Variations in the Spatial Distribution of Sonoluminescing Bubbles in

Feb 8, 2010 - coalescence stability of bubbles, the attenuation of the acoustic wave, and the standing wave ratio. 1. Introduction. There has been a g...
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Variations in the Spatial Distribution of Sonoluminescing Bubbles in the Presence of an Ionic Surfactant and Electrolyte Judy Lee,*,† Ivan U. Vakarelski,‡ Kyuichi Yasui,† Toru Tuziuti,† Teruyuki Kozuka,† Atsuya Towata,† and Yasuo Iida† National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami, Moriyama ku, Nagoya 463-8560, Japan, and Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833, Singapore ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: January 17, 2010

It has been established that the addition of sodium dodecylsulfate (SDS) to water to a concentration of 1 mM increased the integrated sonoluminescence (SL) intensity to a maximum. Moreover, further increase in the SDS concentration to 10 mM decreased the SL intensity to a level comparable to that obtained for water. Photographic images of water and 10 mM SDS have revealed a localized distribution of SL bubbles near the liquid surface. For 1 mM SDS, a homogeneous distribution of SL bubbles was observed throughout the liquid. In this study, a comprehensive investigation was performed to determine the variations in the spatial distribution of SL bubbles as a function of SDS concentration, with and without the addition of sodium chloride (NaCl). It was found that the integrated SL intensity passed through a local minimum as the distribution of SL bubbles transformed from an isolated to a homogeneous distribution at 0.25 and 2.4 mM SDS. Similar transformations in the spatial distribution of SL bubbles within these SDS solutions were also observed upon the addition of a few millimolar NaCl. These variations in the spatial distribution of SL bubbles in aqueous solutions containing an ionic surfactant and electrolyte were believed to be the result of changes in the coalescence stability of bubbles, the attenuation of the acoustic wave, and the standing wave ratio. 1. Introduction There has been a growth in the application of ultrasound in processes such as cleaning,1,2 degradation of pollutants,3,4 and emulsification.5,6 However, the chaotic and complex nature of acoustic cavitation, coupled with the presence of additives such as surfactants and electrolytes, renders the control and optimization of ultrasound systems more difficult. An understanding of cavitation systems is important for the economic and efficient application of ultrasound in industry. This has motivated studies on the influence that various sonication and solute conditions have on the efficiency of ultrasound systems.7-14 The efficiency of ultrasound systems is closely related to the cavitation activity, which can be easily quantified by the sonoluminescence (SL) intensity or sonochemical (SC) efficiency. It has been shown that, under appropriate degassing,15,16 pulsing,12,15,17 and the addition of surfactants,7,18 SL and SC activities can be enhanced. In our recent study,19 a correlation between the enhancements in the SL intensity and the broadening of the spatial distribution of active bubbles was demonstrated. The broadening of the spatial distribution of active bubbles was attributed to an increase in the proportion of the acoustic wave field that is standing wave and population of active bubbles trapped at pressure antinodes. An increase in the standing wave proportion is brought about by reductions in the formation of large coalesced bubbles, which if present can significantly attenuate the acoustic pressure amplitude. This attenuation can lead to the development of strong traveling wave * Corresponding author. Phone: +81-52-736-7215. Fax: +81-52-7367405. E-mail: [email protected]. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Institute of Chemical and Engineering Sciences.

