Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions.

Ultrasonics Sonochemistry xxx (2014) xxx–xxx

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Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions Carlos Cairós a,⇑, Julia Schneider a, Rachel Pflieger b, Robert Mettin a a Christian Doppler Laboratory for Cavitation and Micro-Erosion, Drittes Physikalisches Institut, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany b Institut de Chimie Séparative de Marcoule, ICSM-UMR5257 CNRS/CEA/UM2/ENSCM, BP17171, 30207 Bagnols-sur-Ceze Cedex, France

a r t i c l e

i n f o

Article history: Received 30 November 2013 Received in revised form 12 February 2014 Accepted 10 March 2014 Available online xxxx Keywords: Multibubble sonoluminescence Sodium line emission Bubble dynamics High-speed imaging

a b s t r a c t The sonoluminescence spectra from acoustic cavitation in aqueous NaCl solutions are systematically studied in a large range of ultrasonic frequencies under variation of electrical power and argon sparging. At the same time, bubble dynamics are analysed by high-speed imaging. Sodium line and continuum emission are evaluated for acoustic driving at 34.5, 90, 150, 365, and 945 kHz in the same reactor vessel. The results show that the ratio of sodium line to continuum emission can be shifted by the experimental parameters: an increase in the argon flow increases the ratio, while an increase in power leads to a decrease. At 945 kHz, the sodium line is drastically reduced, while the continuum stays at elevated level. Bubble observations reveal a remarkable effect of argon in terms of bubble distribution and stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. As a consequence, the bubble interactions (splitting, merging) appear enhanced which supports a link between non-spherical bubble dynamics and sodium line emission. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Sonoluminescence (SL) from alkali-metal salt solutions has been widely studied [1–11]. It is important to elucidate the nature of the SL of non-volatile ions, and the origin and location of their reduction and excitation, because it will lead to a better understanding of sonochemistry of non-volatile species in aqueous solutions. Taylor and Jarman [1] reported alkali-metal SL spectra. They observed SL emission in the range of 280–740 nm from a 2 M NaCl solution saturated with different noble gases at frequencies of 16 and 500 kHz, and they found the necessity of the presence of noble gas dissolved into the solution to obtain alkali line emission. The spectra were composed of three characteristic types of emission: a continuum background, the characteristic bands for OH⁄ radical emission, and sodium line emission with a characteristic peak around 589 nm. They were the first to propose that sodium ions enter the bubbles and that sodium emission is essentially chemiluminescence.

⇑ Corresponding author. Tel.: +49 551399981. E-mail address: [email protected] (C. Cairós).

This statement was later confirmed by Lepoint-Mullie et al. [5] among other authors [2,12]. They studied visible emission spectra from aqueous and 1-octanol solutions of NaCl and RbCl with two dissolved gases: Ar and Kr. They detected satellite peaks in the emission spectra corresponding to exciplex transitions of alkalimetal/rare gas molecules. This supported the idea that alkali-metal species emit in the gas phase of collapsing bubbles. Sehgal et al. [2] used the broadening and the shift of SL spectra from KI and NaCl to obtain values of temperature and pressure inside the collapsing bubble. In contrast, Flint and Suslick [4] proposed that the alkali-metal emission originates from the liquid–gas interface surrounding the bubble. According to their explanation, the line profile of alkali-metal atom emission does not give good information on the cavitation condition at bubble collapse. Grieser and Ashokkumar [7], and Ashokkumar et al. [8] pointed out that the bubble/liquid interface region may play an important role in the reduction process of Na+, as a result of their works with NaCl and surfactants. Sunartio et al. [12] concluded in their study that excited sodium emission is primarily from those bubbles which are sonochemically active, but not emitting continuum radiation. Recently Xu et al. [13], Hatanaka et al. [14] and Thiemann et al. [15] were able to observe a spatial separation in the sodium and continuum emission when a solution of sodium sulphate in

http://dx.doi.org/10.1016/j.ultsonch.2014.03.006 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: C. Cairós et al., Effects of argon sparging rate, ultrasonic power, and frequency on multibubble sonoluminescence spectra and bubble dynamics in NaCl aqueous solutions, Ultrason. Sonochem. (2014), http://dx.doi.org/10.1016/j.ultsonch.2014.03.006

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C. Cairós et al. / Ultrasonics Sonochemistry xxx (2014) xxx–xxx

Fig. 1. Experimental setup.

