ARTICLE IN PRESS

Optics & Laser Technology 42 (2010) 1049–1053

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Optics & Laser Technology

0030-39

doi:10.1

� Corr

E-m

journal homepage: www.elsevier.com/locate/optlastec

Influence of different interfaces on laser propulsion in water environment

Bing Han, Jun Chen, Hong-Chao Zhang, Zhong-Hua Shen, Jian Lu, Xiao-Wu Ni �

Department of Science, Nanjing University of Science & Technology, Nanjing 210094, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 4 November 2009

Received in revised form

17 January 2010

Accepted 19 January 2010Available online 4 February 2010

Keywords:

Laser propulsion

Water

Medium interface

92/$ - see front matter & 2010 Elsevier Ltd. A

016/j.optlastec.2010.01.029

esponding author. Tel.: +86 025 84315075(O

ail address: stillhanbingnjust@gmail.com (X.-

a b s t r a c t

The dynamics of laser-induced semispherical cavitation bubbles are investigated by means of an optical

beam deflection method. The bubbles were generated in water in the vicinity of three different

interfaces: including water–air, water–colloid and water–solid, and also in the bulk. Numerical

simulation shows that the propelled surface obtains more energy and longer propulsion from the

semispherical bubble than from the spherical bubble during the first expansion of the bubble.

The collapse time of the quasi-semispherical bubble is significantly less than the Rayleigh collapse time

of the spherical bubble for all the cases considered in this work. The influence of water–air or water–

solid interfaces on the collapse time of a semispherical bubble is similar to that of a spherical bubble. As

the bubble energy grows, the effect of the water–colloid interface changes gradually from that of the

water–solid to that of the water–air interface. In other words, the energy of the bubble dictates whether

the water–colloid interface behaves as a water–solid or a water–air interface.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

There are two kinds of well-investigated laser propulsion modes[1–4]. One is the breath mode for the atmospheric environment. Theother is the ablation mode for the vacuum environment. Weproposed the laser propulsion in water environment [5] based onthe special properties of the laser-induced breakdown in water. Itavoids the main problems that limit the development of laserpropulsion in atmosphere and vacuum. A high-intensity laser isfocused near the surface of the object in water. A plasma of high-temperature and high-pressure will be induced when the powerdensity of the laser focus exceeds the breakdown threshold of water.A high density steam bubble [6] is produced with plasma as theinitial center. Before the final collapse, the bubble usually oscillatesseveral times. Each time the bubble expands from its minimumradius, it radiates a quasi-spherical shock wave. The amplitude ofthe subsequent oscillations and the magnitude of correspondingshock waves decrease quickly [7]. Those shock waves togetherwith the final collapse of the bubble contribute to the resultingpropelling force.

The oscillating features of a laser-induced bubble show whetherthe bubble can radiate strong shock waves and lead to fierce finalimpact. Such features, including oscillation times, maximum radius,and the collapse time, are influenced by many factors. For example,the influences of the water–air and water–solid interfaces onthe spherical bubble have been investigated by many researchers

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); fax: +86 025 84318430.

W. Ni).

[8–10]. The lifecycle, radius and dynamic characters of a laser-induced bubble change with the laser pulse energy. When the laseris focused on the object surface, namely the propelled surface, aquasi-semispherical bubble elongated in the opposite direction ofthe laser beam is induced. It is called ‘‘quasi-semispherical’’, becausethe hemispherical bubble is elongated in the opposite directionof the laser beam. After oscillating more than twice, the quasi-semispherical bubble collapses and leads a liquid jet toward thepropelled surface [11]. Furthermore, if we vary the shape ofthe propelled surface, the shape of the laser-induced bubble andthe interaction properties of the bubble–environment also change,which lead to the change in the temporal and spatial distribution ofthe propelling force.

Therefore, the propelling effect is decided by the dynamicproperties of the bubble. Dynamics of laser-induced quasi-semispherical cavitation bubbles, and the corresponding propul-sion of solid surface in water are investigated in this paper.The bubbles were generated in water in the vicinity of threedifferent interfaces: including water–air, water–colloid andwater–solid, and also in the bulk. The influences of the differentinterfaces on the dynamic properties of the quasi-semisphericalbubble, such as oscillation times, collapse time and energyrelease, are discussed. The influences of water–air and water–solid interfaces on the collapse time of the spherical bubble arecompared to the experimental results in this paper. And therelation between the role of the water–colloid interface andthe initial energy of the quasi-semispherical bubble is analyzed.Suggestions of the repeating frequency and pulse energy of thelaser beam in the laser propulsion are given concerning differentinterfaces.

