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By confining cavitation within a microspace, the dynamic behavior of bubbles, such as bubble coalescence, clustering, and fragmentation, could be obse...
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J. Phys. Chem. C 2007, 111, 19015-19023

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Influence of Surface-Active Solutes on the Coalescence, Clustering, and Fragmentation of Acoustic Bubbles Confined in a Microspace Judy Lee,†,§ Toru Tuziuti,† Kyuichi Yasui,† Sandra Kentish,| Franz Grieser,§ Muthupandian Ashokkumar,*,§ and Yasuo Iida*,† National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan, Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia, and Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: July 11, 2007; In Final Form: October 9, 2007

High-speed imaging was used to capture the effect of surface-active solutes on the behavior of acoustic bubbles, generated in a microspace using low-frequency ultrasound (60 kHz). By confining cavitation within a microspace, the dynamic behavior of bubbles, such as bubble coalescence, clustering, and fragmentation, could be observed directly. It was observed that bubbles coalesced instantly in water; however, in the presence of surface-active solutes (n-propanol and sodium dodecyl sulfate, SDS) the coalescence was hindered. Lowdensity bubble clusters were observed in the presence of 1 mM SDS and 0.1 M n-propanol. When 0.1 M sodium chloride was added to 1 mM SDS solution, the extent of clustering and the density of the clusters enhanced significantly; a similar observation was made at a higher SDS concentration (10 mM). The importance of these results in understanding multibubble sonoluminescence data published previously has been addressed. The collective oscillation of a bubble cluster consisting of different sized bubbles and images of bubbles emitting a fountain of microbubbles have also been presented.

Introduction Acoustic cavitation is the creation or expansion of pre-existing cavities in a liquid to form bubbles in the presence of ultrasound.1 The term cavitation is also used loosely to encompass other ultrasound-induced effects such as bubble growth by rectified diffusion2 and bubble collapse leading to the emission of light, known as sonoluminescence.3,4 Currently, there are two books by Leighton5 and Young1 that provide detailed reviews on this subject. A review on cavitation bubble dynamics is also given by Neppiras6 and Lauterborn et al.7 Bubbles formed through acoustic cavitation that are smaller than the resonance size8 are drawn toward the pressure antinodes (by the primary Bjerknes force) forming branched filament structures or streamers. Within the cluster of bubbles at the pressure antinodes and within the streamers, bubbles dissolve, grow, coalesce, collapse, fragment, and sonoluminesce. Highspeed photography has been employed to study such cavitation bubble dynamics because of the fast and chaotic nature of acoustic cavitation.9-13 The highly complex bubble structure that forms has been referred to as the acoustic Lichtenberg structure by Lauterborn and co-workers12,14-16 because it resembles that of the famous Lichtenberg figures. This structure has recently been simulated numerically by Mettin et al.14 using a particle model approach. Hatanaka et al.17 captured sonoluminescencing bubbles using an intensified charge couple device * Authors for correspondence. E-mail: [email protected]; y.iida@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology (AIST). § Particulate Fluids Processing Centre, School of Chemistry, University of Melbourne. | Particulate Fluids Processing Centre, Department of Chemical and Biomolecular Engineering, University of Melbourne.

(ICCD) video camera and matched them with the bubble distribution of the cavitation structure in water. These authors have also reported images of bubbles coalescing and fragmenting. A common phenomenon in acoustic cavitation is the cyclic cavitation process (stable cavitation).17-19 This is where a bubble grows either by rectified diffusion or via coalescence of tiny bubbles that are drawn together by Bjerknes forces. Once the critical size8 is reached, the bubble appears to explode, releasing and leaving behind a trail of microbubbles. These microbubbles can then coalesce, thus “maintaining a coalescence-growth collapse cycle”.18 The release of these microbubbles is believed to be due to the distortion of the bubble surface by surface waves that become violent, throwing bubbles from the crests of the surface waves.6 To better study such cavitation bubble dynamics, single bubble techniques have been developed.10,20,21 Coupled with the development of high-speed cameras, it is now possible to experimentally isolate a single bubble and follow the collapse and lifetime of the bubble.9-12 Workers using this approach have reported that when the symmetry of the bubble environment is disturbed, that is, when a solid boundary is nearby, bubbles no longer collapse symmetrically.9-11 They observe the development of a high-speed jet and under certain conditions the formation of a counter jet in the opposite direction.9 In the systems mentioned above, water is usually the cavitating medium. However, in many ultrasonic applications, surface-active materials are also present in the solutions. Surfaceactive solutes are added in ultrasonic baths to facilitate the cleaning process.22 Anionic surfactants are also used in the ultrasound aided chemical cleaning of dairy whey-fouled ultrasound filtration membranes,23 polymerization of a styrene in water emulsion under ultrasound,24 and also the sonochemical

