Spatial Distribution Enhancement of Sonoluminescence Activity by

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J. Phys. Chem. B 2008, 112, 15333–15341

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Spatial Distribution Enhancement of Sonoluminescence Activity by Altering Sonication and Solution Conditions Judy Lee,* 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 ReceiVed: July 8, 2008; ReVised Manuscript ReceiVed: September 29, 2008

An intensified charge-couped device (CCD) camera was used to capture raw images of multibubble sonoluminescence, generated by 168 and 448 kHz ultrasound. The effect of various air and surfactant concentrations, and pulse conditions on the acoustic pressure distribution, percentage of standing wave component, the structure of the sonoluminescence activity, and speed of streaming was investigated. It was observed that the enhancement in the sonoluminescence intensity by appropriate degassing, pulsing, and addition of sodium dodecylsulfate were closely related to an expansion in the spatial distribution of sonoluminescence activity. This broadening in the spatial distribution is correlated with a high percentage of standing wave component. This effect stems from the reduction in the attenuation of the acoustic field by inhibiting the formation of large coalesced bubbles. 1. Introduction Acoustic cavitation is an unique phenomenon involving the inertial collapse of micrometer sized bubbles in less than a few microseconds, creating extreme temperatures and pressures inside the bubbles.1 As a consequence of these extreme conditions, light known as sonoluminescence (SL) is emitted, and OH radicals and H2O2 molecules are generated.2 The chemical as well as the physical effects are responsible for the employment of ultrasound in numerous applications such as cleaning,3-5 emulsification,6-8 degradation of pollutants,9-11 and nanoparticle synthesis.12-14 Although applications of ultrasound in the industry appear promising, there is a need to improve the efficiency of these applications before it is economically viable. The efficiency of the application of ultrasound largely depends on the population and spatial distribution of active bubbles within a reactor. Since SL intensity is proportional to the population of active bubbles, the intensity and spatial distribution of SL can be used as a measure of the efficiency of ultrasound. It has been shown that with appropriate degassing,15 pulsing,15-17 and the addition of charged surface active solute,18,19 sonoluminescence and sonochemical activity can be enhanced. However, most of these studies show only the integrated SL intensity and do not provide information on how the spatial distribution of cavitation activity is influenced by sonication conditions or the addition of surface active solutes, and no mechanism have been provided. In addition, various sonication and solute conditions can alter the number and size of bubbles, which in turn can have a significant effect on the local acoustic pressure distribution and hence the SL intensity. In many SL studies, the driving or calorimetric power is often quoted; however, these values represent an averaged value which does not reflect the local acoustic pressure established within the vessel. There are only a few reports in the literature that show images depicting the spatial distribution of SL emission.20-23 Crum et al.20 showed using a 20 kHz horn that SL emissions were emitted from regions near the pressure antinodes in the standing wave * E-mail: [email protected].

field and thus concluded that the SL activity is from “stable” cavitation rather than “transient” cavitation. Generally, acoustic fields may be part standing wave and part traveling wave depending on the reflectivity of the interface as well as the attenuation effect of the medium.24 Leighton et al.21 demonstrated using an intensified charge-coupled device (CCD) camera that large amount of light was produced at the pressure antinodes when there is a high percentage of standing wave component in the acoustic field whereas in the case of a predominately traveling wave field, very little light was detected. These studies were conducted in water and to the authors’ knowledge images of SL structures in the presence of sodium dodecylsulfate (SDS) has only been reported by Sunartio et al.23 The focus of that work was on the emission from electronically excited sodium atoms and not the effect of SDS on the spatial distribution of SL activity, and in addition, the effect of SDS on the acoustic pressure distribution was not investigated. In this paper, an intensified CCD camera was used to capture raw images of SL structure using short exposure times without any added chemicals such as luminol that may otherwise interfere with SDS. The effect of different air and SDS concentrations, and pulse conditions on the spatial distribution of SL activity was investigated. Additional measurements and calculations such as the acoustic pressure distribution, streaming speed, percentage of standing wave component, and magnitude of the radiation forces is presented and discussed. 2. Experimental Details Sodium dodecylsulfate (SDS) was purchased from BDH, special purity grade, and rhodamine B isothiocyanate, from Sigma-Aldrich. The SDS solutions were made by diluting an appropriate volume of 100 mM stock solution with distilled water that has been saturated with air. For every experiment a volume of 1 L was used and performed at room temperature (≈18 °C). Shown in Figure 1 is the rectangular glass vessel, dimensions 12.4 cm × 9.5 cm × 12 cm, used in all the experiments. At the bottom of this vessel is a stainless steel plate with a circular