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B. Han et al. / Optics & Laser Technology 42 (2010) 1049–10531050

2. Experimental setup

The experimental layout based on optical beam deflection isillustrated in Fig. 1. The quasi-semispherical bubbles weregenerated in a glass cuvette by a Q-switched Nd:YAG laser. Thelaser delivered single mode (TEM00) light pulses (1.06mm, 10 ns)to get fixed and controllable bubble center. The oscilloscope wastriggered by the signal from PIN photodiode (rise time 100 ps).The laser beam was perpendicular to a polished surface of thepropelled object placed in the glass cuvette. The object could bemoved with a resolution of 10mm along the x (change thedistance between the detecting beam and the bubble center) andy (change the laser–object interaction point) directions as shownin Fig. 1. In order to prevent linear breakdown in water, the pulsedlaser beam was first expanded by a concave–convex lens group,then focused to the propelled surface by a biconvex lens. The

9

10

8

7

11

12

14

1516

13

17

x

y

6 5 4 3 2 1

Fig. 1. Experimental setup based on optical beam deflection: (1) Nd:YAG laser;

(2) and (3) beam splitter; (4) attenuation group; (5) concave–convex lens group;

(6) focusing lens; (7) propelled object; (8) glass cuvette; (9) He–Ne laser; (10) and

(11) focusing lens; (12) five-dimensional fiber positioner; (13) single-mode

optical fiber; (14) photomultiplier; (15) oscilloscope; (16) PIN photodiode; and

(17) energy meter.

55 mm

2 mm

90°

Dfs or Dfc

Dfr

He-Ne La

dc

30 m30 mm

xView in y direction

Fig. 2. The propelled surface, the distance between the initial bubble center and the di

He–Ne laser beam. The inset shows the quasi-semispherical bubble viewed in y direct

energy intensity of the focus area (with a radius of about 50mm)was several orders higher than the breakdown threshold of water,so a series of phenomena would happen, such as laser-inducedplasma, shock wave, and cavitation bubble. The laser focus wasthe initial center of the bubble. The measurement origin of thebubble radius was fixed on the propelled surface.

The detecting part was formed by 9–14 in Fig. 1. Thecontinuous detecting He–Ne laser beam (0.6328mm) was focusedbefore and after passing through the bubble area. The signal fiberwas fixed on the five-dimensional fiber positioner. The detectinglight signal from the fiber was first transformed into electricsignal and amplified by the photomultiplier, then sent intothe oscilloscope. The fiber was adjusted properly coupling to thedetecting light [11]. That is to say, when there is no plasma, shockwave, or bubble in the path of the detecting light, the output ofthe fiber is half the maximum, which can be judged by theoscilloscope wave amplitude directly. It means that the fiber waspositioned at the linear section of the Gauss shaped detectinglight. Plasma, shock wave and cavitation bubble induced by theNd:YAG laser all change the refractive index of water, whichinduces deflection of the detecting beam. Thus every stage of thelaser-induced bubble, such as generating, oscillating, and collap-sing, can be investigated in detail by analyzing the series of outputwaves of the oscilloscope.

It is possible that the laser propulsion needs to be implemen-ted near different interfaces. A water–colloid interface, whosedegree of freedom (DOF) lies between water–solid and water–air,is possible when the propulsion is implemented in or near theorganism. The colloid used in this paper is made of glutin mainly.Fig. 2 shows the propelled surface (7 in Fig. 1), the distancebetween the initial bubble center and the different interfaces, andthe relative position of the bubble center and the detecting He–Nelaser beam. The propelled surface is made of copper. The initialbubble center is the Nd:YAG laser focus. Dfs, Dfc and Dfr

are distances between the bubble center and the water–air,water–colloid and water–solid interface, respectively. dc is thethickness of the colloid layer.

