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Degreasing of Solid Surfaces by Microbubble Cleaning

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2007 Jpn. J. Appl. Phys. 46 1236

(http://iopscience.iop.org/1347-4065/46/3R/1236)

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Degreasing of Solid Surfaces by Microbubble Cleaning

Makoto MIYAMOTO�, Satoshi UEYAMA, Nobuhide HINOMOTO1,

Tadashi SAITOH, Shigeki MAEKAWA1, and Junji HIROTSUJI

Environmental Technology and Systems Department, Advanced Technology R&D Center, Mitsubishi Electric Corporation,

8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan1Structural Technology Department, Manufacturing Engineering Center, Mitsubishi Electric Corporation,

8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661-8661, Japan

(Received June 2, 2006; revised November 22, 2006; accepted November 30, 2006; published online March 8, 2007)

It is increasingly required to reduce the environmental impact and cost in the field of industrial cleaning. As a substitute forconventional degreasing technology using organic solvents, acids, and alkalis, the authors have developed a new cleaningtechnology that uses microbubbles having an average diameter of about 70 mm. Grease being adsorbed onto a bubble’s surfaceand grease being separated from a solid surface by its buoyancy were captured using a high-speed microscopic video camerato demonstrate the degreasing capability of bubbles. High-density microbubbles were generated by adding a trace amount of aspecific chemical (0.1% weight or less). The cleaning performance using microbubbles was found to be highly improvedcompared with that using normal bubbles. It was also revealed that the grease removal efficiency was strongly dependent onthe viscosity of the grease. Raising the temperature of the cleaning solution is an effective method of improving cleaningperformance by reducing the viscosity. Finally, the degreasing of about 150 machining metal parts at the same time wasdemonstrated to exceed the common target cleaning level (5–20 mg/cm2) in only 2min because of their large surface area.Furthermore, the high degreasing performance was maintained even after repeated use of the cleaning solution because of theseparation of grease due to buoyancy. [DOI: 10.1143/JJAP.46.1236]

KEYWORDS: degreasing, microbubble, cleaning, high-density, additive

1. Introduction

Chlorofluorocarbons (CFC) were primarily used to re-move grease from electronic and mechanical componentsafter machining and cutting processes until 1974, when CFCwere found to induce ozone layer depletion,1) a seriousglobal environmental problem. Chlorinated organic solventssuch as trichloroethane and dichloromethane have been usedas substitutes. However, they also induce global warming2)

although their ozone depletion potential (ODP) is lower thanthose of CFC.3,4) Fluorinated organic solvents and halogen-free organic solvents (hydrocarbons) have started to be used,but they may have a negative impact on the health of theirusers because they are highly volatile, and they also result inhigh-cost cleaning. From the viewpoint of cleaning perform-ance, the target level of degreasing is 5–20 mg/cm2 of theremaining grease after degreasing. This target value has astrongly relation with the efficiency of later processes afterdegreasing.

To reduce the environmental impact of the degreasingprocess, semiaqueous and aqueous cleaning solutions suchas acid and alkali solutions have been increasingly adopted,but these solvents are expensive and their waste solutiontreatment is expensive. In addition, the degreasing processusing semiaqueous and aqueous cleaning solutions con-sumes large amounts of pure water in the subsequent rinsing.Although functional water cleaning such as electrolysiswith alkali water and high-pressure jet cleaning have alsobeen developed for environmentally friendly degreasing,these technologies have been found unsatisfactory in termsof removal efficiency, throughput, or cleaning uniformity.These cleaning methods can be roughly categorized into twogroups in terms of their degreasing mechanism: 1) greasemolecules are dissolved into the cleaning solution by meansof the dissolving properties of the solution or the affinity of

the surfactant, and 2) grease molecules are lifted off froma solid surface by physical force. In both cases, however,grease molecules gradually accumulate in the cleaningsolution, which makes it impossible to use the solutionrepeatedly in a stable manner and maintain uniform cleaningperformance.