Fig. 2. Sonoluminescence spectra from a 3 M NaCl solution at two frequencies: solid line 90 kHz dashed line 945 kHz (both measurements at a power of 10 W and argon flow 46 mL/min).

sulphuric acid was irradiated both with a 20 kHz probe and in an ultrasonic bath. The transfer of non-volatile ions into bubbles is suggested to happen by means of injection of micro-droplets due to surface instabilities, coalescence, bubble fragmentation or

(a)

Lateral view

Transducer

(b)

jetting [15]. From these results it appears likely that in multibubble sonoluminescence it is possible to split the bubble cloud in two emitting bubble populations, the bubbles that can produce Na⁄ emission, and bubbles that can produce continuum emission. Of course, it is rather a different collapse modality that switches the emissions, and the same bubble might produce both emissions in different moments of its life cycle. Besides the abundant research, the exact mechanism is still under debate. The principal aim of this work was to learn more about multibubble sonoluminescence of aqueous NaCl solutions, and in particular to see whether it is possible to affect the proportion of these two conjectured bubble populations through a change of the common experimental parameters: ultrasound frequency, electrical power, and noble gas flow. As a second goal, we tried to find correlations of changes in emission with microscopic changes in bubble dynamics, directly observed by high-speed imaging. Our results show that the presence of argon not only changes the bubble gas content, but definitely as well the bubble dynamics, with a final increase in the proportion of bubbles exciting alkali metals. On the other hand, electrical power has the opposite effect, as an increase in power enhances continuum emission compared with Na⁄ emission.

10000 fps

(d) Maximum diameter

(c)

100000 fps

Bubble tracking - velocity

Binary Image

Fig. 3. (a) Characteristics of the bubble distribution: streamers (dashed arrows) with directions of bubble motion, and typical video observation frame size, (b) and (c) are representative images with spatial resolution at the two framing speeds 10,000 fps and 100,000 fps, and (d) image processing and bubble tracking.



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The voltage at the transducers is controlled via a power amplifier (NF-HSA-4101) and a function generator (Tektronix AFG 3021). An in-house made power meter was placed between amplifier and transducer to measure the real electrical power supplied to the system. Spectroscopic measurements were performed in the range of 250–750 nm with an ActonResearch SP-300i imaging spectrometer coupled to a charge-coupled-device detector (PIXIS 100B, Princeton Instrument). The spectrometer is equipped with two gratings: 600 gr/mm blz. 300 nm and 300 gr/mm blz 500 nm. For measurements in the visible range a high-pass filter blocking wavelengths below 320 nm was used. The wavelength calibration was performed with Hg (Ar) pen ray light sources (LSP 035, LOT). Spectra are corrected against the sensitivity curves of the gratings and CCD as provided by the manufacturer. The slit aperture was 0.5 mm for all experiments and the distance between spectrometer and cell was 10 cm (optimised in previous experiments to obtain the maximum intensity). All solutions were prepared using pure DI water (max. conductivity 0.0005 mS/cm) and sodium chloride (Sigma–Aldrich, analytical grade P98%) to get a concentration of 3 M. 2.2. Methods

Fig. 4. Change in (a) sodium emission (589 nm), (b) continuum, and (c) ratio sodium/continuum, with respect to argon flow (7–96 mL/min). Electrical power (12 W) remains constant for all experiments. Results for all the frequencies are shown (indicated by the symbols and given in kHz).