The variation of the bubble radius with time and the relationbetween the maximum bubble radius and the laser pulse energyare investigated. The bubble radius is in the order of �mm in thispaper, so it is reasonable to neglect the influence of the interfaces

Air

Nd:YAG Laser

Water ser

Colloid

m

y

Nd:YAG Laser

He-Ne Laser

quasi semispherical bubble

(inset)

fferent interfaces, and the relative position of the bubble center and the detecting

ion.

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B. Han et al. / Optics & Laser Technology 42 (2010) 1049–1053 1051

when Dfs or Dfc=50 mm and Dfr=50 mm. In order to obtain thedifferent experimental conditions, including in the bulk, nearwater–air interface, near water–colloid interface and near water–solid interface, we designed different experimental conditions aslisted in Table 1.

3. Experimental results and discussion

Fig. 3(a) shows a typical schematic diagram of bubble radiusR plotted as the function of time in water. Fig. 3(b) shows theoscilloscope wave detected when the He–Ne laser was 0.4 mm offthe propelled surface. The relation between the He–Ne laser andthe bubble radius is shown in Fig. 3(a). Rmax1 (mm) isthe maximum bubble radius of the first oscillation. T1 (ms) is thecollapse time of the first oscillation. In the experiment, thedetecting beam (the He–Ne laser) was moved to vary distance tothe propelled surface. Thus a series of oscilloscope waves asshown in Fig. 3(b) were obtained, from which the R–t figure asshown in Fig. 3(a) can be obtained.

On the assumption that the flow field is infinite with aconstant pressure, and the condensability and viscosity areneglected, the collapse time Tc of a spherical bubble can bederived from the Rayleigh model based on the law of conservationof energy [12], and described as Eq. (1). The assumption above canbe called a Rayleigh hypothesis

Tc ¼ 0:915Rmax

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir

P1�Pv

rð1Þ

where Tc is the collapse time, Rmax is the maximum bubble radius,r is the flow density, PN is the environmental pressure, and Pv isthe saturated vapor pressure. In this paper, the experiment wasimplemented at 20 1C, standard atmospheric pressure, and thebubbles were generated 1–50 mm beneath the water surface,so r is fixed to 1�103 kg/m3, PN is fixed to 1.01�105 Pa, and Pv is

Table 1Different experimental conditions in this paper.

Propelling environment Dfs (mm) Dfr (mm) Dfc (mm) dc (mm)

In the bulk 50 50 – 0

Near water–solid interface 50 [1.1,2.7] – 0

Near water–air interface [1.1,2.7] 50 – 0

Near water–colloid interface – 50 [1.1,2.7] 5

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

T1

Detecting Position

R/m

m

Rmax1

100 200 300 400 500

Time/μs

Fig. 3. (a) Schematic diagram of bubble radius R plotted as a function of time in water. (

is shown in (a).

fixed to 2.33�103 Pa. Because of the Rayleigh hypothesis, Eq. (1)cannot describe the laser-induced quasi-semispherical cavitationbubble in this paper. So Rmax1 and T1 in the different experimentalconditions (Table 1) were detected experimentally when theNd:YAG laser pulse energy was 25 mJ. The experimental resultsare shown in Fig. 4. The calculated Tc (ms) of the bubble far fromthe interfaces (in the bulk) is shown in Fig. 4(b).

The bubble expands rapidly and radiates a shock wave after itreaches the minimum volume. The propelled surface obtains thepropelling force by reflecting the shock wave. Because the energyof the bubble decreases quickly, the energy of the successiveshock waves attenuates greatly. In the final collapse, the bubbletransforms all its energy into the kinetic energy of the liquid jet,which impacts onto the propelled surface and the object ispropelled for the last time. The total bubble energy EB can bedescribed by [12].

EB ¼4p3ðP1�PvÞR

3max ð2Þ

The maximum bubble radius Rmax shows the total energy of abubble. Therefore, the propelling ability of a bubble can berepresented by Rmax. The energy of the quasi-semisphericalcavitation bubble can be described as 1

2EB approximately.If the laser intensity increases too much, the plasma will be

induced in the laser beam path and blocks the laser energy fromreaching the initial focus, namely the laser plasma shielding. Thisphenomenon results in the maximum bubble radius no longerincrease with the laser pulse energy. In this paper, we detectedthe maximum radius Rmax (mm) that a bubble can reach when thepulse energy keeps on increasing. EB (mJ) corresponding to everyRmax were calculated from Eq. (2). The experimental results ofthe different experimental conditions listed in Table 1 and thecorresponding EB are shown in Fig. 5.

