area of the microbubbles as well as negative electronic charges

on their surface may promote the growth of plants because microbubbles can attract

positively charged ions that are dissolved in the nutrient solution.

5.2 FURTHER RESEARCH

The purpose of the dissertation was to elucidate the effects of nanobubble suspensions,

produced with nanobubbles generators, and study the nanobubble formation, size

distribution, coalescence, stability and dynamic behavior. Consequently, gain insight

into the properties of nanobubbles. This study discussed the effects of bulk

nanobubbles on the physicochemical properties of water based on research results

from a variety of experiments.



While their existence has been confirmed, there are many open questions related to

their formation and dissolution processes along with their structures and properties,

which are difficult to investigate experimentally.

It is considered important to examine nanobubbles under a wide range of pH in order to

gain understanding on their charging mechanism.

Moreover, nanobubbles consist of a condensed gaseous phase with a surface tension

smaller than that of an equivalent system under atmospheric conditions, and contact

angles larger than those in the equivalent nanodroplet case. We anticipate that further

study will provide useful insights into the physics of nanobubbles and will stimulate

further research in the field. For that reasons, surface tension and contact angle

measurements should be conducted.



6. ABBREVIATIONS – INITIALS

MNB Micro-Nano Bubbles

NB Nano-Bubbles

VP-LP Vapor Phase-Liquid Phase

BP Boiling Point

MP Melting Point

VP Vapor Pressure

DLS Dynamic Light Scattering

SEM Scanning Electron Microscope



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