In an attempt to completely overcome these difficultiesof existing cleaning methods, the authors focused on thepotential of bubbles to degrease. It was considered thatgrease molecules are adsorbed onto the bubble surface whena bubble, which constitutes a hydrophobic reaction field,is generated in water. A bubble is buoyant and therebypromptly rises to the water surface. When a bubble withadsorbed grease breaks on the water surface, the greaseremains at the interface in the form of a grease film. Byremoving this grease film, grease can be removed and thecleaning solution can be purified at the same time. Theauthors have developed a new microbubble cleaning methodthat is environmentally friendly and cheap and that featureshigh-efficiency grease removal.

2. Microbubbles

A microbubble is a bubble generated in water with adiameter of 100 mm or less. Figure 1 shows photographs ofordinary bubbles and microbubbles. The diameter of anordinary bubble is about 2mm as in Fig. 1 whereas thediameter of a microbubble is about fifty mm, which is smallerthan that of an ordinary bubble by about 2 orders ofmagnitude. Microbubbles have been reported to have thefollowing characteristics.5–7)

(1) A microbubble has a large surface area per unit gasvolume.

(2) Hydrophobic and amphipathic substances build up onthe surface of a bubble.

(3) A microbubble can remain in water for a relativelylong time.

Characteristic (1) is explained in the following way. The�E-mail address: Miyamoto.Makoto@ak.MitsubishiElectric.co.jp

Japanese Journal of Applied Physics

Vol. 46, No. 3A, 2007, pp. 1236–1243

#2007 The Japan Society of Applied Physics

1236

volume, V , of a spherical bubble with a diameter of d isexpressed as

V ¼ �d3=6:

The surface area, S, of the bubble is expressed as

S ¼ �d2:

The surface area per unit volume, therefore, can be ex-pressed as

S=V ¼ 6=d:

Therefore, the smaller the diameter of the bubble, the largerthe surface area for the same amount of volume.

The authors focused on the large surface area and theadsorption capability of the bubble surface for application tothe degreasing process. To apply microbubbles to degreas-ing, which requires the treatment of large amounts of greasewithin a short time, it is important to generate microbubblesat as high density (many microbubbles per unit watervolume) as possible. Microbubbles with a smaller diameterhave a number of advantages. Firstly, the surface area ofmicrobubbles per unit gas volume is larger. Secondly, theyremain in water for a longer period of time, which raises theprobability of contact with grease. Thirdly, they can freelycome in contact with the grease on the various sized andcomplex shapes. Accordingly, increasing the density ofmicrobubbles is important for reducing cleaning time, i.e., toimprove throughput.

This paper will report the following;(1) The generation of high-density microbubbles using

specific additives.(2) The degreasing phenomenon captured using a high-

speed microscopic video camera.(3) The degreasing performance of microbubbles for a

single machining metal part.(4) The repeatability of degreasing performance of the

same cleaning solution for many machining metal parts(about 150 parts).

3. Experimental Procedure

3.1 MaterialsSome alcohol compounds were specially prepared for this

study to generate high-density microbubbles. (The role ofthis additive will be described in detail later.) Linoleic acidand oleic acid were used as single-component fatty acids,and hot-quench oil 809XV manufactured by Nippon Greasewas used as industrial-use grease to investigate the cleaningperformance for single components. Hot-quench oil 809XVis a mineral-based grease used for the quenching processfollowing the thermal treatment of mechanical components.To study the relationship between rease removal efficiencyand grease viscosity, several test-sample greases withvarious viscosities were prepared from 809XV. Ring-shapedstainless-steel test pieces (28mm outer diameter, 15mminner diameter and 9mm high, were used as a representativemachining metal part in a series of cleaning experiments(see Fig. 9). In the cleaning experiments, pure water treatedwith ion-exchange resin and air was used to generate themicrobubbles.