10.1021/jp075431j CCC: $37.00 © 2007 American Chemical Society Published on Web 11/30/2007

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Figure 1. (a) Top view of the microchip with a hexagonal observation “micropool” that was used to observe bubble motions. (b) Side view of the microscopic observation system.

synthesis of gold nanoparticles.25,26 In the sonolytic degradation of pollutants, the pollutant of interest is also often surface-active, that is, sodium dodecylbenzene sulfonate.27 Moreover, most of the biomolecules (e.g., proteins) are surface-active. The surfaceactive properties of surfactants can be expected to affect the bubble-bubble interactions within an acoustic cavitation field. Therefore, an understanding of the effect of surfactants on the acoustic cavitation field is important. For single bubble systems, Ashokkumar and co-workers,28-30 Lee et al.,31 and Crum2 have looked at the effect of surface-active solutes on bubble dynamics. They have found that surface-active solutes can enhance the rate of growth by rectified diffusion and that volatile solutes can quench sonoluminescence. For multibubble systems, indirect methods such as sonoluminescence intensity,32-38 change in total bubble volume,39,40 and acoustic emission spectra34,36,41 have been employed. Results from these studies have suggested that charged surface-active solutes can “de-cluster” bubble clouds,32,42 increase the bubble population via inhibition of bubble coalescence,34 and increase the symmetry of bubble collapse in bubble clouds, which in turn affects the sonoluminescence intensity.41 The use of a cavitation microspace technique, reported by Iida et al.,13,43 may add new information that could not be obtained previously from typical multibubble systems. Iida et al.13,43 have shown evidence of cavitation activity in microspace, which has important implications, especially in the medical field where ultrasound is widely used on human tissues containing microchannels, such as blood vessels.43 They have also demonstrated that by confining cavitation bubbles in microspace one can more readily observe the bubbles under a microscope and examine their dynamics in detail. This method also eliminates the problem of obstructions from other bubbles in the foreground and limited depth of view associated with highspeed photography in large volumes of liquid.44 In this report, high-speed imaging is used to capture the effect of surface-active solutes on the behavior of acoustic bubbles, generated in aqueous solutions contained in a microspace at 60 kHz. Images of the dynamic behavior such as bubble coalescence, clustering, and fragmentation are provided and discussed with respect to multibubble sonoluminescence work published previously.

and a quartz supporting plate of 1-mm thickness.45 In this microchip, a hexagonal observation “micropool” with a depth of 200 µm and 10-mm length was imprinted in the PDMS sheet. The dynamic motions of the bubbles were observed within the micropool. The PDMS sheet and the quartz plate were permanently bonded with oxygen plasma treatment. The optical monitoring and illumination by LED was carried out from the side of the quartz plate, and the ultrasonic oscillation was applied from the PDMS side. PDMS is a flexible, elastic polymer and also an excellent medium for ultrasonic waves. The ultrasound was fed by a miniature horn-type oscillator (Kouwa-giken, made with a bolted Langevin-type oscillator; HEC-1560P4B (Honda electronics)), which was driven by a continuous sinusoidal wave produced by a function generator (NF Corporation, WF1946A) and amplified by a wide-band power amplifier (Honda electronics, L-400BM-L). The diameter of the PZT oscillating disks was 15 mm, and the tip diameter of the horn was 10 mm. The operating frequency of the horn was 59.67 kHz. The input power of the oscillator was monitored by a current probe (Tektronix, P6021) and a voltage reading with a digital oscilloscope (Agilent Technologies, DSO3152A). The bubbles were generated by sonication. The cavitation threshold was around 2 W/cm2. However, once the bubbles were generated in the same experimental setup, the ultrasonic intensity was lowered to 0.50.7 W/cm2. The bubble observations were carried out at this lower acoustic intensity. The bubble oscillations were observed under a microscope (Olympus, BX41) attached with a 5x objective lens and a highspeed video camera (Photoron, FASTCAM-512PCI). A highpower light-emitting diode (Luxeon, LXHL-PW09) was driven by a high-speed bipolar amplifier (NF Corporation, HSA4012) in a pulsed mode at 59.70 kHz for 1000 fps (frames per second) monitoring. The light-emitting diode provides light at around 520 nm and a typical luminous flux of 60 mW. The principle of stroboscopic observation is given by Tian et al.46 The movies captured were analyzed frame by frame, and certain frames were selected to demonstrate bubble coalescence, clustering, and bubble fountain emission behaviors. All of these events were observed frequently and were observed in every experiment conducted.