fields and acoustic streaming effects, causing active bubbles to be forced in the direction of the propagating wave and localize near the liquid surface. This localization of SL bubbles is observed in the case of water where bubbles readily coalesce to form large degassing bubbles. With the addition of 1 mM sodium dodecyl sulfate (SDS), an ionic surfactant which has been shown to retard bubble coalescence, a homogeneous distribution of SL bubbles throughout the liquid was observed. Further increases in the SDS concentration to 10 mM caused the spatial distribution of SL bubbles to return to a similar localized distribution obtained for water. An explanation for this is that, at high SDS concentrations, the dissociated SDS monomers act as excess electrolyte, therefore lowering the electrostatic repulsion barrier for coalescence to occur. This screening of electrostatic repulsion is further supported by the observation that, upon the addition of 0.1 M sodium chloride (NaCl) to 1 mM SDS, the integrated SL intensity was decreased to a level comparable to water. The inhibition of bubble coalescence upon the addition of SDS in the absence of ultrasound is a well studied phenomenon, especially in the stability of emulsions or colloidal systems. Using atomic force microscopy (AFM), it is possible to directly measure interacting forces between two bubbles21 or droplets20 as well as the actual DLVO (Derjaguin, Landau, Vervey, and Overbeek) forces under different conditions. It has been shown that the coalescence stability of emulsion droplets or bubbles rises with the initial increase in SDS concentration but remains unaffected at SDS concentrations above 1 mM.20-23 This is contrary to that observed for acoustic bubbles where a decrease in coalescence stability was seen upon increases in SDS concentrations from 1 to 10 mM. In our previous study,19 we only reported extreme changes in the spatial distributions of SL bubbles for water and 1 and

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10 mM SDS. In this study, we have extended the work to investigate in more detail the evolution of the spatial distribution of SL bubbles as a function of SDS concentration and the effect of NaCl in these surfactant systems. Variations in the bubble coalescence stability is discussed in relation to the electrostatic repulsive force between bubbles and acoustic effects including rectified diffusion, secondary Bjerknes force, and cavitation. 2. Experimental Details Sodium dodecylsulfate (SDS) and sodium chloride (NaCl) were purchased from Sigma-Aldrich, and used as received. The SDS solutions were made by diluting an appropriate volume of 100 mM SDS stock solution with distilled water. For the effect of NaCl, dilution of an appropriate volume of 0.1 M NaCl stock solution was used. For every experiment, a solution volume of 1 L was used and the temperature was controlled at 21.0 ( 0.5 °C with the aid of a cooling coil. The ultrasound was emitted from a ceramic transducer (50 mm diameter) through a stainless steel base fitted at the bottom of a glass vessel (100 mm × 100 mm × 120 mm). The transducer was driven by sinusoidal waves produced from a function generator (NF Corporation, WF1946A) and amplified via a wide-band power amplifier (NF Corporation, HSA4014). The nominal sonication conditions were set to 442 kHz, and an input power of 20 W (1.0 W/cm2) was measured by a power meter (Towa, TDW-6102U). The spatial distribution of sonoluminescing bubbles in the reactor was captured by a cooled CCD camera (BITRAN, BS41 L) with an exposure time of 30 s. The integrated SL intensity was measured using computer software. 3. Results 3.1. Effect of Surfactant. Shown in Figure 1a are images of the spatial distribution of sonoluminescence (SL) for water and different SDS concentrations. In the case of water, strong SL activity was found to be localized near the liquid surface. This area of SL activity shrinks with increasing SDS concentration. Weak emission of SL in the bulk of the liquid begins to appear at 0.25 mM SDS and intensifies until the SL activity becomes uniformly distributed in the vessel at approximately 1 mM SDS. Increasing the SDS concentration to 10 mM reversed the variations in the spatial distribution of SL bubbles observed at lower SDS concentrations. That is, the bulk SL activity diminishes and the SL activity near the liquid surface increases. The relative integrated SL intensity as a function of SDS concentration is plotted in Figure 1b. The plot displays the typical curve obtained whereby the SL intensity increases to a maximum at 1 mM SDS and subsequently decreases to a level comparable to water at 10 mM SDS. However, what has not been reported in previous studies at similar sonication frequencies7,18 is the small decrease in the SL intensity at 0.25 and 2.4 mM SDS. Earlier studies used increments of SDS concentrations larger than 0.5 mM, which is greater than the concentration range in which the small decrease in the SL intensity occurs. This might explain why the small decrease in SL intensity was not detected in previous studies. When Figure 1a and b were compared, it was apparent that the small decrease in the SL intensity occurs at the transition from a localized distribution of SL activity near the liquid surface to a homogeneous distribution of SL activity in the bulk of the liquid. 3.2. Effect of Salt. It has been reported that the addition of 0.1 M NaCl completely suppresses the enhancement in SL intensity observed at 1 mM SDS,7,18 but to the authors’