3.2 Observing degreasing by bubblesFigure 2 is a schematic diagram of the system use to

observe the degreasing mechanism. A miniature observationbath was prepared by clamping a U-shaped silicone rubberpiece (4mm thick) between two sheets of slide glass. Asmall droplet of 809XV was placed on one side of the slideglasses. A syringe needle (SUS 30G: 0.14mm innerdiameter and 0.32mm outer diameter) was inserted intothe miniature bath through the silicone bottom. The

1mm 1mm1mm 1mm1mm 1mm

Microbubbles Ordinary-sized bubbles

Same scale

Fig. 1. Magnified photographs of microbubbles and ordinary bubbles.

Light

Slide glass

Water

Grease with carbon powder

Syringe needle

U-shaped silicone rubber

High-speed microscopic video camera

(a)

(b)

Bubble

Fig. 2. Schematic diagram of the experimental system use to observe the degreasing mechanism: (a) side view and (b) front view.

Jpn. J. Appl. Phys., Vol. 46, No. 3A (2007) M. MIYAMOTO et al.

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observation bath was slightly tilted so that the microbubblesgenerated from the syringe needle could come into contactwith the grease. Air, whose flow rate was controlled at 5mL/min using a mass flow controller, was passed throughthe syringe needle.

A high-speed microscopic video camera, Motion Promodel 10000 Mono manufactured by Roper, was used withan 80–200mm zoom lens and a 75mm Nikon close-up ring.A backlight (LS-M350) manufactured by Sumita was alsoused. During the video shooting, 1000 frames/s were takenwith a shutter speed of 1/10000, and the lens diaphragm waskept open. MiDAS camera driver software was used toconvert the moving pictures into still images.

3.3 Cleaning experimentsFigure 3 shows an outline of the cleaning system used in

the experiments. The system was composed of a cleaningbath, a rinsing bath, an ejector as the microbubble generator,a water-circulating pump, a gas feeding pump, and a controlpanel. Microbubbles were generated in the ejector by mixingpure water and gas whose flow rates were controlled. Theejector is constricted in the middle. The pressure there isreduced because the flow rate of water is accelerated, andthus the gas is automatically sucked in due to the pressurereduction. A cleaning solution containing the microbubbleswas fed into the cleaning bath containing the stainless-steeltest pieces in a basket. The grease removed from the testpieces was drained out into an overflow bath.

Prior to the cleaning experiments, the ring-shaped stain-less-steel test pieces were entirely coated with grease andleft for 10min. One or many stainless-steel pieces wereplaced in the cleaning system and cleaned for a predeter-mined time. After the cleaning processs, the test pieces wererinsed with pure water for 5 to 30 s and then blow-dried withnitrogen gas. The amounts of grease on the test pieces beforeand after the cleaning process were measured using an oilcontent meter (OCMA300 manufactured by Horiba) whosemeasurement principle is IR analysis. The oil content isquantified by detecting the change in infrared ray (IR)absorption on the vasis of the stretching vibration (2940–2860 cm�1) of hydrocarbon-specific –CH2– and –CH3

groups. An extraction solvent, S-316, manufactured byDaikin Industries, Ltd, was used to extract grease from thetest pieces. This solvent is a dimer of chlorofluoroethylenethat has almost no IR absorption around 3000 cm�1. Table Ishows the other cleaning conditions.

3.4 Measurement of microbubble diameterFigure 4 shows the experimental system used to measure

the microbubble diameter. The cleaning solution thatcontained microbubbles was introduced into an acrylic resinplate with an inner gap of 1–3mm. Images of the micro-bubbles contained in the cleaning solution were obtainedusing a digital still camera over the acrylic plate. The imageswere analyzed to calculate the mean diameter of themicrobubbles. The procedures are described in more detailbelow. In this paper, the mean microbubble diameter wasdefined as the average of all analyzed data.(1) The microbubbles rising through the acrylic resin tube

were photographed using a digital still camera (EOSKiss Digital N manufactured by Canon) with amagnification of three.

(2) A grid sheet was placed at the same location as themicrobubbles and the sheet was photographed in thesame manner as above. This grid was used as a scale.