2. Materials and methods 2.1. Material Fig. 1 gives a sketch of the setup. The reactor used in this experiment was in-house made and consists of a stainless steel cube of 6.5 cm side length. The fully filled volume of solution is 250 ml. The reactor has quartz windows (5 cm diameter) in three of its lateral sides (allowing observations in the UV–Vis range). At the fourth lateral side, two different transducers can be coupled. The first one is a sandwich-type Langevin transducer providing, after the correct impedance match (ferrite transformer), three working frequencies: 34.5 kHz, 90 kHz, and 150 kHz. The second transducer is a lead zirconate titanate (PZT) disc (5 cm diameter) that can be operated at two working frequencies: 365 kHz and 945 kHz. The acoustic field generated is basically a focused standing wave with a share of travelling wave away from the transducer. The reactor is also equipped with inlets for gas and for temperature control with a cooling jacket at the bottom connected to a cooling bath.

Multibubble reactors are generally known as potentially instationary and unstable systems. The fluctuating character of the bubble distribution was tried to be compensated with the right number of replicates and averaging, and especially, with enough stabilization time. Adding a flow of gas into solution improved the reproducibility of the system. The continuous bubbling of gas resulted in a more stable bubble distribution during sonication, with higher stability of the bubble streamers. This fact was confirmed with high speed camera observations. It is probably attributed to a stationary high gas concentration in the liquid maintained by the gas flow, which counteracted the permanent degassing effect of cavitation. For every measurement, change of electrical power, argon flow or frequency, a sufficient time for stabilization was taken, until the bulk temperature was constant and the variability in the spectra showed no trend but a random distribution. The stabilization time was approximately 1–2 h for all experiments. Due to different intensities of the emission, exposure times for measurements in the visible and ultraviolet regions of the spectra were 20 s (30 replicates each measurement) and 200 s (5 replicates), respectively. Nevertheless, a correct overlap was obtained at the time of linking both parts of the spectra. The typical shape of the spectra is shown in Fig. 2. Three emissions were followed during the experiments: sodium line emission with a maximum at 589 nm (corrected for the continuum contribution), line emission of hydroxyl radical (OH⁄) at 310 nm (also corrected for the continuum), and continuum emission (taking an average value between 380–480 nm). In the results section, just sodium emission and continuum emission is mentioned because, as first conclusion of this work, we found that OH⁄ line emission and continuum showed the same trends under variation of the factors under study (frequency, argon flow and electrical power). Similar behaviour was found by Gordeychuk et al. [16] in their experiments addressing the changes of SL emission with respect to pressure and temperature. Observations of bubble dynamics were performed with a highspeed camera (Photron SA5) using background illumination and a long distance microscope (Infinity, K2/SC). High quality videos in the right frame rate allowed us to obtain valuable information after simple image processing. The bubble structures appear similar for all the frequencies: streamers of bubbles develop at or near the transducer’s surfaces and move towards the walls of the reactor


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Fig. 5. Consecutives frames from videos recorded in a 3 M NaCl solution (frame rate 100,000 fps, acoustic frequency 90 kHz), at fixed power of 12 W and 20 °C under the influence of a 46 mL/min (a) air flow, and (b) argon flow.

Fig. 6. Consecutive frames from video recorded in a 3 M NaCl solution (frame rate 100 kHz, acoustic frequency 90 kHz) at power of 12 W and 20 °C sparged with an argon flow of 46 mL/min argon flow. Larger bubbles leave a trace of smaller bubbles behind. This phenomenon is frequently observed in solutions with argon.