Figure 1. (a) Spatial distribution of SL bubbles as a function of SDS concentrations. (b) Integrated SL intensity as a function of SDS concentration. The integrated SL intensity is normalized relative to the SL intensity for water. The inset shows the integrated SL intensity at high SDS concentrations. An exposure time of 30 s was used for the SL images.

knowledge, the structure of the spatial distribution of SL bubbles as a function of NaCl concentration was unknown. Shown in Figure 2 is the spatial distribution of SL bubbles for 0.2, 1, and 3 mM SDS as a function of NaCl concentration. At 0.2 mM SDS (Figure 2a), a concentration near the transition observed in Figure 1, the SL activity near the liquid surface broadens with increasing NaCl concentration and approaches the spatial distribution of SL bubbles obtained for water. At 1 mM SDS (Figure 2b), where the spatial distribution of SL bubbles is homogeneously distributed, the addition of NaCl caused the SL intensity throughout the liquid to decrease while the SL activity near the liquid surface increased. At 3 mM SDS (Figure 2c), the addition of NaCl has very little effect on the spatial distribution of SL bubbles. The integrated SL intensity as a function of NaCl concentration for the systems depicted in Figure 2 are plotted in Figure 3. At 0.2 mM SDS, the integrated SL intensity increased with increasing NaCl concentration. At 1 mM SDS, the integrated SL intensity passes through a local minimum at 0.6 mM NaCl. This corresponds to the same NaCl concentration at which the spatial distribution of SL bubbles transforms from a uniform distribution to an isolated distribution depicted in Figure 2b. At 3 mM SDS where the spatial distribution of SL bubbles has already reverted to the same spatial distribution as that of water, the integrated SL intensity remained unaffected by the addition of NaCl. The data presented indicates that the addition of only a few millimolars of NaCl to 0.2 and 1 mM SDS solutions returns

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Figure 2. Spatial distribution of SL bubbles for (a) 0.2 mM SDS, (b) 1.0 mM SDS, and (c) 3.0 mM SDS as a function of NaCl concentration. An exposure time of 30 s was used.

Figure 3. Integrated SL intensity for 0.2 mM SDS, 1.0 mM SDS, and 3.0 mM SDS as a function of NaCl concentration. The integrated SL intensity was normalized relative to the SL intensity for water.

both the spatial distribution of SL bubbles and the integrated SL intensity to those as obtained for water. These concentrations of NaCl are much lower than previously reported studies where 0.1 M was used.7,18 4. Discussion 4.1. Interaction Forces between Bubbles. The variations observed in the spatial distribution of SL bubbles are largely attributed to the changes in bubble sizes brought about by the effect that SDS has on bubble coalescence, in both the presence and absence of NaCl. It has been indicated in previous studies24-26 that bubble coalescence is reduced (increase in