(3) The photographs of microbubbles and the scale wereinput into a personal computer, and magnified asnecessary with the same rate, and then the diameter ofeach microbubble was measured.

4. Results and Discussion

4.1 Generation of high-density microbubbles by adding atiny amount of a specific additive

The generation of high-density microbubbles with adiameter of 100 mm or less is very difficult because thebubbles merge if they contact each other. It has beenreported that bubbles hardly merge with each other in aspiked surfactant induced by surfactant molecules adsorbedonto the bubble surface; this is known as the Marangonieffect.8,9) The tiny amount of additive is considered essentialto increase the density of the microbubbles. The authorsconfirmed that a specific alcohol compound of 10–1000parts per million (ppm) was effective for preventing themerging of bubbles. Figure 5 shows photographs thatcompare the bubbles generated in pure water with the addedalcohol compound of 100 ppm and those without theadditive with the same gas volume. The solution with theadditive became clouded due to the high density of micro-bubbles. On the formation of other hand, the solutionwithout the additive remained transparent due to the largebubbles of several millimeters.

It is not clear how the additive actually works, althoughit has been proven effective. In an attempt to reveal themechanism, the authors observed the behavior of bubblegeneration from micropores using a high-speed microscopicvideo camera. In pure water without the additive, thebubbles were found to be small immediately after they weregenerated, but they rapidly merged and grew in size as theycollided with each other (Fig. 6). On the other hand, in pure

PFlow meter

Water-circulating pump

Valve

Pressure gauge

Microbubble ejector

Cleaning vessel

Cleaning bath

Overflowbath

Fig. 3. Schematic illustration of the experimental cleaning system.

Table I. Cleaning conditions.

Cleaning time (min) 0–5

Additive concentration (wt%) 0–0.1

Cleaning temperature (�C) 20–70

Flow rate of air (L/min) 0–40

Flow rate of solution (L/min) 0–20

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water with the 0.01% alcohol compound, the microbubbleswere found to rebound off each other, similar to balloons,when they collided. The authors tested several kinds ofalcohol compound with similar structural isomers and foundthat they were considerably different in their capability togenerate microbubbles. It is considered that the alcoholmolecules are adsorbed onto the microbubble surface andthereby prevent the microbubbles from merging. It is alsosuggested that a slight difference in chemical structure wasa strong impact on microbubble generation. The importantcharacteristics of the additive include

. solublity in water

. no environmental impact

. not within the scope of the PRTR law or any otherlegislation

. not very volatile

. not very dependent on temperature.Figure 7 shows the diameter distribution of the bubbles

in the cleaning bath. In pure water without the additive,bubbles with a diameter of about 4mm were predominant.When a trace amount of alcohol compound was added topure water, the mean bubble diameter decreased by a factorof 50 or more to 75 mm. Accordingly the surface area of thebubbles is increased by 50 times or more.

4.2 Observation of the degreasing mechanism ofmicrobubble cleaning

In the degreasing process using microbubbles, it is mostimportant that grease, such as machining oil or cutting oil, isadsorbed onto the microbubble surface. In an attempt to

2 mm2 mm 2 mm

Fig. 5. Photographs comparing high-density microbubbles generated

in pure water with a trace amount of additive (left side) and bubbles

generated in pure water without the additive (right side).

Water temperature:28°CTime interval : 500 sec.Shutter speed: 10 sec.

µµ

fusionfusion

Fig. 6. Photographs of bubble fusion obtained with high-speed micro-

scopic video camera.

P

Pressure gauge

PWater flow meter

Water-circulating pump

Valve

P Gas pump

Gas flow meter

LightHigh-speed microscopicvideo camera

Overflowbath

Microbubble generator

Acrylic plate

Cleaning bath

Perista pump

Fig. 4. Schematic illustration of the experimental

system use to measure microbubble diameter.

Jpn. J. Appl. Phys., Vol. 46, No. 3A (2007) M. MIYAMOTO et al.