Experimental Details

Results and Discussion

The chemicals used were high-purity research-grade sodium dodecyl sulfate (SDS), n-propanol, and sodium chloride as purchased from Wako Pure Chemical Industries. The solutions were made using deionized and distilled water. The microscopic observation system for bubble motion in a microspace with ultrasonic agitation is shown in Figure 1, and the details have been given elsewhere.13 Microchips were obtained from Fluidware Technologies. The microchip consisted of a polydimethyl siloxane (PDMS) sheet of 2-mm thickness

Bubble Coalescence and Clustering. Selected frames showing the behavior of bubbles in water, captured at a rate of 500 fps, are shown in Figure 2. In Figure 2a and b, it can be seen that one of the bubbles (25-30 µm) grows by coalescing almost instantaneously with smaller bubbles of approximately 10 µm in diameter. These smaller bubbles are probably drawn toward the “big” bubble by both primary and secondary Bjerknes forces.17,47,48 Using a high-speed camera with a frame speed of 1000 fps, the volume oscillation of the bubbles is clearly

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Figure 2. Coalescence of bubbles in water. Frames from left to right: 0 s, 0.012 s, and 0.018 s, respectively. The scale shown by means of a double-headed arrow is 100 micrometer.

Figure 3. Strobed images of bubble dynamics in water, captured at a rate of 1000 fps. The images show a bubble (A) emitting a stream of microbubbles toward another neighboring bubble (B). The microbubbles emitted from A form the source for the formation of a new bubble (C) near B. Bubbles B and C are attracted through, most probably, the secondary Bjerknes force and coalesce to form bubble D.

demonstrated in Figure 3. Here, bubble A emits a stream of microbubbles toward another neighboring bubble (B). The microbubbles emitted from A form the source for the formation of a new bubble (C) near B. Bubbles B and C are attracted through, most probably, the secondary Bjerknes force and coalesce to form bubble D. It can be seen that bubble A oscillates from a minimum diameter of 30 µm to a maximum bubble diameter of 43.5 µm. There appears to be a hole or dimple on the right side of the bubbles featured in Figure 3. It is difficult to ascertain if there is indeed an indentation or hole, or simply a light illumination effect. In the presence of 1 mM SDS (Figure 4), bubbles tend to cluster at first rather than coalescing immediately (as observed in water). In Figure 4a the three selected frames show several 10-µm-size bubbles being attracted toward a 25-µm bubble to form a small grape-like cluster. With time, the 25-µm bubble increases in size. It is uncertain if this growth is due to rectified diffusion, coalescence, Ostwald ripening (the growth of a larger

bubble by diffusion of gas from smaller adjacent bubbles49), or a combination of all three processes. This effect is better demonstrated in Figure 4b where a bubble 30 µm in diameter grows to a size of 50 µm in 0.3 s while the surrounding cluster of smaller bubbles disappears over time. Sonoluminescence emission was not measured from the microspace system used in this study. Nevertheless, the direct images of bubble clustering and the effect of surface-active solutes can be compared with the speculations arising from the indirect observations of multibubble systems, such as the effect of surface-active solutes on multibubble sonoluminescence (MBSL) for 20 kHz. Many workers have suggested that acoustic bubbles cluster in water at antinodes. The bubbles on the outside of the cluster impede sound from reaching the inner bubbles,5 and this ultimately decreases the number of active bubbles. It has been found that for ionic surfactants such as SDS the steadystate MBSL intensity increases at low concentrations (0.5-4 mM).32,34,35,42 It has been suggested that the adsorption of small

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Figure 4. Effect of 1 mM SDS on bubble coalescence and clustering.