Lee et al. coalescence stability) by the initial increase in SDS concentration up to 1 mM. However, this coalescence stability decreases with further increases in SDS concentration. Yasui13 has shown that a range of ambient bubble radii for SL bubbles exists. Therefore, modifications in the coalescence process consequently result in changes in the population and size of both SL and non-SL bubbles generated. Furthermore, these changes can also have a profound effect on the acoustic radiation forces acting on a bubble and the spatial distribution of the bubbles within the acoustic field. This will be discussed in the next section. In an acoustic field, bubbles are continuously undergoing radial oscillations and rapid translational movements in the order of meters per second. These effects introduce additional dynamic factors which complicate the coalescence process of bubbles in comparison to the coalescence of bubbles or emulsion droplets in the absence of an acoustic field. However, the pronounced variations in bubble stability in the presence of varying SDS and electrolyte concentrations are prompting one to consider the role of the surfactant adsorption and the associated surface forces acting between the bubbles. In the absence of stabilizing surfactants, bubbles readily coalesce as they are brought together by predominately the secondary Bjerknes force, which stems from the acoustic field radiated by pulsating bubbles. This force is usually attractive for bubble pairs of similar sizes.27,28 However, in the presence of a stabilizing surfactant such as SDS, this coalescence process is hindered. The increase in coalescence stability with the addition of SDS to 1 mM can be easily explained by the contribution of two factors: (i) the increase in the bubble’s surface charge controlling the long-range electric double layer (EDL) repulsive interactions and (ii) the increase in short-ranged steric or hydration repulsive interactions associated with the adsorption of surfactants.25 The consecutive decrease in coalescence stability for concentrations above 1 mM SDS in an acoustic field was previously considered to be due to the screening of EDL interactions by excess surfactant molecules.7,29 At concentrations lower than the critical micelle concentration (CMC), the SDS monomers are fully dissociated throughout the solution and act as an excess electrolyte screening the EDL force, e.g., the decrease in the Debye length from 9.6 nm for 1 mM SDS to about 3.6 nm for 8 mM SDS and 1 nm for 1 mM SDS with 0.1 M NaCl.18,30,31 The screening of the EDL forces may be the apparent reason for the decrease in coalescence stability when a substantial amount of added electrolyte is present. Moreover, previously reported bubble void volume32 and light scattering26 measurements both support this decrease in stability. This decrease in coalescence stability of bubbles in an acoustic field from 1 to 8 mM SDS differs from the coalescence stability of emulsion droplets or bubbles that have shown to rise with the initial increase in SDS concentration but remains unaffected at SDS concentrations above 1 mM.20,21,23,33 A simple estimation of the DLVO disjoining pressure between static bubbles will actually reveal that, although the interaction becomes short ranged, the DLVO maximum will increase with increasing SDS concentration, assuming that the bubble’s surface potential does not vary significantly in that concentration range. This suggests that in an acoustic field there exists some short-ranged attractive force imposing on the DLVO forces, thus rendering the barrier to coalescence higher at 1 mM SDS. Segebarth et al.18 have proposed that the role of such a force could be the secondary Bjerknes force. However, we have noticed that the magnitude of the secondary Bjerknes force is generally proportional to the inverted square of the separation

Spatial Distribution of Sonoluminescing Bubbles distance between the center of the bubbles rather than from the bubble surface as assumed by Segebarth et al.18 If they had taken the separation distance from the center of the bubble, the magnitude of the secondary Bjerknes force would not change significantly in the range at which the DLVO forces are in operation. However, the situation may be different under nonlinear conditions and to include nonlinear secondary Bjerknes force in the DLVO force calculations is beyond the scope of this study. However, at low frequencies, Mettin et al.27 have shown that under nonlinear conditions the magnitude of the secondary Bjerknes force increases with increasing bubble size. Bubbles can increase in size by the process of rectified diffusion, and Louisnard et al.34 have theoretically reported a growth rate of an order of micrometers per second for high frequencies. There is experimental evidence showing increases in the rate of growth in the bubble radius by rectified diffusion with the addition of surfactants.35,36 This increase in the rectified diffusion growth rate may be sufficient to cause a stronger attractive secondary Bjerknes force and lower the coalescence stability for bubbles in 8 mM SDS. Another possible effect is that the decrease in the Debye length (at high SDS concentrations or the addition of NaCl) can allow bubbles to reach a separation at which the occurrence of cavitation can lead to the rupture and coalescence of the bubbles, as has been demonstrated at a lower frequency with larger oil droplets.37 4.1.1. Acoustic Radiation Forces. As discussed in the previous section, the addition of SDS with or without the addition of NaCl has a considerable effect on the coalescence process between bubbles. This has significant consequences on the forces which govern the spatial distribution of SL bubbles within the reactor vessel. One of these forces is the radiation force from a standing wave field which directs bubbles at and below the linear resonance size to the antinodes. The other is the radiation force from a traveling wave field which translates bubbles in the direction of the propagating wave and is greatest for bubbles at the linear resonance size. Using different reflective materials, Leighton et al.38 varied the standing and traveling wave proportions to show that strong bands of SL are observed at the antinodes of a standing wave field and very little SL is observed when the proportion of the traveling wave field is high. Under ideal systems where the acoustic wave is perfectly reflected from the air liquid surface, a strong standing wave field is established between the transducer and the reflective liquid surface. However, in a real system, the formation of standing waves is disrupted by the attenuation of acoustic pressure amplitude from large coalesced bubbles and energy dissipation. This attenuation results in a strong traveling wave field and limits the standing waves to be near the liquid surface.19 This effect can be observed in the case of water where bubbles readily coalesce into large bubbles and SL activity is found localized to near the liquid surface.39-41 The addition of SDS to water to a concentration of 1 mM has shown to significantly reduce the formation of large coalesced bubbles,25,26,32 and a homogeneous spatial distribution of SL bubbles is obtained.19 At 10 mM SDS, the decrease in coalescence stability increases the sizes of bubbles and lowers the attenuation of acoustic pressure amplitude. This results in the decrease in the standing wave proportion, causing the active bubbles to be confined to near the liquid surface as observed for water. As the spatial distribution of SL bubbles transforms from localized to a homogeneous distribution with the addition of SDS, the integrated SL intensity actually decreases at the concentration marking the transition from a strong traveling wave to a standing wave field. These small decreases in the SL

