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Development and optimization of acoustic bubble structures at high frequencies
Ultrasonics Sonochemistry 18 (2011) 92–98
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Development and optimization of acoustic bubble structures at high frequencies Judy Lee a,*, Muthupandian Ashokkumar b, Kyuichi Yasui a, Toru Tuziuti a, Teruyuki Kozuka a, Atsuya Towata a, Yasuo Iida a a b
National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama ku, Nagoya 463-8560, Japan School of Chemistry, University of Melbourne, VIC 3010, Australia
a r t i c l e
i n f o
Article history: Received 24 April 2009 Received in revised form 31 January 2010 Accepted 11 March 2010 Available online 17 March 2010 Keywords: Ultrasound frequency Acoustic bubble structure Radiation forces Attenuation Bubble coalescence Surfactants
a b s t r a c t At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Acoustic cavitation is a unique phenomenon where extreme pressures and temperatures are generated upon the collapse of micron size bubbles. These conditions induce physical and chemical effects that are beneficial for a number of industrial applications [1–5]. However, due to the chaotic nature of acoustic cavitation, the control and optimization of the cavitation systems are difficult and have been the subject of a number of studies [6–13]. There are numerous reports in the literature examining the enhancement in integrated sonoluminescence (SL) intensities by the addition of a surface active solute [14,15] and various sonication conditions such as frequency [16,17], power [18,19], pulse repetition frequency [13,20], reactor vessel geometry [8] and liquid height [9,21]. Literature reports [9,22–27] on the spatial distribution of active bubbles and sonochemiluminescence (SCL) structures have demonstrated the importance of acquiring a homogeneous spatial distribution of active bubbles for the enhancement in the integrated SL or SCL intensities. It is further demonstrated that a homogeneous distribution of active bubbles is obtained * Corresponding author. Tel.: +81 52 736 7215; fax: +81 52 736 7405. E-mail address: [email protected] (J. Lee). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.03.004
when a strong standing wave field is established in the system [25,28]. However, these SL and SCL structures reported to date are equilibrium structures observed when steady-states were reached under specific experimental conditions. An understanding into the development of the SL and SCL structures is critical for a better control and employment of ultrasound for industrial applications, but is difficult to capture as a function of time due to the short time scales (milliseconds) and a relatively low number of active bubbles. In the early stages of cavitation, fast photography have revealed that bubbles emerge from some point source and streams toward the antinodes to form a dense cluster and filamentary like structures [19,29,30]. However, these investigations were restricted to low frequencies for the reason that the bubble size and wavelength decrease with increasing frequency, making direct imaging difficult without special high speed imaging systems. The data presented in this manuscript will demonstrate the resemblance between the visible bubble structure and that of the SL bubble structure under equilibrium (steady-state) conditions at high frequencies. Using this association, the development of active bubble structures is indirectly inferred from the development of visible bubble structures at 168, 448 and 726 kHz. This manuscript will further show how acoustic streaming and weakening
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of the standing wave field can cause the spatial distribution of bubbles to become inhomogeneous and how this can be minimized under appropriate pulse ON and OFF ratio.
Transducer 1 (V1)
Transducer 2 Receiver
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Transducer 2 Receiver
2. Experimental method 2.1. Solutions and sonication conditions Sodium dodecylsulfate (SDS) was purchased from Sigma–Aldrich, special purity grade and were used without further purification. The SDS solutions were made by diluting an appropriate volume of 100 mM stock solution with distilled water that has been saturated with air. Three 5 cm in diameter piezo-electric transducers (Kaijo Sonic Corp.) at resonance frequencies of 168, 448 and 726 kHz were used. Signals from the function generator (NF Wavefactory, model 1946A) were amplified by a power amplifier (NF High Speed bipolar Amplifier, HSA4014). The power output to the transducer was measured using a Megasonic power meter (Towa Electronic, model TDW 6102U). For all experiments a power output of 20 W (1.1 W/ cm2), unless stated otherwise, was used. Degassed water was obtained using a membrane degassing unit (Membrana, MiniModule). The level of dissolved air content was determined by a dissolved oxygen meter (Horiba, model D-25). 2.2. Bubble structure images The experimental configuration used was the same as that described in Ref. [25] where a glass vessel which holds 1 L of solution was radiated with ultrasound emitted from the transducer fitted at the bottom. The initial development of bubble structures were captured using a high speed digital compact camera (Casio, Exilim EXFH20) at a rate of 210 frames per second (fps). This corresponds to an exposure time of 5 ms per frame. For equilibrium bubble structures, a slower rate of 30 fps (exposure time 33 ms) was used. In both cases, the sonicated solution was illuminated with a light source from the side of the vessel. 2.3. Void generation rate The void generation rate was measured using a capillary technique. In this technique the void volume generated by sonication as a function of time was measured. This void volume measured was assumed to be equivalent to the total volume of large coalesced bubbles generated for a given sonication time. Further details of this method can be found else where [31]. An approximate volume of 280 mL of solution was used to fill the capillary cell and the void volume as a function of time was recorded. This void volume was found to increase linearly with time and the void generation rate was obtained from the gradient. 2.4. Percentage of attenuation The configuration used to measure the attenuation of the acoustic wave by cavitation bubbles is shown in Fig. 1. The amplitude of the signal sent to transducer 1 from the function generator was modulated between two voltage outputs, V1 and V2. The amplitude of V1 was set to equivalent of 20 W (1.1 W/cm2) driving power and V2 was set to 10% of V1, below the cavitation threshold. A 5 MHz transducer (transducer 2) acted as a passive detector of the acoustic wave from V1 and V2, and the signals were collected by an oscilloscope. The percentage of attenuation was taken as the percentage of decrease in the amplitude of V2 by bubbles created by V1, relative to V2 when the water was degassed to a concentration of 1.0 mg/L where there were no cavitation bubbles present.
Fig. 1. Schematic of the system used to measure attenuation of acoustic waves by bubbles. Transducer 1 is modulated between two output voltages, V1 and V2. The voltage for V1 is equivalent to 20 W driving power, sufficient to create cavitation bubbles. V2 is set at 10% of V1, below the cavitation threshold, and is attenuated by the bubbles created by V1. The acoustic signals is received by transducer 2 and collected by an oscilloscope.
3. Results and discussion 3.1. Comparison between visible bubble structures and invisible SL bubbles structures Shown in Fig. 2(i) are the equilibrium (steady-state) spatial distribution of bubbles generated at frequencies 168, 448 and 726 kHz. The bubbles seen in these images are too large to be sonoluminescing (SL) bubbles, which have been shown theoretically [32] and experimentally [33] to be less than 10 microns in diameter at these frequencies. These large visible bubbles are bubbles that have been expelled from the pressure antinodes and become trapped at adjacent pressure nodes. This expulsion of the bubbles occurs when the bubble size becomes larger than the active size. The increase in the bubble size occurs predominately from the coalescence of sub-resonance size bubbles at the antinodes by the actions of primary and secondary Bjerknes forces. Light scattering measurements monitoring the development of size distribution of bubbles as a function of time have demonstrated this increase in the size and also number of bubbles [34]. Therefore, the spatial distribution of these visible bubbles would constitute an approximate indication of the spatial distribution of active bubbles that are otherwise invisible to the naked eye. The SL images taken by a CCD camera with an exposure time of 30 s are shown in Fig. 2(ii). Comparing the SL images with that of the visible bubbles in Fig. 2(i), a correlation between the structures of two different size of bubbles can be seen along the path of the propagating acoustic wave for all three frequencies.