3.1. Influence of the water–solid interface

Before the final collapse, the quasi-semispherical cavitationbubble usually oscillates thrice when induced near the water–solid interface. Fig. 4 shows that Rmax1 and T1 are both the highest,when the bubbles were induced near the water–solid interface.Fig. 5 shows that Rmax of the bubble induced near the water–solidinterface is also the highest. In addition, the experimental result ofthe influence of the water–solid interface on the collapse time ofthe quasi-semispherical cavitation bubble is the same with theconclusion for a spherical cavitation bubble from Rattray [8], ‘‘the

-20-18-16-14-12-10-8-6-4-202

Osc

illog

raph

Sig

nal/m

v

Time/ μs

Plasma Shock Wave Oscillation Shock Waves

0 100 200 300 400 500

b) Oscilloscope wave detected 0.4 mm off the target surface, the detecting position

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1.01.15

1.20

1.25

1.30

1.35

1.40

1.45

1.50

1.55

Rigid

Colloid

Surface

Rm

ax1

(mm

)

*

Dfr= Dfs = 50 mm

Rmax1 = 1.34 mm

65

70

75

80

85

90

95

100

105

110

115

120

125*

Rigid

Colloid

Surface

T1

(µs)

Dfr / Dfc / Dfs (mm)

*

Dfr = Dfs = 50 mm

T1 = 93 µs

Tc = 123 µs

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Dfr / Dfc / Dfs (mm)

Fig. 4. Influence of the distance between different interfaces and the bubble center (Dfr, Dfc, Dfs) on (a) the first maximum radius Rmax1 (mm) and (b) the first collapse time

T1 (ms) of the bubble. The two star marks show the experimental results of Rmax1 and T1 in water environment far from the interfaces. And the collapse time Tc (ms) of the

first oscillation far from the interfaces calculated from Eq. (1) is also shown in (b). The pulse energy of the laser is 25 mJ.

Dfr / Dfc / Dfs (mm)

Dfr = Dfs = 50 mm

EB = 5.86 mJ

1

2

3

4

5

6

7

8

9

Dfr / Dfc / Dfs (mm)

Rigid

Colloid

Surface

EB

(m

J) *

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.01.02.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

Rigid Colloid Surface

Rm

ax (

mm

)

*

Dfr = Dfs = 50 mmRmax = 3.05 mm

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0

Fig. 5. Influence of the distance between different interfaces and the bubble center (Dfr, Dfc, Dfs) on (a) the maximum bubble radius Rmax (mm) and (b) the bubble energy EB

(mJ) of the maximum radius Rmax calculated from Eq. (2). The two star marks show the experimental results of Rmax and EB in water environment far from the interfaces.

B. Han et al. / Optics & Laser Technology 42 (2010) 1049–10531052

existence of a water–solid interface prolongs the collapse time ofa spherical cavitation bubble’’.

3.2. Influence of the water–air interface

When the pulse energy was high and Dfs was small, the laser-induced shock waves led to serious unsteadiness of the water–airinterface and even spattered, which made the detecting beam shakeseriously. Thus, several values of Rmax (Fig. 5) were unable to bedetected when the bubbles were induced too close to the water–airinterface. Figs. 4 and 5 show that Rmax1, T1 and Rmax are all the lowestwhen the bubbles were induced near the water–air interface. Theexperiment shows that the oscillation times of the bubble inducednear the water–air interface is also the lowest (twice). The reason isthat such bubbles expend their energy on lifting the water–airinterface through its expanding stage. This explanation can besupported by the experimental results that Rmax of such bubbleswere a little bit bigger than Dfs. In addition, the experimental result ofthe influence of the water–air interface on the collapse time is thesame with the conclusion for a spherical cavitation bubble fromRattray [8], ‘‘the existence of a water–air interface shortens thecollapse time of a spherical cavitation bubble’’.