10.1021/jp8060224 CCC: $40.75  2008 American Chemical Society Published on Web 11/11/2008

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Lee et al. of dissolved oxygen using a dissolved oxygen meter (Horiba, model D-25).

Figure 1. Dimensions of the reactor vessel used to capture the sonoluminescence images. The lengths shown are in centimeters and are not drawn to scale. The image obtained by the ICCD camera is shown with the dotted box indicating the imaged area relative to the reactor vessel.

opening of 8 cm in diameter. Two piezo-electric transducers (Kaijo Sonic Corp.) at resonance frequencies of 168 and 448 kHz were used. Both transducers are 5 cm in diameter and are glued to a circular stainless steel plate with a diameter of 10 cm (6 and 2 mm in thickness for 448 and 168 kHz, respectively) and attached to the bottom of the vessel. Signals from the function generator (NF Wavefactory, model 1946A) is amplified by a power amplifier (Honda Electronics, model L400BM-H) and fed through an impedance matching unit (Tokyo HY-Power, Model MT-300-HD) before reaching the transducer. The power output to the transducer was measured using a Megasonic power meter (Towa Electronic, model TDW 6102U), and the power and voltage were monitored on the oscilloscope (HewlettPackard, model 54645A). For all SL experiments, a power output of 1.1 W/cm2 was used. The local acoustic amplitude was measured using a PVDF hydrophone (Generex, model MH28-10) with a sensing element of 1 mm in diameter. The level of dissolved air content was measured by the concentration

The sonoluminescence images were captured by an intensified CCD camera (Andor Technology, model DH501-18F-01). The exposure time was set such that 2 × 105 acoustic cycles were captured. For pulse mode operations, the ICCD camera was gated to synchronize with the ultrasound pulses. SL images from the top and the side were imaged (Figure 1). The SL image obtained from the top shows a symmetrical ring structure, this symmetry was observed for the two frequencies used in this study. Therefore, with a line of symmetry along the vertical axis at the center of the vessel, only one-half of the SL image was taken for the side view. The speed of streaming was quantified by injecting 0.5 mL of 2 mM rhodamine B isothiocyanate dye and filming the movement of the dye toward the liquid surface during sonication. The videos were then analyzed and the average speed at which the dye is flowing toward the liquid surface was calculated. A frequency of 448 kHz and power of 1.1 W/cm2 was used. 3. Results and Discussion 3.1. Radiation Forces. The spatial distribution of SL activity depends on the distribution of active bubbles in the vessel during sonication. This distribution of active bubbles primarily depends on the radiation forces, which can drive bubbles to a certain location depending on the size of the bubble and whether it is driven in a standing or a traveling wave field. For small amplitude oscillations, bubbles less than the resonance size are driven by the standing wave field to the antinodes, whereas bubbles in the traveling wave field is transported in the direction of the propagating sound wave.24 The force from the standing and traveling wave as a function of bubble sizes can be calculated using eqs 1 and 2:24

Figure 2. Magnitude of the force from the standing wave and traveling wave as a function of bubble radius at different ratios of standing and traveling wave for 168 and 448 kHz.