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Figure 4. The spatial distributions of SL bubbles in Figures 1a and 2b were separated into two regions: SL approximately 4 cm below the liquid surface and the remaining SL in the bulk of the liquid. The SL near the liquid surface is believed to be predominately resonance size bubbles driven by the radiation force from the traveling wave field, and that in the bulk of the liquid is attributed to mainly subresonance bubbles that are trapped at the antinodes of the standing wave field. (a) Variations in SL intensity from the two regions as a function of SDS concentration. (b) Variations in SL intensity from the two regions as a function of NaCl concentration in the presence of 1 mM SDS. In both parts a and b, the total integrated SL intensity is also included.

intensity at the transitions in the wave field can be explained by a closer examination into the spatial distribution of SL bubble profiles in Figure 1a. The spatial profiles of SL activity can be separated into two regions: SL approximately 4 cm below the liquid surface and the remaining SL in the bulk of the liquid. The SL near the liquid surface is believed to be predominately resonance size bubbles driven by the traveling wave field toward the liquid surface. Those in the bulk are attributed to resonance and subresonance bubbles that are driven by the primary Bjerknes force to the antinodes in the standing wave field. The variations in SL intensity in these two regions and the total integrated SL as a function of SDS concentration are plotted in Figure 4a. In water, the tendency for bubbles to coalesce gives rise to a larger and broader size distribution of bubbles, with a mean bubble radius of approximately 40 µm.26 These large bubbles would cause considerable attenuation of acoustic pressure amplitude and result in the force from the traveling wave field to dominate. As discussed earlier, in a strong traveling wave field, a bubble at the linear resonance size experiences a strong driving force toward the air-liquid surface and is then trapped by the standing wave field near the liquid surface, leaving very little SL activity in the bulk of the liquid. The addition of SDS to a concentration of 0.1 mM was shown to