Figure 5. Strobed images showing synchronized bubble volume oscillations of a bubble cluster in the presence of 1 mM SDS. Images were captured at a rate of 1000 fps.

amounts of ionic surfactant molecules at the bubble/solution interface may “decluster” the bubbles due to electrostatic repulsion between the “charged” bubbles. Such a cluster/decluster effect may be a dominant factor at higher frequencies (for example, 500 kHz), where several antinodes exist in the sonication medium. Tronson et al.35 showed that the relative enhancement in MBSL is very small at 20 kHz, which is comparable to the frequency used in this study. At 20 kHz and under the experimental conditions used in this study,35 the cluster/de-cluster effect may not be significant. Results shown in Figure 2 suggest that the bubbles do not cluster in water but undergo rapid coalescence. When 1 mM SDS is present in the solution, the extent of coalescence is significantly reduced as seen in Figure 4. This leads to an increase in the number of active bubbles, which is the probable cause of the slight enhancement in MBSL intensity observed in the Tronson et al.35 study rather than the de-clustering effect. Figure 5 shows magnified images of a cluster undergoing volume oscillations in 1 mM SDS solution. It is apparent from

the images that both the “big” bubble at the center of the cluster and the surrounding smaller bubbles oscillate in synch despite their differences in size that would give them different resonance frequencies and different nonlinear oscillation frequencies.50 This collective oscillation was also reported by Lauterborn and Koch7,50 using high-speed holography and acoustic cavitation spectra. The photographic evidence provided in this report thus provides further evidence for the coherence of bubble oscillations. The above discussion suggests that a slight increase in MBSL at low SDS concentrations might be due to a decrease in bubble coalescence caused by SDS adsorption at the bubble/solution interface. Whether this observation is valid for MBSL data at higher frequencies will be explored in our future investigations. Price et al.38 observed a slight enhancement in MBSL intensity at 20 kHz in the presence of long-chain alcohols, such as n-hexanol, which is surface-active but not charged. They suggested that the adsorption of alcohols at the bubble/solution interface hinders bubble coalescence and this resulted in an

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Figure 6. Effect of 0.1 M propanol on bubble coalescence and clustering.

Figure 7. Effect of 10 mM SDS on bubble coalescence and clustering.

increase in the number of active bubbles leading to an increase in the MBSL intensity. The pictures shown in Figure 6 support this interpretation. It is observed that the behavior of the bubbles in n-propanol solution is similar to that of 1 mM SDS solution, where coalescence is hindered and a low-density bubble cluster is formed. As discussed above, the bubbles within a low-density cluster could be active and generate sonoluminescence (SL). In the same study discussed above, Tronson et al.35 observed a decrease in the MBSL intensity at lower SDS concentrations in the presence of a background electrolyte or at higher SDS concentrations. They suggested that the presence of an electrolyte (NaCl or excess SDS) brings the bubbles in close contact (electrostatic repulsion is negated), leading to an increased clustering and acoustic impedance that ultimately decreases the number of active bubbles (and hence SL). To gain further insight into this hypothesis, high-speed photographic images were recorded in aqueous solutions containing 10 mM and 1 mM SDS + 0.1 M NaCl, and the results are shown in Figures 7 and 8. It is clear from the pictures shown in Figures 7 and 8 that, for high SDS concentration and in the presence of salt at low SDS concentration, there appears to be a greater population of micrometer-sized bubbles forming larger and denser clusters compared to 1 mM SDS. It can be seen in Figure 7a that a stream of tiny bubbles are drawn toward

a larger bubble that already has some clustering associated with it. In Figures 7b and 8a, two small clusters of bubbles are formed near one another and mutual attraction causes the two small clusters to join, forming a larger cluster of bubbles. Returning back to the discussion on MBSL, it may be suggested that the increased cluster density, in 10 mM SDS or 1 mM SDS + 0.1 M NaCl solutions, is responsible for lowering the MBSL intensity observed by Tronson et al.35 under similar experimental conditions. Fountain of Bubbles. In all of the systems studied, a stable emission of microbubbles, like a fountain, was observed. Selected images for each system are featured in Figure 9. It is observed that when a bubble has grown to a certain size streams of microbubbles are released from a bubble, like a fountain. In the presence of surface-active solutes, a greater number of microbubbles are ejected. In some cases, the microbubbles are drawn back to the “parent” bubble. There are instances where the smaller of two bubbles that are joined together ejects the microbubbles. In the presence of surfactants, there are occasions where the bubbles are sprayed in all directions (see Figure 9b). A similar release of micrometer-sized bubbles was also observed in aqueous solutions containing 10 mM SDS and 1 mM SDS + 0.1 M NaCl. For 10 mM SDS and 1 mM SDS + 0.1 M NaCl, a swirl of tiny bubbles is emitted and when the bubble

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Figure 8. Effect of 1 mM SDS + 0.1 M NaCl on bubble coalescence and clustering.