(Fig. 3a). High speed camera recordings were performed at two principal frame rates: 10,000 fps for streamers overview and velocity measurements (approx. field of view of 4 3 mm2), and 100,000 fps for bubble size quantification and detailed view (approx. field of view of 1.5 1 mm2), as it is shown in Fig. 3b and c. The bubble size was estimated through image processing as follows: first the images were converted to binary (black and white),

which isolated the dark bubbles from the background, and second, particle analysis to n random frames with a large enough time interval between them (to assure analysis of different bubbles) was performed, using the tools provided by the free software image J [17]. The size is finally expressed as diameter at maximum expansion of the bubble (Dmax). Due to the limitation of the exposure time which could not be smaller than 1 ls, the values for



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Fig. 7. Histograms of bubble size showing the bubble distribution at three argon flow rates (7, 46, 92 mL/min) at 90 kHz and 12 W power.

Fig. 8. Histograms showing the change in the bubble velocities with respect to the argon flow rate (7, 46, 92 mL/min) at 90 kHz and 12 W power.

945 kHz are less precise. Velocity measurements were performed using a manual tracking software also integrated in image J. The tracked bubbles were semi-randomly selected to collect information for all kinds of bubbles (differences in size, velocity, etc.) Due to resolution limitations only qualitative information was obtained for the overall bubble density in the structures, since the number of streamers is missing in our experimental data. For every measurement, about 5–10 videos (replicates) were recorded at different points in the cell (trying to get information from different places of the reactor, especially near the walls and in the middle of the reactor). 3. Results and discussion 3.1. Effect of argon flow We recorded the emission spectra of a 3 M NaCl solution at five different argon flows: 7, 23, 46, 60 and 96 mL/min (12 W of power and room temperature, 20 °C), and for the five frequencies. Fig. 4 shows the evolution of the emission with the changes in argon flow for (a) the Na emission line at 589 nm, (b) continuum emission (average value between 350–450 nm) and (c) ratio of emissions (sodium emission/continuum). The sodium emission increases with an increase in argon flow, and it is higher for low frequencies (34.5 and 90 kHz). Continuum emission, however, decreases for all frequencies, with the exception of 34.5 kHz. The effective ratio between emissions increases for all frequencies. High-speed camera observations show significant differences between bubble streamers obtained under air (either in an air-saturated solution or under air flow) and under argon flow. The difference is illustrated in Fig. 5 that shows a sequence of a bubble streamer for two videos. The first sequence (a) is a 3 M NaCl solution sonicated at 90 kHz, under 46 mL/min air flow. The second sequence (b) is a video recorded under the same conditions but

changing the air flow by argon flow. In both videos we observe streamers of running bubbles (from right to left) with frequent collisions and bubble merging and splitting. One big difference emerges from the simple observation of these frames. The frames corresponding to air flow show a clearer background than the frames from argon flow. This cloudy grey background in Fig. 5b corresponds to smaller or out of focus bubbles. We can elucidate that from the videos because they also oscillate and show a beat oscillation with the recording frequency (Fig. 5b frames 1, 6, 15 for example). In Fig. 6, we show 15 frames of a video recorded with an argon sparged solution at 46 mL/min, under the same conditions than before (video from Fig. 5b), but with a higher magnification and a lower frame rate. Here, we observe the typical phenomenon that is changing the bubble cloud characteristics in presence of argon. Streamers of bubbles are composed of several bubbles of different sizes, and the bigger bubbles tend to decompose or let a tale of smaller (and apparently passive) bubbles behind. These bubbles then grow by collision with neighbouring bubbles and possibly also by rectified diffusion and (at least some of them) again turn into active cavitation bubbles. This phenomenon occurred in all the videos we recorded under argon flow, and it is similar to the observations reported before by Hatanaka et al. [14] and Choi et al. [11] in sulphuric acid and NaCl-ethylene glycol solutions, respectively, where small daughter bubbles were ejected from larger ones. We could quantify the effect of the argon flow on the dynamics of the cloud through the analysis of the bubble size distribution and the velocity of the bubbles. Fig. 7 shows histograms of bubble size distribution for the minimum, the medium and the maximum argon flow. Data were obtained from the analysis of 30 random frames picked from three different videos for each flow. The average size of the bubbles remains roughly constant, but the distribution of sizes suffers a stretch effect. It goes from the narrow and mean centred distribution of the low (or non-existent) argon flow,