Enhancement of Sonoluminescence Activity 2 3 PTW Vok 〈FTW〉 ) 2 R 2Fω 2 o

〈FSW〉 )

o

2

βtot ω

({ ( ) } ( 1-

3PSWkRεoaVo sin(2ky) 2Ro

ω ωo

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2 2

+ 2βtot

ω ωo2

)) 2

(1)

(ωo2 - ω2)

√(ωo2 - ω2)2 + (2βtotω)2

(2)

where FTW and PTW are the time averaged force and acoustic pressure for the traveling wave, FSW and PSW are the time averaged force and acoustic pressure for the standing wave, Vo is the equilibrium bubble volume, k is the wavenumber, βtot is the dissipative constant, ω is the angular frequency, ωo is the resonance angular frequency, Ro is the bubble radius, and F is the density of the liquid. βtot is evaluated using the approximation Q ) ωo/2βtot where Q is the quality factor and a value of 10 is used.24 Acoustic waves are not always perfectly reflected and with the presence of attenuation effects, the acoustic wave field is usually partially standing and partially traveling wave.24 Therefore, for simplicity, it is assumed that PTW + PSW ) acoustic pressure amplitude in the medium. Assuming small amplitude oscillations and acoustic pressure amplitude of 1 bar, the magnitude of the force exerted by different percentages of standing and traveling wave for 168 and 448 kHz is shown in Figure 2. Only bubble sizes below the linear resonance radius is shown and it can be observed that the size of bubbles are smaller for the higher frequency. In addition, the magnitude of the force is lower for the higher frequency. However, beside these differences the force curves for different percentages of standing and traveling wave are similar for the two frequencies. At high percentage of standing wave, a bubble below the resonance size is strongly influenced by the force from the standing wave. In this case, bubbles below the resonance size are more likely to be driven toward the pressure antinodes. When both the standing and traveling waves are in equal proportion, forces from the standing wave component dominates for bubbles below the resonance size and as the bubble size approaches the resonance size, force from the traveling wave component dominates. If the percentage of standing wave is less than 30%, the force on a bubble below the resonance size from the standing wave component becomes weaker than the force from the traveling wave component. This means that active bubbles are more likely to be driven toward the liquid surface by the force from the traveling wave component. The percentages of the traveling and standing wave component can be determined experimentally using acoustic pressure distribution measurements and eq 3:24

% standing wave )

Pantinode - Pnode ×100 Pantinode + Pnode

(3)

where Pantinode and Pnode are the pressure maximum at the pressure antinode and the pressure minimum at the adjacent pressure node, respectively. 3.2. Acoustic Pressure Distribution. The acoustic pressure along the line of symmetry down the center of the vessel was measured and the results are presented in Figure 3 for 168 and 448 kHz. In the case of strongly degassed water (1 mg/L of oxygen), distinct antinodes and nodes can be observed as a

function of distance from the transducer for both frequencies. However, for saturated water, strong attenuation of the acoustic pressure is observed. This attenuation of acoustic pressures arises from the presence of large coalesced bubbles which can absorb and scatter the acoustic waves.25,26 For degassed water, the number of bubbles is very low due to a lower number of gas nuclei27 and higher acoustic pressure threshold for rectified diffusion28 in degassed mediums. This reduces the frequency of bubble coalescence and hence decreases the formations of large coalesced bubbles. As the dissolved air concentration increases, so does the number of bubbles and the frequency of bubble coalescence. This leads to formation of large coalesced bubbles and attenuation of acoustic pressure. For 1 mM SDS and 10 mM SDS, there appears to be very little attenuation of the acoustic pressure in the vessel compare to the acoustic pressure distribution obtained with saturated water. At the SDS concentrations used in this study, the concentration of dissolved air is not affected and remains the same concentration as air saturated water. However, SDS is a surface active agent that has shown to inhibit the coalescence of bubbles both in the absence29 and presence30,31 of ultrasound. This inhibition will prevent the formation of large coalesced bubbles that would otherwise attenuate the acoustic pressure. These large coalesced bubbles have been quantified in the study by Lee et al.28 and Sunartio et al.30,31 involving the measurement of the total bubble volume (∆VT) using a capillary tube system. This total bubble volume was determined by measuring the volume of liquid displaced in the capillary tube after a fixed period of sonication. This rise in the volume is due to the generation of large bubbles via bubble coalescence in the liquid, therefore the volume displaced in the capillary tube was assumed to be equivalent to the total bubble volume generated in the liquid. It was shown that for air saturation below 50%, ∆VT is very low and as the air concentration increases to saturation level, ∆VT increases significantly. It was also shown for 1 mM SDS that the ∆VT dropped to 20% of the ∆VT for saturated water. This decrease in the population of large coalesced bubbles is also supported by visual observations. For 10 mM SDS, the ∆VT study reported an increase in the ∆VT relative to ∆VT for 1 mM SDS, suggesting an increase in the population of large coalesced bubbles. This increase in ∆VT becomes larger with increasing frequency. The effect from the increase in ∆VT for 10 mM SDS is noticeable in the small decrease in the acoustic pressure relative to 1 mM SDS shown in Figure 3 for 448 kHz. For 168 kHz, the small increase in bubble size for 10 mM SDS is not enough to attenuate the acoustic pressure as shown in Figure 3, instead a slight increase in the acoustic pressure is observed. 3.3. Spatial Distribution of SL Activity. Standing waves are important for SL activity as it allows the accumulation of active bubbles and growth of bubble nuclei at the antinodes that are fixed at certain locations. The correlation between SL activity and high percentage of standing wave was demonstrated by Leighton et al.21 They showed that when a large ratio of the wave is traveling, very little light was detected. Therefore, using eq 3 and the acoustic pressure data in Figure 3, the percentage of the standing wave in the sound field for strongly degassed water, saturated water, and 1 and 10 mM SDS was evaluated and an average percentage between the height 0-3, 3-6, and 6 cm to the liquid surface is presented in Figure 4. These values will be used to explain the SL structures obtained for different solute conditions. Note that the percentages of standing wave were calculated based on the acoustic pressure data shown in Figure 3 where the measurements were taken along the central