Figure 9. Selected images of the release of a fountain of microbubbles from a parent bubble in water in the absence and presence of surface-active solutes: (a) water, (b) 1 mM SDS and (c) 0.1 M propanol.

becomes erratic trails of small bubbles are left behind (Figure 10). These bubbles can then re-cluster or be drawn to another cluster of bubbles. The fountain of microbubbles displays some resemblance to the jetting phenomenon observed when a bubble undergoes asymmetric collapse.9-11 This asymmetry arises from the existence of a solid boundary or bubbles nearby. Because of the experimental setup, it is difficult to determine the location of the bubbles with respect to the surrounding walls. However, the position of the observations appears to be at the bottom of the hexagonal observation micropool (near the top of the PDMS sheet) and this may have created the asymmetric surrounding

needed for jetting to occur during bubble collapse. In addition to the solid boundary, the presence of other bubbles in the vicinity of a collapsing bubble can also interfere with the symmetry of the field surrounding the bubble. The photographs shown in Figures 9 and 10 clearly indicate that the generation of fountain of bubbles is significantly different in solutions containing surface-active solutes compared to that observed in water. The density of “fountain bubbles” is much higher in the presence of surfactants. This increase in the fountain of bubbles in the presence of surfactants may be related to a decrease in bubble coalescence and increase in bubble clustering. However, support from the literature cannot be

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Figure 10. At high SDS concentration and in the presence of 0.1 M NaCl, there is an increase in the population of microbubbles. Often a trail of microbubbles remains when a cluster of bubble undergoes erratic motions.

Figure 11. Strobed images of a bubble in water that is attached to the side wall of the hexagonal observation micropool. The single bubble then turns into two smaller bubbles, which then appear to undergo shape oscillations. Four bubbles were observed to be released between the centers of the two bubbles, labeled A-D. The images were taken at a frame rate of 3000 fps.

provided because there is very little information on this phenomenon and none to the authors’ knowledge on the effect of surface-active solutes. Thus, the discussion has been limited to describing the evolution of bubbles and clusters near an interface in water. Figures 11 and 12 are strobed images of cavitating bubbles that are actually on the side of the hexagonal observation micropool wall. In Figure 11, a bubble with a diameter of 50 µm along the longest axis (at 0 s) appears to collapse at time 0.002 s. There appears to be a force coming from the left that is causing the bubble to collapse asymmetrically and involute the left side of the bubble wall to form what could be a cuspshaped jet-like projection, seen as a faint line through the center of the bubble (parallel to the wall). It is usually reported in literature that the direction of the jet formed is directed toward the solid boundary for laser-induced cavitation bubbles.9,11

However, in the case of bubble collapse induced by shock waves the jet formed has been found to be in the direction perpendicular to the shock front.10,51 In Figure 11, there is a possibility that there is a shock wave from a nearby collapsing bubble on the left. At time 0.003 s, there appears to be the presence of a second bubble, which is clearly observed at time 0.004 s. This pair of bubbles then proceeds to undergo shape oscillations and ejects bubbles A-D from the center. This fountain-like effect is demonstrated clearly in Figure 12 at time 0.019 s. There is initially a cluster of oscillating bubbles, very similar to the “ghost effect”, a term given by Howkins52 for a white concentrated spot consisting of small cavitating bubbles thought to be held together by attractive hydrodynamic forces. At certain instances, a stable cluster is formed as shown at time 0.001 and 0.019 s. At 0.019 s there appears to be a split at the center, and one can only speculate

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Figure 12. Strobed images of a cluster of bubbles that is cavitating in water adjacent to the side wall of the hexagonal observation micropool. At time 0.001 and 0.019 s, the individual bubbles can be clearly observed. At time 0.019 s, a fountain of bubbles can be seen emitted from the center of the bubbles. The images were taken at a frame rate of 3000 fps.