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retard the coalescence of bubbles and reduce the mean bubble radius from 40 to 20 µm.26 This would lower the attenuation of acoustic pressure amplitude and the traveling wave proportion. This lowering of the traveling wave proportion is manifested by the decrease in SL intensity near the liquid surface. Although concurrently there is an increase in the standing wave proportion, the lack of increase in SL intensity in the bulk of the liquid suggests that there is only a small population of subresonance bubbles that can be trapped at the antinodes of the standing wave field. This retardation of bubble coalescence at low SDS concentrations can be reversed by the addition of NaCl to screen the electrostatic repulsion. This was demonstrated by the increase in SL near the surface with increasing NaCl concentration for 0.2 mM SDS shown in Figures 2 and 3. As the concentration of SDS increases from 0.25 to 1 mM SDS, the SL intensity in the bulk of the liquid increases. It has been reported that the extent of bubble coalescence inhibition reaches a maximum at 1 mM SDS, and light scattering experiments have revealed that the mean bubble size is less than 4 µm.26 This is less than the linear resonance size for a frequency of 442 kHz. This increase in the population of subresonance size bubbles in conjunction with an increase in the standing wave proportion is believed to be the cause for the increase in SL activity in the bulk of the liquid. As the SDS concentration increases above 1 mM, the decrease in coalescence stability discussed in the previous section leads to the loss of subresonance bubbles as they coalesce to form larger bubbles. Furthermore, the increase in the size of bubbles will effectively increase both the attenuation of acoustic pressure amplitude and the proportion of the wave field that is traveling wave. These effects led to the subsequent decrease in the SL activity in the bulk of the liquid and increase in the SL activity near the liquid surface. As shown in Figure 4a, the small decrease in SL intensity at 2.4 mM SDS, at the transition from a homogeneous to a localized distribution, is due to the faster decay in SL intensity in the bulk of the liquid compared to the slower increase in SL intensity near the liquid surface. A similar effect was observed when NaCl is added to 1 mM SDS shown in Figure 4b. That is, the SL intensity passes through a minimum as the spatial distribution of SL bubbles progresses from a homogeneous distributed SL to an isolated spatial distribution of SL bubbles near the liquid surface. This is brought about by the increase in the size of bubbles as the addition of NaCl decreases the electrostatic repulsion and lowers the barrier for bubble coalescence to occur. 5. Conclusion In this study, the behavior of ultrasound induced SL intensity in aqueous solutions containing SDS and NaCl was reported. It was found that, in addition to the enhancement in the integrated SL intensity at 1 mM SDS that is usually reported, we found that the integrated SL intensity passed through two local minimums at 0.25 and 2.4 mM SDS. These two SDS concentrations corresponded to the concentration at which the spatial distribution of SL bubbles transforms from an isolated to a homogeneous distribution, and vice versa. These variations were due to changes in the standing and traveling wave component of the wave field, which is dependent on the extent of attenuation of the acoustic pressure amplitude by large coalesced bubbles. Therefore, the variations observed in the spatial distribution of SL bubbles provide an insight into the effect SDS has on coalescence stability in an acoustic field. The importance of electrostatic repulsion was demonstrated by the addition of NaCl.