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to the scattered and minimum centred of the higher argon flows. These graphs reveal the effect of argon into solution: the population of active bubbles increases due to the fragmentation observed in Figs. 5 and 6, and as a result, the bubble cloud has an increase in the number of smaller bubbles, but also, bubbles up to double of the size appear, in a greater number as we rise the argon flow (Fig. 7c). The velocity of the bubbles suffers a similar change. The bubbles under an argon flow, due to frequent collisions, merging and splitting have a heterogeneous profile of velocity, especially if we compare with the homogeneous profile of the bubbles under a low argon flow. This is shown in Fig. 8, and histograms of air flow or air saturated solutions appear practically equal to the minimum argon flow histogram and are not shown.

3.2. Effect of electrical power

Fig. 9. Sonoluminescence intensity of (a) sodium peak, (b) continuum, and (c) ratio of these emissions, with respect to real electric power applied to the transducer; frequencies indicated in kHz. Argon flow (46 mL/min) and temperature (20 °C) were constant in all the experiments.

Fig. 10. Bubble average velocities for different powers at 90 kHz, together with SL emission of sodium and continuum (Ar flow 46 ml/min).

The real electrical power supplied to the transducer strongly influences the intensity of the SL emission, as well as the dynamics of the bubbles. Fig. 9 shows changes in sonoluminescence emission registered with respect to the electrical power, for all applied ultrasound frequencies and constant argon flow and temperature. In this case, an increase in electrical power, generally, produces an increase in continuum and Na⁄ line. For all the frequencies the emission increases and apparently tends to saturate, except for 90 kHz which shows a maximum followed by a decrease in sonoluminescence emission. We expect this behaviour for all the others frequencies (see below), but probably limitations of the amplifier impede the system to reach bigger powers and hence, the maximum in emission cannot be achieved. The maximum at 90 kHz is slightly different for both emissions, i.e. the maximum for Na⁄ emission occurs around 8 W and the continuum emission peaks around 12 W. If we focus on the ratio Na⁄/continuum emission, it is clear that increasing the power is promoting the continuum versus the sodium emission for all the frequencies. From the high-speed recordings we cannot judge well the change of bubble sizes due to power increase. However, the bubble densities and the number of streamers increase. This comes together with a reduction of the number of bubble interaction phenomena like coalescence, merging and splitting. While this is an obvious fact from visual observation of the cavitation field, it was impossible to quantify this at the field of view and resolution provided by the high-speed camera. Thus we cannot provide quantitative information about the number of streamers or the global bubble density, nor the frequency of bubble collisions. Nevertheless, from the videos we were able to measure the velocity of the bubbles. Generally, the velocities of bubble translation increase with power and can reach several m/s. Only at 90 kHz the velocity of the bubbles reaches a maximum around the same values of power as the continuum sonoluminescence emission (Fig. 10). On the one hand, this fact shows the strong correlation between the characteristics of the bubble dynamics and the sonoluminescence. On the other hand, the existence of a maximum in SL emission reflects a saturation effect (and even decrease) of sound pressure beyond a certain electrical power provided to the transducer. This phenomenon is well known in high power ultrasonics [18], and it is mainly due to shielding of the sound wave by an increase in bubble density in front of the transducer [19,20]. The position of this maximum might, of course, vary in function of the other variables in the system like frequency, dissolved gas content, or temperature. In any case, we consider this behaviour generic in acoustic cavitation and exclude a special situation for 90 kHz driving, as we would expect observations similar to 90 kHz at the other frequencies at even more elevated power, which was not realisable here.



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Fig. 11. Change in Na⁄ and continuum sonoluminescence emission with respect to: power, (a) and (b), and argon flow, (c) and (d).