3.3. Influence of the water–colloid interface

Fig. 4(b) shows that when pulse energy was small, the collapsetime was longer near a water–colloid interface than in the bulk.That is to say, when the bubble energy is small, the water–colloidinterface is more like a water–solid interface. Asshauer et al. [13]and Kodama and Tomita [14] also found that the collapse time ofa spherical bubble induced near the soft-target (similar to awater–colloid interface) was longer than that of the bubbleinduced in infinite flow. However, Fig. 5(a) shows that Rmax wassmaller near the water–colloid interface than both in the bulk andnear the water–solid interface. That is to say, when the bubbleenergy is big, the water–colloid interface is more like a water–airinterface, because the bubble could expend its energy on liftingthe water–colloid interface through its expanding stage.

3.4. Control of the laser pulse

Both the spherical and the nonspherical (e.g. quasi-semispherical)bubble can be induced for laser propulsion in water. This can becontrolled by focusing the laser near or on the propelled surface.Define the distance between the bubble center and the propelled

ARTICLE IN PRESS

0.00

15

30

45

60

75

γ = 1.1γ = 0

Tot

al E

nerg

y (μ

J)

0.6 1.2 1.8 2.4 3.0 3.6 4.2 4.8 5.4 6.0

Time (μs)

Fig. 6. Numerical results of the energy that the propelled surface obtained from

the first-expansion of the bubble, when g=1.1 and 0.

B. Han et al. / Optics & Laser Technology 42 (2010) 1049–1053 1053

surface as L, and the maximum bubble radius as Rmax, g=L/Rmax. Fig. 6shows the numerical results of the energy that a propelled surface(copper, radius 2 mm, 113 mg) obtains from the first expansion of thebubble in water when g=1.1 (spherical) and g=0 (quasi-semispherical), laser pulse energy is 20 mJ. The starting time of theenergy rise of g=1.1 is 0.57ms later than that of g=0. The initiallocation of the laser focus is 1.1 mm away from the propelled surfacewhen g=1.1. So the 0.57ms is the time that the shockwave needs totransmit from the laser focus to the propelled surface. It is noticed inFig. 6 that the peak energy of g=0 is higher than that of g=1.1, and theattenuation velocity of g=0 is slower than that of g=1.1. That is to say,the propelled surface obtains more energy and longer propulsionwhen g=0 during the bubble first expansion. The reason is that in thecase of g=0, the propelled surface obtains energy not only fromreflecting the shockwave, but also from the counterforce on theexpanding bubble.

The temporal and spatial distributions of the propelling forcefrom a laser-induced bubble influenced by the bubble oscillationproperties, which are different in different propulsion environ-ments, such as in the bulk, near water–air interface, near water–colloid interface and near water–solid interface. Therefore, it isnecessary to design different laser pulse sequences for differentpropulsion environments. For example, a bubble induced bythe former laser beam will scatter the later laser beam. This willstop the later laser energy from reaching the initial focus on thepropelled surface. Thus, the collapse time is important forthe design of repeating frequency of laser. Take another example,if we keep rising the laser pulse energy when the propulsion isimplemented near an interface like the water–colloid, theenergy utilization efficiency will decrease, because the bubblewill lose its energy through lifting the interface. If it isunavoidable to propel the object near an interface, such as

liquid–air, liquid–colloid, or two kinds of liquids, it is advisable totake high-repeat-frequency and low-pulse-energy.

4. Conclusions

The propelled surface obtains more energy and longerpropulsion from the semispherical bubble than from the sphericalbubble during the first expansion of the bubble. The collapse timeof the quasi-semispherical bubble is less than the Rayleighcollapse time of the spherical bubble for all the cases consideredin this work, including in the bulk, near water–air interface, nearwater–colloid interface and near water–solid interface. Thecollapse time of a quasi-semispherical cavitation bubble isprolonged by a water–solid interface, while the effect of awater–air interface is opposite. The maximum bubble radius isbigger when near the water–solid interface than in the bulk, whilethe effect of a water–air interface is opposite. As the laser energygrows, the effect of the water–colloid interface changes graduallyfrom that of the water–solid interface to that of the water–airinterface. In other words, the energy of the bubble decideswhether the water–colloid interface behaves as a water–solid or awater–air interface. It is necessary to design different repeatingfrequency and pulse energy of the laser pulse sequence when thepropulsion is implemented near different medium interfaces.

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