Figure 13. Strobed images of a bubble in water that is attached to the side wall of the hexagonal observation micropool. The bubble splits into two smaller bubbles then merges into one. The images were taken at a frame rate of 3000 fps.

if this is due to jetting; however, what is evident is the ejection of a stream of bubbles from this center. This splitting was observed for another bubble and better shown in Figure 13. It is uncertain if the phenomenon of the fountain of bubbles is the result of jetting or counter jets because most reports in the literature on jet formation are from either laser- or shock-waveinduced bubble collapse at a higher energy density. The ejection of microbubbles in an ultrasonic field has been documented by Leighton47 where he presented a series of still frames showing the transfer of fragmented microbubbles from the surface of a bubble to another neighboring bubble followed by coalescence. Another system that exhibits similarities to the fountain of bubbles from a stable mother bubble is that conducted by Makuta et al. .53 In their system, they introduced a gas bubble at the tip of a needle in an ultrasonic field immersed in a highly viscous medium. A surface wave propagates from the outer perimeter toward the center of the bubble where a stream of uniform-sized microbubbles are released.53 These are the only studies to our knowledge that report the steady release of microbubbles from the surface of a “parent” bubble in the presence of an ultrasonic field. Conclusions In this paper, we have provided a direct evaluation of the effect of surface-active solutes on bubble clustering at low ultrasound frequency (60 kHz). The information presented clarifies some issues related to previous speculation on the declustering and clustering effects generated by charged surfactants under similar experimental condtions. A slight enhancement in MBSL observed at 20 kHz in the presence of low concentrations of SDS might be related to hindrance to bubble coalescence caused by the adsorption of the surfactant at the bubble/solution interface. A low-density bubble cluster produced in 1 mM SDS or 0.1 M propanol solution does not affect the activity of the bubbles present within this cluster. The density of the cluster increases with increasing SDS concentration and also with the addition of 0.1M NaCl. It is suggested that the formation of such high-density clusters may decrease the number of active

bubbles. Sonoluminescence emission from bubbles in the microspace needs to be measured to directly relate the effect of bubble clustering on multibubble sonoluminescence. A steady release of microbubbles from a parent bubble, like a fountain, has been observed. Acknowledgment. This work is partly supported by a Grant from the Ministry of Education, Culture, Sports, Science and Technology, Japan. We also acknowledge the financial support from the Particulate Fluids Processing Centre, a Special Research Centre of the Australian Research Council for infrastructure and funding. References and Notes (1) Young, F. R. CaVitation; Imperial College Press: London, 1999. (2) Crum, L. A. J. Acoust. Soc. Am. 1980, 68, 203. (3) Lohse, D. Nature 2002, 418, 381. (4) Brenner, M. P.; Hilgenfeldt, S.; Lohse, D. ReV. Mod. Phys. 2002, 74, 425. (5) Leighton, T. G. The Acoustic Bubble; Academic Press Limited: Cambridge, 1994. (6) Neppiras, E. A. Phys. Rep. 1980, 61, 159. (7) Lauterborn, W.; Kurz, T.; Mettin, R.; Ohl, C. D. Experimental and Theoretical Bubble Dynamics. In AdVances in Chemical Physics; Prigogine, I., Rice, S. A., Eds.; John Wiley & Sons, Inc., 1999; Vol. 110; p 295. (8) Yasui, K. J. Acoust. Soc. Am. 2002, 112, 1405. (9) Lauterborn, W.; Bolle, H. J. Fluid Mech. 1975, 72, 391. (10) Dear, J. P.; Field, J. E. J. Fluid Mech. 1988, 190, 409. (11) Vogel, A.; Lauterborn, W.; Timm, R. J. Fluid Mech. 1989, 206, 299. (12) Lauterborn, W.; Ohl, C. D. Ultrason. Sonochem. 1997, 4, 65. (13) Iida, Y.; Tuziuti, T.; Yasui, K.; Towata, A.; Kozuka, T. Ultrason. Sonochem. 2007, 14, 621. (14) Mettin, R.; Luther, S.; Ohl, C. D.; Lauterborn, W. Ultrason. Sonochem. 1999, 6, 25. (15) Parlitz, U.; Mettin, R.; Luther, S.; Akhatov, I.; Voss, M.; Lauterborn, W. Philos. Trans. R. Soc. London, Ser. A 1999, 357, 313. (16) Lauterborn, W.; Kurz, T.; Geisler, R.; Schanz, D.; Lindau, O. Ultrason. Sonochem. 2007, 14, 484. (17) Hatanaka, S.; Yasui, K.; Tuziuti, T.; Kozuka, T.; Mitome, H. Jpn. J. Appl. Phys. 2001, 40, 3856. (18) Hughes, D. E.; Nyborg, W. L. Science 1962, 138, 108. (19) Neppiras, E. A.; Fill, E. E. J. Acoust. Soc. Am. 1969, 46, 1264. (20) Lauterborn, W. Appl. Phys. Lett. 1972, 21, 27.

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