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demonstrate the adsorption of grease onto the microbubblesurface, the authors have developed an experimental systemusing a high-speed microscopic video camera to visualizehow grease is removed from a solid surface. Figure 8 showstypical images, (1–6) obtained from the video. In thisexperiment, a slide glass was stained using hot-quench oil809XV, and bubbles were slowly generated one afteranother from a syringe needle placed underneath the slideglass while the behavior of the machine oil and bubbleswas recorded using a high-speed microscopic video camera(2000 frames/s).

A bubble leaving the tip of the syringe needle is adsorbedonto the surface of the machine oil on the slide glass (1). Thebubble stays there for some time while the machine oilspreads around its surface (2). The bubble then rises to thepure water surface, where there is no machine oil. Thismeans that the adsorption can be attributed to the interactionbetween the bubble and the machine oil. A bubble remainingon the surface of the grease on the slide glass grows larger asit collides and merges with another bubble (3). This steptook place repeatedly and the bubble grew further. When the

bubble grew to a certain size, it was desorbed from thesurface of the grease on the slide glass due to its buoyancy(4). The surface of the bubble was found to have lost itsluster and was coated with a thin grease film. In some cases,a bubble took a lump of grease with it when it rose (5).When arriving at the pure water surface, the bubble stayedthere for a while, and then broke to leave the grease on thepure water surface (6). After the bubble desorbed the grease,the amount of grease remaining on the slide glass decreased.Within a few seconds, the grease on the slide glass wastotally removed.

The schematic diagram shown on the right side of Fig. 8illustrates grease adsorption onto the bubble surface. Grease,which is intrinsically hydrophobic, is unstable in water.When gas in the form of a bubble comes in contact with thegrease in the water, grease molecules are adsorbed onto thebubble. This is the initial degreasing stage of microbubblecleaning. Once hydrophobic grease completely covers thebubble surface, it is speculated that the grease molecules,which are in a high-energy state at the grease/waterinterface, move one after another toward the bubble surfaceto cover it with thin layer of grease to stabilize themolecules. Eventually, grease molecules accumulate aroundthe bubble to form a thick film, which can be observed usinga high-speed microscopic video camera. The use of a high-speed microscopic video camera enables us to observe thisgrease adsorption onto the bubble surface.

It was also revealed in this experiment that the grease ona solid surface can be removed using a number of micro-bubbles, although each microbubble can adsorb only a smallamount of grease. This means that it is important to increasethe density of the microbubbles as much as possible toremove a large amount of grease on a solid surface within apractical length of time. In the above experiments, the greaseremoval capability of the microbubble cleaning procedurewas verified only for grease on a flat surface. In actual cases,however, the components to be treated with microbubble aremore complex and irregular in shape, being convex and/or

0

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Diameter distribution of bubbles (µm)

Freq

uenc

y (%

)

Additive conc. : 0.01wt%Gas flow rate : 1 L/minWater flow rate : 7 L/minWater temperature : 25°C

Fig. 7. Diameter distribution of bubbles in cleaning bath.

Air/water interface

grease

Syringe needle

Separated grease

Adsorbed oil onto a bubble

water

gas

Oil molecule

Fig. 8. Images obtained from the video to show

degreasing process using bubbles.

Jpn. J. Appl. Phys., Vol. 46, No. 3A (2007) M. MIYAMOTO et al.

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concave. To treat such components in a uniform manner, it isessential to make the microbubbles even smaller.

4.3 Verification of basic performance of microbubblecleaning

To obtain qualitative degreasing results, the authors usedthree kinds of grease: linoleic acid, oleic acid, and hydro-phobic grease. Firstly, the authors tested the grease removalefficiency when a single stainless-steel test piece wascleaned. Figure 9 shows photographs of a test piece coveredwith hydrophobic grease before and after 10 s microbubblecleaning. The thick layer of hydrophobic grease on the testpiece was perfectly removed by microbubble cleaning. Itwas found, by observing the wettability of the water dropletsleft on the test piece after the rinsing process, that the surfaceof the test piece became to hydrophilic.