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Fig. 2. (i) Visible bubble structure and (ii) SL bubble structure as a function of frequency for saturated water and driving power of 20 W. For 448 kHz, the effect of stabilizing the surface fluctuations with a sheet of silicone foam is shown. Exposure times for bubble structure and SL images were 33 ms and 30 s, respectively.
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For 168 kHz the structure of the visible bubbles are gathered at several horizontal planes orthogonal to the propagating acoustic wave in a standing wave pattern. The density of the visible bubbles is higher near the liquid surface than toward the transducer at the base of the vessel. The SL image for 168 kHz shows a similar standing wave pattern with a strong SL intensity near the liquid surface and a weak SL intensity near the transducer. For higher frequencies, the visible bubbles are localized to near the liquid surface and this is reflected in the SL bubble structure. A reduction in the surface fluctuations or altering the reflectivity at the boundary normal to the incident wave have shown to increase the standing wave field [28,35,36]. Therefore, to further demonstrate the relationship between the spatial distribution of visible bubbles and that of the SL bubbles, a sheet of silicone foam was floated on the liquid to stablilize the liquid surface and increase reflectivity. The results are depicted in Fig. 2 and it shows that the standing wave pattern for both the visible and SL bubbles were increased by the addition of the silicon foam. Although the spatial distribution of the visible bubbles show close resemblance to the structure of SL bubbles, this is only limited to the paths of the acoustic beam, which has a cross-sectional diameter of 5 cm normal to the direction of wave propagation. In addition to this, if coalescence is hindered or inhibited, active bubbles may exist in the system without any visible bubbles. Under this condition, SL images are needed to determine the nature of the SL structure. Nevertheless, provided that coalescence is not hindered and sub-resonance bubbles eventually reach a size that becomes detectable via coalescence, it is possible to monitor the occurrence of these visible bubbles to indirectly deduce the development of active bubbles. 3.2. Development of visible bubble structure 3.2.1. Continuous sonication Shown in Fig. 3 are a series of selected frames documenting the initial development of the bubble structure at 168, 448 and 726 kHz. The colour intensity has been inverted to improve the
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Fig. 3. Development of the bubble structure in saturated water as a function of time for different frequencies. Time in seconds is denoted above the images.
visibility of the bubbles, which appears dark against the light background. The random appearance of visible bubbles at earlier times of 0.24 s and 0.71 s for 168 kHz reflected the general concept that it is the random pre-existing bubble nuclei that act as weak points in the liquid which initiates cavitation [37,38]. As discussed earlier, these visible bubbles are bubbles that have been expelled from nearby antinodes due to predominately the coalescence of active bubbles by Bjerknes forces. With time, as the population and size of the bubbles increases, the standing wave structure becomes more defined and extends across the entire vessel. What is not depicted in these still photographs is the rapid translational motion of bubbles from the antinodal plane to the next nodal plane. This translational motion observed is usually in an upward direction. With the slower frame rate of 30 fps, used for the images shown in Fig. 2(i), these motions are seen as faint streaks between the nodes for 168 kHz. A recent report by Mettin et al. [39] simulated the translational motion of different bubbles which can exist in a high-frequency standing wave. In their study, the initial translational motion of bubbles, starting from the antinode toward the next nodal or some intermediate position, is shown to be only a few hundred acoustic cycles. This rapid movement can account for the blurred streaks that appears to originate from the antinodes in the photographs for 168 kHz. The development of the bubble structure is rather different for higher frequencies. For 448 kHz, bubbles appear along the vertical axis of the propagating acoustic wave in a standing wave pattern stretching from the liquid surface down toward the transducer. At time 0.12 s bubbles at the bottom of the vessel appear to experience an upward force causing them to stream toward the liquid surface. The profile of this force is made apparent by the contour of the bubbles advancing toward the liquid surface observed at 0.13 s and 0.16 s and disrupts the standing wave structure. The profile suggests that the force is weak at the centre, resulting in the two leading bubble fronts indicated by the arrows. In addition, the force appears to be slightly focused towards the central axis. The images also show that with time, the bubbles at the liquid surface spreads out toward the vessel wall and occasionally a burst of bubbles appears near the bottom of the vessel but is then forced towards the liquid surface. For 726 kHz, visible bubbles first emerge near the transducer and increases in number and size with time. Contrary to the other two lower frequencies where the emergence of the bubble structure was observed randomly along the path of the acoustic wave, for 726 kHz the emergence of bubbles were limited to near the base of the transducer. The development of a force is observed at 0.16 s and with time the bubbles are propelled towards the liquid surface, similar to that observed at 448 kHz. The profile of the force is also made apparent by the outline of the bubbles and similar to that observed at 448 kHz, the spatial distribution of the bubble structure shrinks towards the surface and spreads toward the vessel wall. 3.2.2. Importance of attenuation by large non-resonance bubbles In this study, the observed streaming of bubbles toward the liquid surface is believed to be mainly due to the attenuation of the acoustic energy [40]. This attenuation results in the development of an energy gradient or force and causes the fluid to move in the direction of the propagating acoustic wave. This fluid movement is commonly known as acoustic streaming [37] and is a nonlinear phenomenon that is dependent on both frequency and nonlinear property of the fluid itself. Higher frequencies attenuate more than lower frequencies and therefore most reports in the literature on acoustic streaming are in the MHz range [41–44]. Increasing nonlinearity of fluids, such as increasing fluid viscosity, have also been shown to enhance acoustic streaming [41,43]. The size of bubbles or void fraction can also increase the nonlinearity
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Fig. 4. The effect of 1 mM SDS and 10 mM SDS on the formation of visible bubbles at frequencies 168, 448 and 726 kHz, with a driving power of 20 W.
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Fig. 5. Void generation rate at 168, 448 and 726 kHz for water, 1 mM SDS and 10 mM SDS. A driving power of 20 W was used for all systems.