In conclusion, an increase of power leads to a decrease of the ratio of sodium line emission against continuum emission. The promotion of continuum radiation goes along with an increase of bubble velocities and apparently less bubble interaction. We like to remark the possibility of evaluating bubble speed as additional observable in sonoreactor systems, indicating for instance the shielding effect for a given frequency. 3.3. Effect of frequency

Fig. 12. Average maximum diameter and average velocity with respect to the applied frequency at 10 W of power and 46 mL/min of argon flow.

In Fig. 11 we summarise results for the whole set of frequencies, varying the electrical power and the argon flow. The lower frequencies generally yield larger sodium emission than the higher frequencies, in particular at high argon flow. At different frequencies the effect of power, argon flow or other variables is similar, for example, localisation of emission maximum or magnitude of the change. The order of magnitude of the continuum emission remains essentially the same for all the frequencies, but the sodium emission suffers a drastic decay at the highest frequency of

Fig. 13. ‘‘Miller’s arrays’’ of trapped degassing bubbles at (a) 365 kHz and (b) 945 kHz (power 12 W, argon flow 46 ml/min).


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945 kHz. Let us note here that the efficiencies of the transducers are unknown and might vary. Therefore the acoustic power delivered to the cavitating liquid might have been different for the various frequencies studied in spite of equal readings of real electrical power. However, from the bubble dynamics observations like bubble densities and the velocity data (Fig. 12 below) we conjecture only weaker variations of efficiency. From the high speed camera observations we determined values of diameter and velocity of the bubbles. As it was expected, bubble sizes decrease when increasing the frequency [21]. Here we show as well that the opposite happens with the mean bubble velocity, growing with the frequency and reaching peak values of 15 m/s for 945 kHz. Average values were obtained over 300–500 bubbles for 10 W of electrical power and 46 mL/min argon flow, and the results are shown in Fig. 12. Apart from the smallest radius and fastest velocity values being reached at 945 kHz, we did not see any dramatic change in the dynamics of the visible bubbles that might explain the significant decrease in Na⁄ emission for the higher frequency. Thus, a higher shape stability of the smaller bubbles might prevent droplet injection in spite of the higher translational velocities which are expected to support injection (e.g. by jetting). Furthermore, in the global bubble field some difference can be seen at 365 kHz and 945 kHz. Although the bubble structures remain essentially similar for high and low frequencies (streamers of bubbles running from the transducer towards the walls of the reactor), at the two higher frequencies we have the presence of aligned trapped larger gas bubbles near the pressure nodes, see Fig. 13. These trapped bubbles result from degassing and do not appear for the smaller frequencies. They might effectively dampen the sound field and/or interact with the active bubble streamers, finally influencing the population of active bubbles. The phenomenon is well known and has been reported by Miller for the MHz region [22]. Depending on cuvette size (large compared to acoustic wavelength) and dissolved gas content of the liquid (near saturation), such lines or arrays can also be realised at lower frequencies [23,24], but the aspect of their stability is not yet fully understood. Here, the large trapped bubbles might be influencing streamer dynamics and SL emission, but this hypothesis needs further investigation. 4. Conclusions The effects of argon flow, real electrical power and frequency on sonoluminescence have been investigated with respect to sodium line and continuum emission. Both types of emission can be influenced by change of the experimental parameters: While an increase of argon flow enhances line emission and reduces continuum (apart from 34.5 kHz), the increase of power enhances the continuum more than the sodium emission. This results in an effective decrease of the ratio of both types of SL. At 945 kHz, the sodium line is drastically reduced, while the continuum stays at elevated level.