Next, the authors investigated the difference in greaseremoval efficiency for a test piece treated using micro-bubbles compared with a test piece rinsed in solutionwithout any bubbles and a test piece treated using ordinary-sized bubbles (i.e., without the additive). A ring-shapedstainless-steel piece was completely coated with hydro-phobic grease that was designed for thermal treatment andconsidered difficult to remove. The test pieces was placed ata predetermined distance from the bubble generator andtreated for 1min. Figure 10 shows the experimental results.The vertical axis shows the density of the residual grease perunit surface area of the test piece. The test piece was coated

with grease of 1280 mg/cm2 before cleaning. The amount ofgrease was reduced by half when the test piece was rinsedwith pure running water. 185 mg/cm2 of grease was left onthe test piece treated with pure water with ordinary-sizedbubbles; the average bubble diameter was 2000–3000 mm.The grease removal efficiency was about 80%, which wasnot satisfactory. After the cleaning using microbubbleswhose average diameter was 80 mm, the residual grease wasdecreased to 20.5 mg/cm2. The grease removal efficiency ofmicrobubble cleaning is higher than that of the rinsing withpure running water by a factor of 60 or more and higher thanthat of ordinary-sized-bubble cleaning by a factor of about10. Figure 11 shows the residual amounts of linoleic acidand oleic acid after microbubble cleaning. As in the case ofhydrophobic grease, microbubble cleaning was found to behighly effective for removing both linoleic acid and oleicacid compared with ordinary-sized-bubble cleaning. As aresult of these experiments, microbubble cleaning is de-monstreated to have outstanding grease removal efficiency.

4.4 Further improvement of grease removal efficiencyMicrobubble cleaning should reduce the residual grease

density down to 10 mg/cm2 or less to meet the generalrequirements of industry. Since the degreasing mechanismof microbubble cleaning is based on grease adsorption ontothe microbubble surface, it is easy to consider that the greaseremoval efficiency depends on the viscosity of the grease.The authors studied the grease removal efficiency ofmicrobubble cleaning using different greases of almost thesame composition with various viscosities. Figure 12 showsthe experimental results. The grease removal efficiency ofmicrobubble cleaning was found to be strongly dependenton the viscosity of the grease. This result suggests thatdecreasing the viscosity of the grease should be effective forimproving the grease removal efficiency of microbubblecleaning. In general, the viscosity of grease strongly dependson temperature. The authors studied the temperature depen-dence of grease viscosity and grease removal efficiency,using hot-quench oil 809XV. Figure 13 show the exper-imental results. The grease removal efficiency was foundto improve by a factor of 4 or more when the temperature ofthe cleaning solution was raised to about 40 �C. When thetemperature was raised further, the grease removal efficiencyshowed a gradual improvement. It was demonstrated in these

Water droplet

(a) (b)

Grease

Fig. 9. Photographs of a test piece stained with hydrophobic grease (a)

before and (b) after 10 s microbubble cleaning.

570

1280

18520.5

0200400600800

100012001400

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Fig. 10. Efficiency of removal of persistent hydrophobic grease.

0102030405060708090

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Rem

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ease

den

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(µg

/cm

2 )

Microbubbles

Ordinary-sized bubbles

Fig. 11. Efficiency of removal of oleic acid and linoleic acid in cleaning

solutions with and without microbubbles.

Jpn. J. Appl. Phys., Vol. 46, No. 3A (2007) M. MIYAMOTO et al.

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experiments that raising the temperature of the cleaningsolution has a positive impact on microbubble cleaning.The viscosity of grease was decreased with the increase intemperature. The viscosity and grease removal efficiencywere found to have a good correlation. This means that thisimprovement of grease removal efficiency can be attributedto the decrease in grease viscosity.