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of the fluid [45]. It has been shown that for water the ratio of B/A, a parameter used to evaluate the nonlinearity of fluids, can reach a value of about 104 near a void fraction of 10 4 compare to a B/A value of 5 for a void fraction less than 10 8 [45]. This drastic increase in the nonlinearity of fluids with an increase in the void fraction is largely due to the distortion and attenuation of the acoustic wave by large bubbles. Bubbles at resonance are usually considered to have the greatest attenuation of the acoustic wave [46–48]. However, these studies are usually performed at frequencies of a few Hz and have a wavelength much greater than the diameter of bubbles. For acoustic frequencies in the hundreds of kHz or MHz, coalescence can lead to the formation of large bubbles in the order of hundreds of microns [34]. These large bubbles are non-resonating and have shown to distort acoustic waves [49] and scatter high-frequency wave fields strongly [50]. It has been shown that these non-resonance bubbles scatter significantly more than the resonating bubbles by virtue of their size rather than via large amplitude pulsations [50,51]. Therefore, the streaming of the bubbles observed as a function of time in Fig. 3 can be attributed to the fluid movement brought about by the increase in the attenuation of the acoustic energy as the population of large bubbles rises with time. The size and population of the large non-resonance bubbles as a function of time can be quantified by the void generation rate. The attenuation of the acoustic wave can also be measured. In order to further substantiate the effect of the large non-resonance bubbles, a surfactant was added to inhibit bubble coalescence and decrease the population of large non-resonance bubbles. The ability of the surfactant sodium dodecylsulfate (SDS) at inhibiting bubble coalescence in the presence of an acoustic wave has been reported [25,31]. Fig. 4 shows the effect of 1 mM and 10 mM SDS on the formation of large visible bubbles. It can be seen that there are no visible bubbles under all three frequencies for 1 mM SDS. For 10 mM SDS, visible bubbles are present but to a lesser extent compare to the case of water shown in Fig. 2(i). The increase in the size of bubbles at high SDS concentrations is due to the dissociated SDS monomers acting as excess electrolyte which in turn lowers the electrostatic repulsion barrier for coalescence to occur. The results for the void rate and percentage of attenuation under different frequencies and SDS concentrations are shown in Figs. 5 and 6, respectively. The strong correlation between the two figures and also the effect of decreasing bubble coalescence by the addition of SDS further demonstrates that large non-resonance bubbles can significantly attenuate the acoustic wave. The lack of streaming observed for 168 kHz is because there is very little attenuation of the acoustic wave. Attenuation increases dramatically at 448 kHz and 726 kHz despite an increase of only 2–3 times the void generation rate at 168 kHz. Similarly, 10 mM SDS at 448 kHz exhibited stronger streaming of bubbles compared to water at 168 kHz despite having a lower void generation rate. This is because higher frequencies attenuate to a greater extent, in the order of square of the frequency [37]. Dahnke et al. [52] have demonstrated theoretically that at low frequencies, a large void fraction
50 40 30 20 10 0 Saturated Water
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Fig. 6. Percentage of attenuation in saturated water, 1 mM SDS and 10 mM SDS at 168, 448 and 726 kHz. A driving power of 20 W was used for all systems.
of 10 2 is required to significantly decrease the acoustic pressure distribution. The streaming of the bubbles observed may also be attributed to acoustic streaming, a nonlinear acoustic phenomena. However, it was pointed out by Mitome et al. [53] that acoustic radiation pressure can act on bubbles and induce fluid movements which differ from those brought about by acoustic streaming. This acoustic radiation pressure can drive bubbles under linear conditions [37]. In a standing wave field, the radiation force drives bubbles below the resonance size to the antinodes and in a traveling wave field, the radiation force drives bubbles at resonance in the direction of the propagating wave. It is the radiation force from a traveling wave field that can cause the bubbles to stream toward the liquid surface. In a real system, due to the attenuation and reflectivity at the boundary, the wave field can be partially standing and partially traveling. Therefore, in order to distinguish whether acoustic streaming or radiation force from the traveling wave is the driving force behind the streaming of bubbles toward the liquid surface, we have altered the proportion of the standing and traveling wave in the acoustic wave field. The proportion of these wave fields can be manipulated by altering the reflectivity of the boundary normal to the incident acoustic wave. Leighton et al. [28] and Tuziuti et al. [35] reported an increase in the standing wave field by increasing the reflectivity of the boundary. This is demonstrated in Fig. 2 for 448 kHz with and without a sheet of silicone foam. These two images demonstrate the increase in the spatial distribution of the standing wave pattern when the liquid surface is stabilized with a sheet of silicone foam. The development of the bubble structure with the liquid surface stabilized by a sheet of silicone foam for 448 kHz and 726 kHz is shown in Fig. 7. For 448 kHz, the development of the bubble structure in Fig. 7 did
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Air/liquid surface stabilized with silicone foam
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Fig. 7. The effect of stabilizing the surface fluctuations by a sheet of silicone foam on the development of the bubble structure in saturated water as a function of time for 448 kHz and 726 kHz. Time in seconds is denoted above the images.
not display the strong streaming of bubbles as that observed in Fig. 3 where the liquid surface was not stabilized. This suggests that the wave field in Fig. 3 is mostly traveling wave and it is the radiation force from the traveling wave field that is driving the bubbles toward the liquid surface. For 726 kHz, increasing the standing wave proportion appears to have weakened the streaming of the bubbles, but there is sufficient streaming to cause the bubble structure to be isolated to near the liquid surface. The fact that significant streaming of bubbles is still present despite reducing the proportion of traveling wave field implies that the streaming observed at 726 kHz is probably due to acoustic streaming effects. 3.2.3. Power effect The effect of driving acoustic power on the spatial structure of bubbles is shown in Fig. 8. For 168 kHz, no strong streaming of the bubbles toward the liquid surface is observed across all driving powers investigated. For higher frequencies, as the driving power increases there exists a driving power at which streaming of the bubbles toward the liquid surface is observed. Increasing driving power can increase the population of large bubbles and increase nonlinearity and attenuation effects. The lack of streaming of the bubbles at 168 kHz is another demonstration that lower frequencies attenuate to a lesser extent. However, the onset of the streaming of bubbles occurs at a lower acoustic power for 448 kHz than
for 726 kHz. As discussed previously, the main force responsible for the streaming of bubbles for 448 kHz and 726 kHz is different. It is possible that the cause of this difference in the onset of the streaming of bubbles is the extent at which the driving power affects the radiation force from the traveling wave field and acoustic streaming.
3.2.4. Pulsed sonication Pulsed sonication have shown to effect the SL intensity [11], sonochemical efficiency [13,20,54] as well as the spatial distribution of active bubbles [25] when compared to a continuous system. In order to explore the effect of pulsing on bubble structure development, further experiments were carried out under pulsed sonication conditions at 448 kHz. Shown in Fig. 9 are a series of still photographs illustrating the development of the bubble structure under the pulse condition of 4000 cycles ON and 20,000 cycles OFF. The development of the bubble field under pulsed sonication is slower compare to the continuous case and allows the development of the bubble field to be fully captured. At 0.83 s visible bubbles begin to emerge and with time, a standing wave pattern forms throughout the liquid. At 6.67 s it can be seen that a force is present and the standing wave pattern is disrupted, and bubbles are forced toward the liquid surface. The commencement of this streaming occured later compared to that of the continuous case which was less than 1 s. This is because the growth in the size of bubbles via coalescence or rectified diffusion occurs during sonication and by pulsing, this growth is reduced and thus lessens the formation of large bubbles [34]. Furthermore, dissolution of bubbles would occur during the pulse OFF duration and decrease the size of the bubbles [20]. This decrease in the coalescence and size of the bubbles would reduce the degree in which the acoustic pressure is attenuated. The effect of increasing pulse ON or OFF duration is shown graphically in Fig. 10 with the vertical axis as increasing pulse OFF duration and the horizontal axis as increasing pulse ON duration. It can be seen that for a given pulse OFF duration, as the pulse ON duration increases the development of the bubble structure resembles the development observed as a function of time. Similar development in the bubble structure is observed with increasing pulse OFF duration for a given fixed pulse ON duration. As discussed previously, for 448 kHz the force causing the translational movements of bubbles toward the liquid surface is believed to be the radiation force from the traveling wave field. With increasing pulse ON duration or decreasing pulse OFF duration, the formation of large coalesced bubbles increases. This results in the increase in the traveling wave proportion and hence the rise in the radiation force. It is possible to categorize Fig. 10 into three regions according to SL images reported in the literature [25,34]. Region A is where SL is isolated to the liquid surface, region B is where the SL is
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Fig. 8. Equilibrium bubble structure in saturated water for 168, 448 and 726 kHz as a function of power.