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Figure 3. Mapping of the acoustic pressure field along the axis of symmetry along the center axis of the vessel for degassed water (1 mg/L), air saturated water, and 1 and 10 mM SDS solutions at a frequency of 168 and 448 kHz and a power of 0.26 W/cm2.

Figure 4. Percentage of standing wave averaged between height of 0-3, 3-6, and 6-9.3 cm from the transducer for degassed water (1 mg/L), saturated water, and 1 and 10 mM SDS for 168 and 448 kHz. The percentages were calculated using eq 3 and data from Figure 3.

axis of the vessel. These values may not correspond to the entire SL regions captured and also a lower power was used in order to minimize damages to the hydrophone from cavitating bubbles, nevertheless, a close correlation between the percentages evaluated and the SL activity will be shown. 3.3.1. Effect of DissolWed Air Concentration. Shown in Figure 5a is one-half of the SL structure formed for air saturated water sonicated in continuous mode at 448 kHz and at a power of 1.1 W/cm2. The SL structure appears to be in the shape of half a cone with the most intense SL located near the surface of the liquid. As the concentration of dissolved air (quantified by the level of dissolved oxygen) decreases, a rather different SL structure was observed. This transformation in the spatial distribution of SL is demonstrated from Figure 5b-f. As the level of dissolved oxygen decreases to approximately 50% saturation, there appears to be an expansion of the SL areas toward the transducer, producing a rather homogeneous distribution along the center of the vessel Figure 5d. This active area was confined to zones directly above the transducer, with very little SL near the vessel wall. With the SL intensity still evenly distributed along the center, further decrease in the dissolved oxygen concentration below 50% saturation level resulted in a decrease in the SL intensity until the intensity of the SL became too weak for the ICCD camera to detect (Figure 5f).