Lee et al. The results also reveal that the interaction between bubbles in the presence of SDS in an acoustic field cannot be completely explained by the general perception of coalescence stability for droplets or bubbles in the absence of ultrasound. A more complete analysis of bubble interactions in which the surface forces are coupled with the hydrodynamic interaction and bubble deformation is needed for further evaluation of the bubbles’ coalescence stability.20,22 It will be an interesting but challenging task to extend this static field theory to the case of bubbles interacting in an acoustic field where dynamic effects such as rectified diffusion, Bjerknes forces, and absorption of surfactants and ions at the bubble interface are taken into account. Acknowledgment. The authors acknowledge the funding from the JSPS Postdoctoral Fellowship program for foreign researchers and from the Ministry of Education, Culture, Sports, Science and Technology of Japan (Project No. 1907765). References and Notes (1) Mason, T. J. Philos. Trans. R. Soc. London, Ser. A 1999, 357, 355– 369. (2) Muthukumaran, S.; Kentish, S.; Lalchandani, S.; Ashokkumar, M.; Mawson, R.; Stevens, G. M.; Grieser, F. Ultrason. Sonochem. 2005, 12, 29–35. (3) Cheng, J.; Vecitis, C. D.; Park, H.; Mader, B. T.; Hoffmann, M. R. EnViron. Sci. Technol. 2008, 42, 8057–8063. (4) Nanzai, B.; Okitsu, K.; Takenaka, N.; Bandow, H.; Tajima, N.; Maeda, Y. Ultrason. Sonochem. 2009, 16, 163–168. (5) Kentish, S. E.; Wooster, T.; Ashokkumar, M.; Balachandran, S.; Mawson, R.; Simons, L. InnoVatiVe Food Sci. Emerging Technol. 2008, 9, 170–175. (6) Solans, C.; Izquierdo, P.; Nolla, J.; Azemar, N.; Garcia-Celma, M. J. Curr. Opin. Colloid Interface Sci. 2005, 10, 102–110. (7) Ashokkumar, M.; Hall, R.; Mulvaney, P.; Grieser, F. J. Phys. Chem. B 1997, 101, 10845–10850. (8) Ashokkumar, M.; Lee, J.; Iida, Y.; Yasui, K.; Kozuka, T.; Tuziuti, T.; Towata, A. Phys. Chem. Chem. Phys. 2009, 11, 10118–10121. (9) Asakura, Y.; Nishida, T.; Matsuoka, T.; Koda, S. Ultrason. Sonochem. 2008, 15, 244–250. (10) Koda, S.; Kimura, T.; Kondo, T.; Mitome, H. Ultrason. Sonochem. 2003, 10, 149–156. (11) Brotchie, A.; Grieser, F.; Ashokkumar, M. J. Phys. Chem. C 2008, 112, 10247–10250. (12) Tuziuti, T.; Yasui, K.; Lee, J.; Kozuka, T.; Towata, A.; Iida, Y. J. Phys. Chem. A 2008, 112, 4875–4878. (13) Yasui, K. J. Acoust. Soc. Am. 2002, 112, 1405–1413. (14) Yasui, K. J. Chem. Phys. 2002, 116, 2945–1954. (15) Denzhkunov, N. V.; Francescutto, A.; Ciuti, P.; Mason, T. J.; Iernetti, G.; Kulak, A. I. Ultrason. Sonochem. 2000, 7, 19–24. (16) Tuziuti, T.; Yasui, K.; Iida, Y.; Sivakumar, M.; Koda, S. J. Phys. Chem. A 2004, 108, 9011–9013. (17) Henglein, A.; Ulrich, R.; Lilie, J. J. Am. Chem. Soc. 1989, 111, 1974–1979. (18) Segebarth, N.; Eulaerts, O.; Reisse, J.; Crum, L. A.; Matula, T. J. J. Phys. Chem. B 2002, 106, 9181–9190. (19) Lee, J.; Yasui, K.; Tuziuti, T.; Kozuka, T.; Towata, A.; Iida, Y. J. Phys. Chem. B 2008, 112, 15333–15341. (20) Dagastine, R. R.; Manica, R.; Carnie, S. L.; Chan, D. Y. C.; Stevens, G. M.; Grieser, F. Science 2006, 313, 210–213. (21) Vakarelski, I. U.; Lee, J.; Dagastine, R. R.; Chan, D. Y. C.; Stevens, G. M.; Grieser, F. Langmuir 2008, 24, 603–605. (22) Manor, O.; Vakarelski, I. U.; Tang, X.; J., O. S. S.; Stevens, G. W.; Grieser, F.; Dagastine, R. R.; Chan, D. Y. C. Phys. ReV. Lett. 2008, 101, 024501. (23) Xu, Q.; Nakajima, M.; Ichikawa, S.; Nakamura, N.; Roy, P.; Okadome, H. J. Colloid Interface Sci. 2009, 332, 208–214. (24) Lee, J.; Ashokkumar, M.; Kentish, S.; Grieser, F. J. Am. Chem. Soc. 2005, 127, 16810–16811. (25) Sunartio, D.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2007, 129, 6031–6036. (26) Iida, Y.; Ashokkumar, M.; Tuziuti, T.; Kozuka, T.; Yasui, K.; Towata, A.; Lee, J. Ultrason. Sonochem. 2010, 17, 473–479. (27) Mettin, R.; Akhatov, I.; Parlitz, U.; Ohl, C. D.; Lauterborn, W. Phys. ReV. E 1997, 56, 2924–2931. (28) Doinikov, A. A.; Zavtrak, S. T. Phys. Fluids 1995, 7, 1923. (29) Tronson, R.; Ashokkumar, M.; Grieser, F. J. Phys. Chem. B 2002, 106, 11064–11068.

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