Fig. 9. Initial bubble field development in saturated water for a pulse setting of 4000 cycles ON and 20,000 cycles OFF at 448 kHz and a driving power of 20 W.
J. Lee et al. / Ultrasonics Sonochemistry 18 (2011) 92–98
Increasing Pulse Off Cycle
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dominately caused by acoustic streaming. It is shown that there is an optimum pulse ON and OFF ratio at which the disruption of the standing wave field can be minimized.
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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 number 1907765). The authors would like to thank Les Gamel for the capillary cell. MA acknowledges the award of AIST Visiting Fellowship.
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Fig. 10. Visible bubble structure for different pulse ON and OFF cycles captured in saturated water at 448 kHz and a driving power of 20 W. Three regions have been identified according to the SL images reported in the literature. Region A is where SL is found isolated to the liquid surface. Region B is where SL is homogeneously distributed. Region C is where no SL activity was detected.
homogeneously distributed in the vessel and region C is where no SL activity is detected. Comparing the SL images to that of the visible bubbles, the bubble structure agrees well with the reported SL structures [25,34] provided that visible bubbles are observed. Fig. 10 demonstrates that there exists an optimum range of pulse ON and OFF ratio for strong standing wave field and homogeneous distribution of active bubbles, which has been shown to translate to a higher sonochemical efficiency [9,55]. If the pulse ON/OFF ratio is in region A, the pulse ON is sufficiently long or the pulse OFF is considerably short such that the system effectively behaves like a continuous system. If the pulse ON/OFF ratio is in region C, the pulse ON duration is inadequate for the generation of detectable bubbles. At the boundary between C and B where there is very few visible bubble, it is not possible to determine the structure of SL bubbles based on optical photographs. 4. Conclusion This study demonstrates that there is a resemblance between the structure of the SL bubbles at the antinodes and that of the coalesced visible bubbles at the nodes. This association between the visible and SL bubbles made it possible to indirectly infer the development of active cavitation bubbles by monitoring the development of large coalesced bubbles. It is thus concluded that at the initial stages of sonication, the wave field is predominately of a strong standing wave and the bubbles are trapped at the antinode planes extending from the liquid surface down to the transducer. As the growth in the population and size of bubbles progresses with increasing sonication time, attenuation of the acoustic pressure increases. This subsequently led to the development of acoustic streaming and radiation force from the traveling wave field which disrupts the standing wave field, and drives bubbles to the liquid surface. The disruptive forces were weaker for 168 kHz compared to 448 kHz and 726 kHz. This was found to be due to lower void volume generated and that higher frequencies attenuate to a greater extent. It is demonstrated in this study that at 448 kHz, the streaming of the bubbles observed is predominately due to the increase in the traveling wave proportion and weakening of the standing wave field. At 726 kHz, the streaming of bubbles is pre-
[1] C. Petrier, Y. Jian, M.-F. Lamy, Environ. Sci. Technol. 32 (1998) 1316–1318. [2] T.J. Mason, Phil. Trans. R. Soc. Lond. A 357 (1999) 355–369. [3] K. Okitsu, A. Yue, S. Tanabe, H. Matsumoto, Y. Yobiko, Langmuir 17 (2001) 7717–7720. [4] S. Muthukumaran, K. Yang, A. Seuren, S. Kentish, M. Ashokkumar, G.M. Stevens, Sep. Purif. Technol. 39 (2004) 99–107. [5] S.E. Kentish, T. Wooster, M. Ashokkumar, S. Balachandran, R. Mawson, L. Simons, Innov. Food Sci. Emerging Technol. 9 (2008) 170–175. [6] P.R. Birkin, J.F. Power, A.M.L. Vincotte, T.G. Leighton, Phys. Chem. Chem. Phys. 5 (2003) 4170–4174. [7] B. Nanzai, K. Okitsu, N. Takenaka, H. Bandow, N. Tajima, Y. Maeda, Ultrason. Sonochem. 16 (2009) 163–168. [8] S. Koda, T. Kimura, T. Kondo, H. Mitome, Ultrason. Sonochem. 10 (2003) 149– 156. [9] Y. Asakura, T. Nishida, T. Matsuoka, S. Koda, Ultrason. Sonochem. 15 (2008) 244–250. [10] D.J. Casadonte Jr., M. Flores, C. Petrier, Ultrason. Sonochem. 12 (2005) 147– 152. [11] N.V. Dezhkunov, A. Francescutto, P. Ciuti, T.J. Mason, G. Lernetti, A.I. Kulak, Ultrason. Sonochem. 7 (2000) 19–24. [12] S. Muthukumaran, S. Kentish, S. Lalchandani, M. Ashokkumar, R. Mawson, G.M. Stevens, F. Grieser, Ultrason. Sonochem. 12 (2005) 29–35. [13] T. Tuziuti, K. Yasui, J. Lee, T. Kozuka, A. Towata, Y. Iida, J. Phys. Chem. A 112 (2008) 4875–4878. [14] N. Segebarth, O. Eulaerts, J. Reisse, L.A. Crum, T.J. Matula, J. Phys. Chem. B 106 (2002) 9181–9190. [15] M. Ashokkumar, R. Hall, P. Mulvaney, F. Grieser, J. Phys. Chem. B 101 (1997) 10845–10850. [16] G.J. Price, M. Ashokkumar, F. Grieser, J. Am. Chem. Soc. 126 (2004) 2755–2762. [17] R. Tronson, Ph.D, The University of Melbourne, 2004. [18] D. Sunartio, M. Ashokkumar, F. Grieser, J. Phys. Chem. B 109 (2005) 20044– 20050. [19] S. Hatanaka, K. Yasui, T. Tuziuti, T. Kozuka, H. Mitome, Jpn. J. Appl. Phys. 40 (2001) 3856–3860. [20] A. Henglein, R. Ulrich, J. Lilie, J. Am. Chem. Soc. 111 (1989) 1974–1979. [21] P.R. Birkin, T.G. Leighton, J.F. Power, M.D. Simpson, A.M.L. Vincotte, P.F. Joseph, J. Phys. Chem. A 107 (2003) 306–320. [22] S. Hatanaka, K. Yasui, T. Tuziuti, H. Mitome, Jpn. J. Appl. Phys. 39 (2000) 2962– 2966. [23] T. Tuziuti, K. Yasui, Y. Iida, Ultrason. Sonochem. 12 (2005) 73–77. [24] D. Sunartio, K. Yasui, T. Tuziuti, T. Kozuka, Y. Iida, M. Ashokkumar, F. Grieser, Chem. Phys. Chem. 8 (2007) 2331–2335. [25] J. Lee, K. Yasui, T. Tuziuti, T. Kozuka, A. Towata, Y. Iida, J. Phys. Chem. B 112 (2008) 15333–15341. [26] V. Renaudin, N. Gondrexon, P. Boldo, C. Petrier, A. Bernis, Y. Gonthier, Ultrason. Sonochem. 1 (1994) s81–s85. [27] F. Trabelsi, H. Ait-lyazidi, J. Berlan, P.-L. Fabre, H. Delmas, A.M. Wilhelm, Ultrason. Sonochem. 