The change in the SL structure can be explained by the percentage of standing and traveling wave established in the vessel shown in Figure 4. For saturated water, the percentage of standing wave on average is below 30%, this is caused by the attenuation from large coalesced bubbles. Indeed, the SL captured for saturated water show no SL activity near the transducer, but strong SL activity confined toward the surface of the liquid (Figure 5a). It has been explained by Tuziuti et al.32,33 using light scattering technique that standing waves are established near the liquid surface which can trap active bubbles that have been transported there by the traveling wave. However, a low percentage of standing wave was calculated below the liquid surface as shown in Figure 4 for 448 kHz. This may be because for high frequencies the distance between the adjacent antinodes and nodes are very close and due to the finite sensing area of the hydrophone, an averaging effect is obtained. For degassed systems where there are very few large coalesced bubbles present to attenuate the acoustic wave, a high percentage of standing wave averaging around 50% is obtained. Figure 2 shows that at 50% standing wave, bubbles below the resonance size are driven by the standing wave field. This is reflected in the SL obtained for air concentrations below 50% where the SL is homogeneously distributed. As discussed above, as the concentrations of dissolved air increases, excessive coalescence of bubbles can lead to degas bubbles that can significantly attenuate the acoustic wave, resulting in a low percentage of standing waves. However, the number of bubble nuclei available for cavitation activity is also proportional to the concentration of dissolved air.27 When the medium is totally degassed, insufficient number of bubble nuclei27 and a higher threshold for rectified diffusion28 resulted in undetectable SL activity. As the air concentration increases, the number of bubble nuclei and frequency of bubble coalescence increases. This coalescence of bubbles provides a faster path way, compare to the rate of rectified diffusion, for small bubbles to reach the active size. This is responsible for the increase in the SL activity when the oxygen concentration is increased to 4.2 mg/L. As the air concentration increases further toward the saturation level, the number of bubbles increases

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Figure 5. Images showing the effect of dissolved oxygen concentration on the SL structure taken from the side of the vessel: (a) 8.5, (b) 7.0, (c) 5.6, (d) 4.2, (e) 3.4, and (f) 2.9 mg/L. The white dotted lines above and below the SL structure denotes the liquid surface and the transducer position, respectively. Continuous sonication at a frequency of 448 kHz and a power of 1.1 W/cm2 was applied. Exposure time was set to collect 2 × 105 acoustic cycles. The center axis of the vessel is located on the left side of the images.

dramatically and excessive coalescence of bubbles can lead to the loss of active bubbles as large degas bubbles. The lower acoustic pressure at high air concentrations can also reduce the range of bubble sizes that can overcome the Blake threshold pressure,24 resulting in a lower number of active bubbles. 3.3.2. Effect of Increasing Pulse-off Time. Keeping the pulse-on time fixed the effect of increasing pulse-off time on the spatial distribution of the SL intensity for 168 and 448 kHz is displayed in Figure 6. For 168 kHz, because of a longer wavelength, strong bands of SL from the antinodes can be clearly distinguished whereas with the higher frequency of 448 kHz, bands of SL from the antinodes are difficult to separate. The change in the spatial distribution of SL with increasing pulse-off time for the two frequencies is similar to that observed with decreasing dissolved oxygen concentration illustrated in Figure 5. That is, the area of SL activity, initially isolated toward the liquid surface, extends toward the transducer with increasing pulse-off time. The same mechanism used to explain the effect of air concentration on the SL structure may also be responsible for the initial increase in the SL intensity observed with increasing pulse-off time in Figure 6. This enhancement in the SL intensity via pulsing has been reported in the literature.15,17 This enhancement via pulsing is attributed to the suppression in the generation of large coalesced bubbles as discussed in a report by Tuziuti et al.17 Pulsing can also increase the number of active bubbles if the pulse-off time is short enough for bubbles to survive and act as further cavitation nuclei for the subsequent pulse. Conversely, when the pulse-off time is too long, bubbles will dissolve and reduce the bubble population, which will result in the decrease in the SL.16,34 Analogous to the maximum SL activity reached at approximately 50% air saturation, the SL activity shown in Figure 6 increases to a maximum at a pulse-off time of 340 ms for 168 kHz and at a much shorter pulse-off time of 40 ms for 448 kHz. Further increase past this “critical” pulse-off time resulted in the decrease in the level of SL activity via dissolution effect. In a study by Lee et al.,34 the pulse-off time were correlated with the lifetime of a bubble of a certain size, this allowed the size distribution of the bubbles to be estimated. Therefore, the decrease in the SL intensity occurring at shorter pulse-off time