4.5 Grease removal efficiency when many pieces aretreated

The experiments presented in the previous sectionsdemonstrate clearly that microbubble cleaning is effectivewhen a single test piece is treated. In industrial applications,however, several hundred components may needto becleaned effectively within a few minutes. In microbubblecleaning, the temperature of the cleaning solution needs tobe maintained at 50 degrees to maximize its grease removalefficiency.

The authors studied the grease removal efficiency ofmicrobubble cleaning when a number of test pieces weretreated at the same time in repeated batch treatments.Figure 14 shows the experimental results. No difference wasfound in grease removal efficiency between the cases when asingle test piece was treated and when 100 test pieces weretreated at the same time. In other words, microbubblecleaning results in sufficiently high grease removal efficien-cy even when treating a number of components at the same

time. Even after the repeated use of the cleaning solution,grease removal efficiency was found to be almost un-changed. The total organic carbon (TOC) concentrationwas periodically measured after repeating the microbubblecleaning several times. The TOC concentration was foundto remain constant, being independent of the number oftimes of microbubble cleaning. The microbubbles alsoremain unchanged throughout the repeated treatments. Inanother experiment, when grease of about 1% of the totalweight of the cleaning solution was added to the cleaningsolution, the grease removal efficiency was unchanged. Thisoutstanding stability of the microbubble cleaning solution isattributed to the mechanism that constantly separates greasefrom the cleaning solution. As shown in Fig. 15, the greaseremoved from the surface of the test pieces is effectively andefficiently transported from the cleaning bath to the overflowbath. In an attempt to study the effectiveness of this oilseparation mechanism, the authors mixed grease into purewater (with the additive) in the form of an emulsion byultrasonic irradiation, and treated the mixture by the micro-bubble cleaning process. As shown in Fig. 16 (‘‘w/’’indicates ‘‘with’’ and ‘‘w/o’’ indicates ‘‘without’’, in thisfigure), it is found that microbubbles are a capable ofperfectly removing the grease mixed in the form of anemulsion. Microbubble cleaning was found to be effective inkeeping the cleaning solution unchanged even after repeateduse due to its efficient oil separation mechanism.

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Coefficient of grease viscosity (Pa··s)

Rem

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Fig. 12. Grease removal efficiency as a function of coefficient of grease

viscosity.

020406080

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02468101214161820

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Pa

·· s)

Grease removal efficiency

Viscosity

Rem

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µ2 )

Fig. 13. Wter temperature dependence of grease

removal efficiency and coefficient of grease viscos-

ity for the same grease.

0

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)

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Before cleaning3659 g/cmµ 2

Fig. 14. Grease removal efficiency in repeated batch treatments cleaning

for multiple test pieces.

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5. Conclusions

. The use of an additive has made it possible to generatehigh-density microbubbles with a very large totalsurface area.

. Grease adsorption onto the microbubble surface wasobserved using a high-speed microscopic video camera.It was demonstrated by this observation that grease ona solid surface was removed by two mechanisms: 1)grease was adsorbed onto the bubble surface and 2) thebubbles removed the grease from a solid surface as theywere desorbed and rose up due to their buoyancy.

. Microbubble cleaning was proven effective for remov-ing grease that is difficult to remove by conventionalcleaning methods. The grease removal efficiency wasfound to strongly depend on the viscosity of the grease.

. It was demonstrated that the grease removal efficiencycan be increased by increasing the temperature of thecleaning solution to decrease the viscosity of thegrease.

. Microbubble cleaning was found to be effective forremoving grease from a number of machining metalparts at the same time. It was also found to remaineffective even when the cleaning solution is usedrepeatedly.

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297.

Cleaning bath

Overflowbath

Separated grease

Fig. 15. Photograph of separated grease in overflow bath when cleaning

multiple test pieces.

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Before w/Microbubble treatment

w/o Microbubble treatment

TOC

con

c. in

cle

anin

g so

lutio

n[m

g/L_

TOC

]

Fig. 16. Grease separation using microbubbles in cleaning solution mixed

with the grease in the form of an emulsion.

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