3 (1996) s125–s130. [28] T.G. Leighton, M.J.W. Pickworth, A.J. Walton, P.P. Dendy, Phys. Med. Biol. 33 (1988) 1239–1248. [29] U. Parlitz, R. Mettin, S. Luther, I. Akhatov, M. Voss, W. Lauterborn, Phil. Trans. R. Soc. Lond. A 357 (1999) 313–334. [30] R. Mettin, S. Luther, C.D. Ohl, W. Lauterborn, Ultrason. Sonochem. 6 (1999) 25– 29. [31] J. Lee, S.E. Kentish, M. Ashokkumar, J. Phys. Chem. B 109 (2005) 5095–5099. [32] K. Yasui, J. Acoust. Soc. Am. 112 (2002) 1405–1413. [33] A. Brotchie, F. Grieser, M. Ashokkumar, Phys. Rev. Lett. 102 (2009) 084302. [34] Y. Iida, M. Ashokkumar, T. Tuziuti, T. Kozuka, K. Yasui, A. Towata, J. Lee, Ultrason. Sonochem. 17 (2010) 473–479. [35] T. Tuziuti, K. Yasui, T. Kozuka, A. Towata, Y. Iida, J. Phys. Chem. A 111 (2007) 12093–12098. [36] M. Ashokkumar, J. Lee, Y. Iida, K. Yasui, T. Kozuka, T. Tuziuti, A. Towata, Phys. Chem. Chem. Phys. 11 (2009) 10118–10121. [37] T.G. Leighton, The Acoustic Bubble, Academic Press Limited, Cambridge, 1994. [38] F.R. Young, Cavitation, Imperial College Press, London, 1999. [39] R. Mettin, A.A. Doinikov, Appl. Acoust. 70 (2009) 1330–1339.
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Contents lists available at ScienceDirect
Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultsonch
Development and optimization of acoustic bubble structures at high frequencies Judy Lee a,*, Muthupandian Ashokkumar b, Kyuichi Yasui a, Toru Tuziuti a, Teruyuki Kozuka a, Atsuya Towata a, Yasuo Iida a a b
National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama ku, Nagoya 463-8560, Japan School of Chemistry, University of Melbourne, VIC 3010, Australia
a r t i c l e
i n f o
Article history: Received 24 April 2009 Received in revised form 31 January 2010 Accepted 11 March 2010 Available online 17 March 2010 Keywords: Ultrasound frequency Acoustic bubble structure Radiation forces Attenuation Bubble coalescence Surfactants
a b s t r a c t At high ultrasound frequencies, active bubble structures are difficult to capture due to the decrease in timescale per acoustic cycle and size of bubbles with increasing frequencies. However the current study demonstrates an association between the spatial distribution of visible bubbles and that of the active bubble structure established in the path of the propagating acoustic wave. By monitoring the occurrence of these visible bubbles, the development of active bubbles can be inferred for high frequencies. A series of still images depicting the formation of visible bubble structures suggest that a strong standing wave field exists at early stages of wave propagation and weakens by the increase in the attenuation of the acoustic wave, caused by the formation of large coalesced bubbles. This attenuation is clearly demonstrated by the occurrence of a force which causes bubbles to be driven toward the liquid surface and limit standing wave fields to near the surface. This force is explained in terms of the acoustic streaming and traveling wave force. It is found that a strong standing wave field is established at 168 kHz. At 448 kHz, large coalesced bubbles can significantly attenuate the acoustic pressure amplitude and weaken the standing wave field. When the frequency is increased to 726 kHz, acoustic streaming becomes significant and is the dominant force behind the disruption of the standing wave structure. The disruption of the standing wave structure can be minimized under certain pulse ON and OFF ratios. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Acoustic cavitation is a unique phenomenon where extreme pressures and temperatures are generated upon the collapse of micron size bubbles. These conditions induce physical and chemical effects that are beneficial for a number of industrial applications [1–5]. However, due to the chaotic nature of acoustic cavitation, the control and optimization of the cavitation systems are difficult and have been the subject of a number of studies [6–13]. There are numerous reports in the literature examining the enhancement in integrated sonoluminescence (SL) intensities by the addition of a surface active solute [14,15] and various sonication conditions such as frequency [16,17], power [18,19], pulse repetition frequency [13,20], reactor vessel geometry [8] and liquid height [9,21]. Literature reports [9,22–27] on the spatial distribution of active bubbles and sonochemiluminescence (SCL) structures have demonstrated the importance of acquiring a homogeneous spatial distribution of active bubbles for the enhancement in the integrated SL or SCL intensities. It is further demonstrated that a homogeneous distribution of active bubbles is obtained * Corresponding author. Tel.: +81 52 736 7215; fax: +81 52 736 7405. E-mail address: [email protected] (J. Lee). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2010.03.004
when a strong standing wave field is established in the system [25,28]. However, these SL and SCL structures reported to date are equilibrium structures observed when steady-states were reached under specific experimental conditions. An understanding into the development of the SL and SCL structures is critical for a better control and employment of ultrasound for industrial applications, but is difficult to capture as a function of time due to the short time scales (milliseconds) and a relatively low number of active bubbles. In the early stages of cavitation, fast photography have revealed that bubbles emerge from some point source and streams toward the antinodes to form a dense cluster and filamentary like structures [19,29,30]. However, these investigations were restricted to low frequencies for the reason that the bubble size and wavelength decrease with increasing frequency, making direct imaging difficult without special high speed imaging systems. The data presented in this manuscript will demonstrate the resemblance between the visible bubble structure and that of the SL bubble structure under equilibrium (steady-state) conditions at high frequencies. Using this association, the development of active bubble structures is indirectly inferred from the development of visible bubble structures at 168, 448 and 726 kHz. This manuscript will further show how acoustic streaming and weakening
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of the standing wave field can cause the spatial distribution of bubbles to become inhomogeneous and how this can be minimized under appropriate pulse ON and OFF ratio.