for 448 compare to 168 kHz suggests that bubbles generated at the higher frequency are smaller in size. 3.3.3. Effect of SDS Concentration. Shown in Figure 7 are the structures of SL viewed from above for different concentrations of SDS. For saturated water, a ringlike structure is observed for both 168 and 448 kHz. This ringlike structure has been reported in other studies35-37 and is explained by Birkin et al.35 to be related to the Bessel function character of the acoustic modes in the vessel. However, the addition of 1 mM SDS caused the SL ring to enlarge toward the vessel wall for 168 kHz and in contrast to this, for 448 kHz the addition of 1 mM SDS resulted in a strong SL activity at the center of the ring. At higher SDS concentration of 10 mM, the SL structure returned to almost identical ring structure observed with saturated water. To the authors’ knowledge, this change in the ring structure by the addition of SDS has not been reported in the literature. The effect of SDS on the spatial distribution of SL activity viewed from the side is shown in Figure 8. For saturated water, again the side views show strong SL intensities toward the liquid surface whereas in areas toward the transducer there were no detectable SL activities. With the addition of 1 mM SDS (Figure 8b), the SL activity is broadened from the liquid surface toward the transducer, expanding the regions of SL activity as seen with the effect of degassing and pulsing. This enlargement in the SL activity coincides with the increase in the integrated SL intensity reported in the literature for 1 mM SDS.18,19 The enlargement in the SL activity is largely due to a higher percentage of standing wave established as shown in the calculated percentages for 1 mM SDS in Figure 4. A high percentage of standing wave obtained under the addition of 1 mM SDS is due to the reduction in the attenuation resulted from a lower number of large coalesced bubbles. This increases the force from the standing wave component and drives active bubbles to the antinodes. The decrease in the number of large coalesced bubble is due to the inhibition of excessive bubble coalescence, resulting in an increase in the number of smaller bubbles. With the addition of 10 mM SDS, integrated SL intensities have been reported to decrease to an intensity comparable to water.18,19 Similar behavior was observed for 448 kHz where

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Figure 6. Images showing the effect of increasing pulse-off time on the SL structure generated in saturated water at a frequency of 168 and 448 kHz and at a power of 1.1 W/cm2. For 168 kHz, the pulse-off times are (a) 90, (b) 340, (c) 680, (d) 790, and (e) 800 ms. The pulse-on time was fixed at 5 ms and exposure time was set to capture 100 pulses. For 448 kHz, the pulse off times are (f) 1.5, (g) 10, (h) 40, (i) 110, and (j) 135 ms. The pulse-on time was fixed at 12 ms, and the exposure time was set to capture 100 pulses. The white dotted lines above and below the SL structure denotes the liquid surface and the transducer position, respectively. The center of the vessel is located on the left side of the images.

the spatial distribution of SL activity obtained for 10 mM SDS (Figure 8c) is almost identical to that obtained for saturated water (Figure 8a). That is, both systems show strong SL activities being restricted to areas below the liquid surface and with very low percentage of standing wave component of less than 50%. However, for 168 kHz, strong SL activity is observed near the transducer as observed for 1 mM SDS with high percentage of standing wave. Indeed, the standing wave percentages show that on average the percentage of standing wave for 10 mM SDS is

above 50%, higher than that obtained for saturated water, but less than the percentages calculated for degassed and 1 mM SDS. The reason for the SL structure at 10 mM SDS to resemble that of saturated water for the higher frequency is attributed to an increase in the population of large coalesced bubbles at the higher frequency as shown by the ∆VT study by Sunartio et. al,31 discussed in section 3.2. The attenuation of acoustic pressure by the increase in the bubble size at 10 mM SDS

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Figure 8. Images showing the effect of SDS on the SL structure taken from the side of the vessel for 168 and 448 kHz: (a) water, (b) 1 mM SDS, and (c) 10 mM SDS. For both frequencies, continuous sonication at a power of 1.1 W/cm2 was used. Exposure time was set to collect 2 × 105 acoustic cycles. The white dotted lines above and below the SL structure denotes the liquid surface and the transducer position, respectively. The center axis of the vessel is located on the left side of the images.