On the level of individual bubbles, we see a remarkable effect of argon in terms of bubble stability: larger bubbles of non-spherical shapes form and eject small daughter bubbles which in turn populate the whole liquid. This leads to higher bubble densities and broader bubble size and velocity distributions. As a consequence, the bubble interactions (splitting, merging) appear enhanced. All these phenomena support a link between non-spherical bubble dynamics and sodium line emission. Measurements of bubble velocities show a substantial increase with frequency (up to 15 m/s). Furthermore, we see a high correlation of mean bubble velocity to SL emissions and thus this might serve as independent observable for reactor characterisation. Acknowledgements The financial support by the Austrian Federal Ministry of Economy, Family and Youth and the Austrian National Foundation for Research, Technology and Development is gratefully acknowledged. References [1] K.J. Taylor, P.D. Jarman, Aust. J. Phys. 23 (1970) 319–334. [2] C. Sehgal, R.P. Steer, R.G. Sutherland, R.E. Verrall, J. Chem. Phys. 70 (1979) 2242–2248. [3] T.J. Matula, R.A. Roy, P.D. Mourad, W.B. McNamara III, K.S. Suslick, Phys. Rev. Lett. 75 (1995) 2602–2605. [4] E.B. Flint, K.S. Suslick, J. Phys. Chem. 95 (1991) 1484–1488. [5] F. Lepoint-Mullie, N. Voglet, T. Lepoint, R. Avni, Ultrason. Sonochem. 8 (2001) 151–158. [6] P.K. Choi, K. Funayama, Jpn. J. Appl. Phys. 46 (2007) 4768–4770. [7] F. Grieser, M. Ashokkumar, Adv. Colloid Interface Sci. 89–90 (2001) 423–438. [8] M. Ashokkumar, T. Vu, F. Grieser, in: Proc. 18th Int. Cong. Acoust. 4 (2004) 2935–2936. [9] S. Abe, P.K. Choi, Jpn. J. Appl. Phys. 48 (2009) (07GH02.1-3). [10] Y. Hayashi, P.K. Choi, J. Phys. Chem. B 116 (2012) 7891–7897. [11] Y. Sawada, Y. Takeuchi, P.K. Choi, Proc. 20th Int. Cong. Acoust. (ICA2010), p. 540. [12] D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, M. Ashokkumar, F. Grieser, Chem. Phys. Chem. 8 (2007) 2331–2335. [13] H. Xu, N.C. Eddingsaas, K.S. Suslick, J. Am. Chem. Soc. 131 (2009) 6060–6061. [14] S. Hatanaka, S. Hayashi, P.-K. Choi, Jpn. J. Appl. Phys. 49 (2010) (07HE01). [15] A. Thiemann, dissertation Georg-August-University Göttingen, 2011; R. Mettin, A. Thiemann, C. Cairos, F. Holsteyns, A. Troia, in: Proceedings of the Eighth International Symposium on Cavitation (CAV 2012), Singapore, 2012, 769–772; A. Thiemann, F. Holsteyns, R. Mettin, submitted to Ultrasonics Sonochemistry. [16] T.V. Gordeychuk, M.V. Kazachek, Opt. Spectrosc. 106 (2) (2009) 238–241. [17] http://rsb.info.nih.gov/ij/. [18] L.D. Rozenberg (Ed.), High Intensity Ultrasonic Fields, Plenum Press, New York, 1971. [19] M.M. van Iersel, N.E. Benes, J.T.F. Keurentjes, Ultrason. Sonochem. 15 (2008) 294–300. [20] O. Louisnard, Ultrason. Sonochem. 19 (2012) 66–76. [21] A. Brotchie, F. Grieser, M. Ashokkumar, Phys. Rev. Lett. 102 (2009) 084302. [22] D. Miller, J. Acoust. Soc. Am. 62 (1977) 12. [23] A. Otto, T. Nowak, R. Mettin, F. Holsteyns, A. Lippert, in: M.M. Boone (Ed.), NAG-DAGA 2009 International Conference on Acoustics, Rotterdam, Deutsche Gesellschaft für Akustik e.V. (DEGA), Berlin, 2009, pp. 1350–1353. [24] A. Thiemann, T. Nowak, R. Mettin, F. Holsteyns, A. Lippert, Ultrason. Sonochem. 18 (2011) 595–600.


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