Transducer 1 (V1)
Transducer 2 Receiver
Transducer 1 (V2)
Transducer 2 Receiver
2. Experimental method 2.1. Solutions and sonication conditions Sodium dodecylsulfate (SDS) was purchased from Sigma–Aldrich, special purity grade and were used without further purification. The SDS solutions were made by diluting an appropriate volume of 100 mM stock solution with distilled water that has been saturated with air. Three 5 cm in diameter piezo-electric transducers (Kaijo Sonic Corp.) at resonance frequencies of 168, 448 and 726 kHz were used. Signals from the function generator (NF Wavefactory, model 1946A) were amplified by a power amplifier (NF High Speed bipolar Amplifier, HSA4014). The power output to the transducer was measured using a Megasonic power meter (Towa Electronic, model TDW 6102U). For all experiments a power output of 20 W (1.1 W/ cm2), unless stated otherwise, was used. Degassed water was obtained using a membrane degassing unit (Membrana, MiniModule). The level of dissolved air content was determined by a dissolved oxygen meter (Horiba, model D-25). 2.2. Bubble structure images The experimental configuration used was the same as that described in Ref. [25] where a glass vessel which holds 1 L of solution was radiated with ultrasound emitted from the transducer fitted at the bottom. The initial development of bubble structures were captured using a high speed digital compact camera (Casio, Exilim EXFH20) at a rate of 210 frames per second (fps). This corresponds to an exposure time of 5 ms per frame. For equilibrium bubble structures, a slower rate of 30 fps (exposure time 33 ms) was used. In both cases, the sonicated solution was illuminated with a light source from the side of the vessel. 2.3. Void generation rate The void generation rate was measured using a capillary technique. In this technique the void volume generated by sonication as a function of time was measured. This void volume measured was assumed to be equivalent to the total volume of large coalesced bubbles generated for a given sonication time. Further details of this method can be found else where [31]. An approximate volume of 280 mL of solution was used to fill the capillary cell and the void volume as a function of time was recorded. This void volume was found to increase linearly with time and the void generation rate was obtained from the gradient. 2.4. Percentage of attenuation The configuration used to measure the attenuation of the acoustic wave by cavitation bubbles is shown in Fig. 1. The amplitude of the signal sent to transducer 1 from the function generator was modulated between two voltage outputs, V1 and V2. The amplitude of V1 was set to equivalent of 20 W (1.1 W/cm2) driving power and V2 was set to 10% of V1, below the cavitation threshold. A 5 MHz transducer (transducer 2) acted as a passive detector of the acoustic wave from V1 and V2, and the signals were collected by an oscilloscope. The percentage of attenuation was taken as the percentage of decrease in the amplitude of V2 by bubbles created by V1, relative to V2 when the water was degassed to a concentration of 1.0 mg/L where there were no cavitation bubbles present.
Fig. 1. Schematic of the system used to measure attenuation of acoustic waves by bubbles. Transducer 1 is modulated between two output voltages, V1 and V2. The voltage for V1 is equivalent to 20 W driving power, sufficient to create cavitation bubbles. V2 is set at 10% of V1, below the cavitation threshold, and is attenuated by the bubbles created by V1. The acoustic signals is received by transducer 2 and collected by an oscilloscope.
3. Results and discussion 3.1. Comparison between visible bubble structures and invisible SL bubbles structures Shown in Fig. 2(i) are the equilibrium (steady-state) spatial distribution of bubbles generated at frequencies 168, 448 and 726 kHz. The bubbles seen in these images are too large to be sonoluminescing (SL) bubbles, which have been shown theoretically [32] and experimentally [33] to be less than 10 microns in diameter at these frequencies. These large visible bubbles are bubbles that have been expelled from the pressure antinodes and become trapped at adjacent pressure nodes. This expulsion of the bubbles occurs when the bubble size becomes larger than the active size. The increase in the bubble size occurs predominately from the coalescence of sub-resonance size bubbles at the antinodes by the actions of primary and secondary Bjerknes forces. Light scattering measurements monitoring the development of size distribution of bubbles as a function of time have demonstrated this increase in the size and also number of bubbles [34]. Therefore, the spatial distribution of these visible bubbles would constitute an approximate indication of the spatial distribution of active bubbles that are otherwise invisible to the naked eye. The SL images taken by a CCD camera with an exposure time of 30 s are shown in Fig. 2(ii). Comparing the SL images with that of the visible bubbles in Fig. 2(i), a correlation between the structures of two different size of bubbles can be seen along the path of the propagating acoustic wave for all three frequencies.
168 kHz
448 kHz
726 kHz
448 kHz
(i)
(ii)
Free air/liquid surface
Air/liquid surface stabilized with silicone foam
Fig. 2. (i) Visible bubble structure and (ii) SL bubble structure as a function of frequency for saturated water and driving power of 20 W. For 448 kHz, the effect of stabilizing the surface fluctuations with a sheet of silicone foam is shown. Exposure times for bubble structure and SL images were 33 ms and 30 s, respectively.
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For 168 kHz the structure of the visible bubbles are gathered at several horizontal planes orthogonal to the propagating acoustic wave in a standing wave pattern. The density of the visible bubbles is higher near the liquid surface than toward the transducer at the base of the vessel. The SL image for 168 kHz shows a similar standing wave pattern with a strong SL intensity near the liquid surface and a weak SL intensity near the transducer. For higher frequencies, the visible bubbles are localized to near the liquid surface and this is reflected in the SL bubble structure. A reduction in the surface fluctuations or altering the reflectivity at the boundary normal to the incident wave have shown to increase the standing wave field [28,35,36]. Therefore, to further demonstrate the relationship between the spatial distribution of visible bubbles and that of the SL bubbles, a sheet of silicone foam was floated on the liquid to stablilize the liquid surface and increase reflectivity. The results are depicted in Fig. 2 and it shows that the standing wave pattern for both the visible and SL bubbles were increased by the addition of the silicon foam. Although the spatial distribution of the visible bubbles show close resemblance to the structure of SL bubbles, this is only limited to the paths of the acoustic beam, which has a cross-sectional diameter of 5 cm normal to the direction of wave propagation. In addition to this, if coalescence is hindered or inhibited, active bubbles may exist in the system without any visible bubbles. Under this condition, SL images are needed to determine the nature of the SL structure. Nevertheless, provided that coalescence is not hindered and sub-resonance bubbles eventually reach a size that becomes detectable via coalescence, it is possible to monitor the occurrence of these visible bubbles to indirectly deduce the development of active bubbles. 3.2. Development of visible bubble structure 3.2.1. Continuous sonication Shown in Fig. 3 are a series of selected frames documenting the initial development of the bubble structure at 168, 448 and 726 kHz. The colour intensity has been inverted to improve the
168 kHz
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Fig. 3. Development of the bubble structure in saturated water as a function of time for different frequencies. Time in seconds is denoted above the images.