Figure 7. Images showing the effect of SDS on the SL structure taken from the top of the vessel for 168 and 448 kHz: (a) image of the vessel from the top, (b) water, (c) 1 mM SDS, and (d) 10 mM SDS. For both frequencies, continuous sonication at a power of 1.1 W/cm2 was used. Exposure time was set to collect 2 × 105 acoustic cycles. The white dotted lines denote the vessel wall.

relative to 1 mM SDS lowers the percentage of standing wave field and causes the force from the traveling wave to dominate. This results in the transport of active bubbles to the liquid surface where they are trapped by the strong standing wave field. For 168 kHz, ∆VT study31 shows that only a small increase in

the volume of large coalesced bubbles is observed at low frequencies. This small increase in the volume of large coalesced bubbles is not enough to significantly attenuate the acoustic pressure and the percentages of standing wave remains high. However, the amplitude at the nodes do not fall to zero, this indicates the presence of a weak traveling wave component.24 This may explain why the SL activity and percentage of standing wave is highest below the liquid surface for 10 mM SDS at 168 kHz. The effect of SDS on bubble sizes is demonstrated in Figure 9 when various SDS solutions are pulsed at the critical pulseoff time, which is 380 ms for 168 kHz and 40 ms for 448 kHz. The critical pulse-off times were determined from Figure 6 as the point at which longer pulse-off times resulted in a decrease in SL intensity due to dissolution of bubbles in saturated water. When pulsed at this duration, no SL was detected for 1 mM SDS under both frequencies (Figure 9b). This demonstrates that bubbles generated in 1 mM SDS are smaller than the bubbles

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Figure 10. Schematic depicting the dissolution of bubbles during the pulse-off time in the system shown in Figure 9. (a) For saturated water, bubbles are generally larger at 168 than for 448 kHz; therefore, the critical pulse-off time is longer for 168 kHz. (b) The addition of 1 mM SDS inhibits bubble coalescence, resulting in a population of smaller bubbles which dissolves away during the pulse-off time. (c) For 10 mM SDS, there is an increase in the size of the bubbles. This increase is larger for 448 than for 168 kHz; therefore, bubbles at 448 kHz are able to survive during the critical pulse-off time and act as further cavitation nuclei in the subsequent pulse.

TABLE 1: Average Speed of Dye in the Liquid Brought about by Streaming Effects under Different Air and Surfactant Concentrationsa average speed of injected dye [cm/s] degassed (1 mg/L oxygen) degassed (3.8 mg/L oxygen) saturated water 1 mM SDS in saturated water 10 mM SDS in saturated water a

Figure 9. Images from the side showing the effect of SDS on the SL structure when pulsed at the critical pulse-off time for 168 and 448 kHz: (a) water, (b) 1 mM SDS, (c) 10 mM SDS, and (d) 1 mM SDS + 0.1 M NaCl. For 168 kHz, the pulse-on time was fixed at 12 ms and pulse-off time fixed at 340 ms. For 448 kHz, the pulse-on time was fixed at 5 ms and pulse-off time fixed at 40 ms. Pulsed sonication at a power of 1.1 W/cm2 was used for both frequencies. Exposure time was set to capture 100 pulses. The white dotted lines above and below the SL structure denotes the liquid surface and the transducer position, respectively. The center axis of the vessel is located on the left side of the images.

generated in saturated water and therefore, dissolves away faster during the pulse-off time compare to the dissolution of larger bubbles in saturated water. This effect is illustrated in Figure 10a and b. This results in inadequate number of bubbles remaining before the next pulse arrives to act as further cavitation bubbles and SL is decreased. For 10 mM SDS at 168 kHz, very little SL was detected when the critical pulse-off time of 380 ms was used (Figure 9c for 168 kHz), this is because the bubble sizes are not large enough to survive through the pulse-off period (illustrated in Figure 10c for 168 kHz), resulting in low SL activity observed. However, for 10 mM SDS at 448 kHz, the increase in the bubble sizes discussed earlier allows bubbles to survive through the critical pulse-off period of 40 ms, which is much shorter than

no streaming 3.9 ( 0.6 5.0 ( 0.5 2.2 ( 0.6 11.7 ( 0.2

A frequency of 448 kHz and power of 1.1 W/cm2 were used.