visibility of the bubbles, which appears dark against the light background. The random appearance of visible bubbles at earlier times of 0.24 s and 0.71 s for 168 kHz reflected the general concept that it is the random pre-existing bubble nuclei that act as weak points in the liquid which initiates cavitation [37,38]. As discussed earlier, these visible bubbles are bubbles that have been expelled from nearby antinodes due to predominately the coalescence of active bubbles by Bjerknes forces. With time, as the population and size of the bubbles increases, the standing wave structure becomes more defined and extends across the entire vessel. What is not depicted in these still photographs is the rapid translational motion of bubbles from the antinodal plane to the next nodal plane. This translational motion observed is usually in an upward direction. With the slower frame rate of 30 fps, used for the images shown in Fig. 2(i), these motions are seen as faint streaks between the nodes for 168 kHz. A recent report by Mettin et al. [39] simulated the translational motion of different bubbles which can exist in a high-frequency standing wave. In their study, the initial translational motion of bubbles, starting from the antinode toward the next nodal or some intermediate position, is shown to be only a few hundred acoustic cycles. This rapid movement can account for the blurred streaks that appears to originate from the antinodes in the photographs for 168 kHz. The development of the bubble structure is rather different for higher frequencies. For 448 kHz, bubbles appear along the vertical axis of the propagating acoustic wave in a standing wave pattern stretching from the liquid surface down toward the transducer. At time 0.12 s bubbles at the bottom of the vessel appear to experience an upward force causing them to stream toward the liquid surface. The profile of this force is made apparent by the contour of the bubbles advancing toward the liquid surface observed at 0.13 s and 0.16 s and disrupts the standing wave structure. The profile suggests that the force is weak at the centre, resulting in the two leading bubble fronts indicated by the arrows. In addition, the force appears to be slightly focused towards the central axis. The images also show that with time, the bubbles at the liquid surface spreads out toward the vessel wall and occasionally a burst of bubbles appears near the bottom of the vessel but is then forced towards the liquid surface. For 726 kHz, visible bubbles first emerge near the transducer and increases in number and size with time. Contrary to the other two lower frequencies where the emergence of the bubble structure was observed randomly along the path of the acoustic wave, for 726 kHz the emergence of bubbles were limited to near the base of the transducer. The development of a force is observed at 0.16 s and with time the bubbles are propelled towards the liquid surface, similar to that observed at 448 kHz. The profile of the force is also made apparent by the outline of the bubbles and similar to that observed at 448 kHz, the spatial distribution of the bubble structure shrinks towards the surface and spreads toward the vessel wall. 3.2.2. Importance of attenuation by large non-resonance bubbles In this study, the observed streaming of bubbles toward the liquid surface is believed to be mainly due to the attenuation of the acoustic energy [40]. This attenuation results in the development of an energy gradient or force and causes the fluid to move in the direction of the propagating acoustic wave. This fluid movement is commonly known as acoustic streaming [37] and is a nonlinear phenomenon that is dependent on both frequency and nonlinear property of the fluid itself. Higher frequencies attenuate more than lower frequencies and therefore most reports in the literature on acoustic streaming are in the MHz range [41–44]. Increasing nonlinearity of fluids, such as increasing fluid viscosity, have also been shown to enhance acoustic streaming [41,43]. The size of bubbles or void fraction can also increase the nonlinearity
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168 kHz
448 kHz
726 kHz
1 mM SDS
10 mM SDS
Fig. 4. The effect of 1 mM SDS and 10 mM SDS on the formation of visible bubbles at frequencies 168, 448 and 726 kHz, with a driving power of 20 W.
9
168 kHz
448 kHz
726 kHz
8 VoidRate [ µL/s]
7 6 5 4 3 2 1 0 Water
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Fig. 5. Void generation rate at 168, 448 and 726 kHz for water, 1 mM SDS and 10 mM SDS. A driving power of 20 W was used for all systems.
70
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60 Percentage Attenuation
of the fluid [45]. It has been shown that for water the ratio of B/A, a parameter used to evaluate the nonlinearity of fluids, can reach a value of about 104 near a void fraction of 10 4 compare to a B/A value of 5 for a void fraction less than 10 8 [45]. This drastic increase in the nonlinearity of fluids with an increase in the void fraction is largely due to the distortion and attenuation of the acoustic wave by large bubbles. Bubbles at resonance are usually considered to have the greatest attenuation of the acoustic wave [46–48]. However, these studies are usually performed at frequencies of a few Hz and have a wavelength much greater than the diameter of bubbles. For acoustic frequencies in the hundreds of kHz or MHz, coalescence can lead to the formation of large bubbles in the order of hundreds of microns [34]. These large bubbles are non-resonating and have shown to distort acoustic waves [49] and scatter high-frequency wave fields strongly [50]. It has been shown that these non-resonance bubbles scatter significantly more than the resonating bubbles by virtue of their size rather than via large amplitude pulsations [50,51]. Therefore, the streaming of the bubbles observed as a function of time in Fig. 3 can be attributed to the fluid movement brought about by the increase in the attenuation of the acoustic energy as the population of large bubbles rises with time. The size and population of the large non-resonance bubbles as a function of time can be quantified by the void generation rate. The attenuation of the acoustic wave can also be measured. In order to further substantiate the effect of the large non-resonance bubbles, a surfactant was added to inhibit bubble coalescence and decrease the population of large non-resonance bubbles. The ability of the surfactant sodium dodecylsulfate (SDS) at inhibiting bubble coalescence in the presence of an acoustic wave has been reported [25,31]. Fig. 4 shows the effect of 1 mM and 10 mM SDS on the formation of large visible bubbles. It can be seen that there are no visible bubbles under all three frequencies for 1 mM SDS. For 10 mM SDS, visible bubbles are present but to a lesser extent compare to the case of water shown in Fig. 2(i). The increase in the size of bubbles at high SDS concentrations is due to the dissociated SDS monomers acting as excess electrolyte which in turn lowers the electrostatic repulsion barrier for coalescence to occur. The results for the void rate and percentage of attenuation under different frequencies and SDS concentrations are shown in Figs. 5 and 6, respectively. The strong correlation between the two figures and also the effect of decreasing bubble coalescence by the addition of SDS further demonstrates that large non-resonance bubbles can significantly attenuate the acoustic wave. The lack of streaming observed for 168 kHz is because there is very little attenuation of the acoustic wave. Attenuation increases dramatically at 448 kHz and 726 kHz despite an increase of only 2–3 times the void generation rate at 168 kHz. Similarly, 10 mM SDS at 448 kHz exhibited stronger streaming of bubbles compared to water at 168 kHz despite having a lower void generation rate. This is because higher frequencies attenuate to a greater extent, in the order of square of the frequency [37]. Dahnke et al. [52] have demonstrated theoretically that at low frequencies, a large void fraction
50 40 30 20 10 0 Saturated Water
1 mM SDS
10 mM SDS
Fig. 6. Percentage of attenuation in saturated water, 1 mM SDS and 10 mM SDS at 168, 448 and 726 kHz. A driving power of 20 W was used for all systems.