the critical pulse-off period for 168 kHz, to further act as bubble nuclei for the subsequent pulse (illustrated in Figure 10c for 448 kHz). Another method of quantifying the strength of the standing and traveling wave component is to measure the speed of acoustic streaming. A study by Tuziuti et al.17,38 suggested that acoustic streaming can enhance the transportation of bubbles to the surface and enhance the number of active bubbles trapped by the standing waves near the liquid surface. This acoustic streaming is the flow of liquid in the direction of the propagating pressure waves and increases with increasing attenuation and frequency.24 Although acoustic streaming is usually observed at frequencies in the order of MHz, acoustic streaming at 500 kHz have been reported by Mitome et al.39 They described this streaming as “quasi” acoustic streaming, brought about by the movement of bubbles induced by the radiation force. The radiation force responsible for the streaming would be the traveling wave component as it drives bubbles in the direction of the propagating wave. Conversely, a high percentage of standing wave will result in a slower streaming speed. The magnitude of streaming was quantified by measuring the speed of moving dye in the solution during sonication. The effect of different air and surfactant concentrations on the average speed of the moving dye is tabulated in Table 1. No streaming was observed when the water is strongly degassed and as the air concentration increases, streaming developed as indicated by the increase in the streaming speeds measured. The addition of 1 mM SDS decreased the speed of streaming, however, the addition of 10 mM SDS increased the streaming speed to 11.7 cm/s, nearly twice as fast as that of saturated water. The cause behind this significant increase in the streaming speed at 10

Enhancement of Sonoluminescence Activity mM SDS may be attributed to an increase in the number of bubbles near the resonance size. As shown in Figure 2, bubbles near resonance size are strongly driven by the traveling wave component. An increase in the number of bubbles being driven by the traveling wave may increase the flow of liquid as shown in other studies on the effect of bubbles on the speed of acoustic streaming.39,40 Comparing the streaming speeds in Table 1 with the SL images in Figures 5 and 8, it is evident that the systems exhibiting a wider area of SL activity and high percentage of standing wave component (50% degassed and 1 mM SDS) have a slower streaming speed compare to the systems where strong SL activity was found to be isolated toward the liquid surface and where a high traveling wave component was calculated (saturated water and 10 mM SDS). 4. Conclusions SL intensity and distribution is proportional to the number and spatial distribution of active bubbles. The number of active bubbles depends critically on processes such as nucleation, rectified diffusion, bubble coalescence and dissolution, whereas the distribution of the active bubbles can be influenced by the size of the bubbles and the radiation forces generated in the vessel. It is shown in this study that gas and surface active solute concentrations and pulsing affects the formation of large bubbles from excessive bubble coalescence. It is found in this study that strong SL emission near the liquid surface observed for water is due to the damping of standing waves by large coalesced bubbles and the increase in the transport of active bubbles to the liquid surface by the traveling wave component. The increase in the total SL intensity by appropriate degassing, pulsing and addition of 1 mM SDS corresponded to an expansion in the spatial distribution of SL. This behavior is brought about by the reduction in the excessive coalescence of bubbles resulting in a lower population of large degassing bubbles and lower attenuation of the acoustic pressure. This increases the standing wave component in the sound field and results in active bubbles to be strongly driven toward the antinodes, producing a broader spatial distribution of SL activity. At 10 mM SDS, there is a slight increase in the size of bubbles toward the resonance size, which is strongly driven by the force from the traveling wave component. This increase in the number of bubbles driven by the traveling wave component is shown by a faster streaming speed measured and is the cause for the SL structure to be isolated toward the liquid surface as observed in the case of saturated water. Acknowledgment. The authors thank 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 number 1907765). References and Notes (1) Ohl, C. D.; Kurz, T.; Geisler, R.; Lindau, O.; Lauterborn, W. Phil. Trans. R. Soc. Lond. A 1999, 357, 269. (2) Yasui, K.; Tuziuti, T.; Sivakumar, M.; Iida, Y. J. Chem. Phys. 2005, 122, 224706.

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