of 10 2 is required to significantly decrease the acoustic pressure distribution. The streaming of the bubbles observed may also be attributed to acoustic streaming, a nonlinear acoustic phenomena. However, it was pointed out by Mitome et al. [53] that acoustic radiation pressure can act on bubbles and induce fluid movements which differ from those brought about by acoustic streaming. This acoustic radiation pressure can drive bubbles under linear conditions [37]. In a standing wave field, the radiation force drives bubbles below the resonance size to the antinodes and in a traveling wave field, the radiation force drives bubbles at resonance in the direction of the propagating wave. It is the radiation force from a traveling wave field that can cause the bubbles to stream toward the liquid surface. In a real system, due to the attenuation and reflectivity at the boundary, the wave field can be partially standing and partially traveling. Therefore, in order to distinguish whether acoustic streaming or radiation force from the traveling wave is the driving force behind the streaming of bubbles toward the liquid surface, we have altered the proportion of the standing and traveling wave in the acoustic wave field. The proportion of these wave fields can be manipulated by altering the reflectivity of the boundary normal to the incident acoustic wave. Leighton et al. [28] and Tuziuti et al. [35] reported an increase in the standing wave field by increasing the reflectivity of the boundary. This is demonstrated in Fig. 2 for 448 kHz with and without a sheet of silicone foam. These two images demonstrate the increase in the spatial distribution of the standing wave pattern when the liquid surface is stabilized with a sheet of silicone foam. The development of the bubble structure with the liquid surface stabilized by a sheet of silicone foam for 448 kHz and 726 kHz is shown in Fig. 7. For 448 kHz, the development of the bubble structure in Fig. 7 did
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Air/liquid surface stabilized with silicone foam
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Fig. 7. The effect of stabilizing the surface fluctuations by a sheet of silicone foam on the development of the bubble structure in saturated water as a function of time for 448 kHz and 726 kHz. Time in seconds is denoted above the images.
not display the strong streaming of bubbles as that observed in Fig. 3 where the liquid surface was not stabilized. This suggests that the wave field in Fig. 3 is mostly traveling wave and it is the radiation force from the traveling wave field that is driving the bubbles toward the liquid surface. For 726 kHz, increasing the standing wave proportion appears to have weakened the streaming of the bubbles, but there is sufficient streaming to cause the bubble structure to be isolated to near the liquid surface. The fact that significant streaming of bubbles is still present despite reducing the proportion of traveling wave field implies that the streaming observed at 726 kHz is probably due to acoustic streaming effects. 3.2.3. Power effect The effect of driving acoustic power on the spatial structure of bubbles is shown in Fig. 8. For 168 kHz, no strong streaming of the bubbles toward the liquid surface is observed across all driving powers investigated. For higher frequencies, as the driving power increases there exists a driving power at which streaming of the bubbles toward the liquid surface is observed. Increasing driving power can increase the population of large bubbles and increase nonlinearity and attenuation effects. The lack of streaming of the bubbles at 168 kHz is another demonstration that lower frequencies attenuate to a lesser extent. However, the onset of the streaming of bubbles occurs at a lower acoustic power for 448 kHz than
for 726 kHz. As discussed previously, the main force responsible for the streaming of bubbles for 448 kHz and 726 kHz is different. It is possible that the cause of this difference in the onset of the streaming of bubbles is the extent at which the driving power affects the radiation force from the traveling wave field and acoustic streaming.
3.2.4. Pulsed sonication Pulsed sonication have shown to effect the SL intensity [11], sonochemical efficiency [13,20,54] as well as the spatial distribution of active bubbles [25] when compared to a continuous system. In order to explore the effect of pulsing on bubble structure development, further experiments were carried out under pulsed sonication conditions at 448 kHz. Shown in Fig. 9 are a series of still photographs illustrating the development of the bubble structure under the pulse condition of 4000 cycles ON and 20,000 cycles OFF. The development of the bubble field under pulsed sonication is slower compare to the continuous case and allows the development of the bubble field to be fully captured. At 0.83 s visible bubbles begin to emerge and with time, a standing wave pattern forms throughout the liquid. At 6.67 s it can be seen that a force is present and the standing wave pattern is disrupted, and bubbles are forced toward the liquid surface. The commencement of this streaming occured later compared to that of the continuous case which was less than 1 s. This is because the growth in the size of bubbles via coalescence or rectified diffusion occurs during sonication and by pulsing, this growth is reduced and thus lessens the formation of large bubbles [34]. Furthermore, dissolution of bubbles would occur during the pulse OFF duration and decrease the size of the bubbles [20]. This decrease in the coalescence and size of the bubbles would reduce the degree in which the acoustic pressure is attenuated. The effect of increasing pulse ON or OFF duration is shown graphically in Fig. 10 with the vertical axis as increasing pulse OFF duration and the horizontal axis as increasing pulse ON duration. It can be seen that for a given pulse OFF duration, as the pulse ON duration increases the development of the bubble structure resembles the development observed as a function of time. Similar development in the bubble structure is observed with increasing pulse OFF duration for a given fixed pulse ON duration. As discussed previously, for 448 kHz the force causing the translational movements of bubbles toward the liquid surface is believed to be the radiation force from the traveling wave field. With increasing pulse ON duration or decreasing pulse OFF duration, the formation of large coalesced bubbles increases. This results in the increase in the traveling wave proportion and hence the rise in the radiation force. It is possible to categorize Fig. 10 into three regions according to SL images reported in the literature [25,34]. Region A is where SL is isolated to the liquid surface, region B is where the SL is
168 kHz 1W
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11W
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11W
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11W
0
0.33
0.83
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3.33
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10.0
448 kHz
726 kHz 15W
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Fig. 8. Equilibrium bubble structure in saturated water for 168, 448 and 726 kHz as a function of power.
Fig. 9. Initial bubble field development in saturated water for a pulse setting of 4000 cycles ON and 20,000 cycles OFF at 448 kHz and a driving power of 20 W.
J. Lee et al. / Ultrasonics Sonochemistry 18 (2011) 92–98
Increasing Pulse Off Cycle
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dominately caused by acoustic streaming. It is shown that there is an optimum pulse ON and OFF ratio at which the disruption of the standing wave field can be minimized.
C
Acknowledgment 20000
10000
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 number 1907765). The authors would like to thank Les Gamel for the capillary cell. MA acknowledges the award of AIST Visiting Fellowship.
B
5000
A References
500
1000
2000 5000 10000 Increasing pulse On cycle
Fig. 10. Visible bubble structure for different pulse ON and OFF cycles captured in saturated water at 448 kHz and a driving power of 20 W. Three regions have been identified according to the SL images reported in the literature. Region A is where SL is found isolated to the liquid surface. Region B is where SL is homogeneously distributed. Region C is where no SL activity was detected.
homogeneously distributed in the vessel and region C is where no SL activity is detected. Comparing the SL images to that of the visible bubbles, the bubble structure agrees well with the reported SL structures [25,34] provided that visible bubbles are observed. Fig. 10 demonstrates that there exists an optimum range of pulse ON and OFF ratio for strong standing wave field and homogeneous distribution of active bubbles, which has been shown to translate to a higher sonochemical efficiency [9,55]. If the pulse ON/OFF ratio is in region A, the pulse ON is sufficiently long or the pulse OFF is considerably short such that the system effectively behaves like a continuous system. If the pulse ON/OFF ratio is in region C, the pulse ON duration is inadequate for the generation of detectable bubbles. At the boundary between C and B where there is very few visible bubble, it is not possible to determine the structure of SL bubbles based on optical photographs. 4. Conclusion This study demonstrates that there is a resemblance between the structure of the SL bubbles at the antinodes and that of the coalesced visible bubbles at the nodes. This association between the visible and SL bubbles made it possible to indirectly infer the development of active cavitation bubbles by monitoring the development of large coalesced bubbles. It is thus concluded that at the initial stages of sonication, the wave field is predominately of a strong standing wave and the bubbles are trapped at the antinode planes extending from the liquid surface down to the transducer. As the growth in the population and size of bubbles progresses with increasing sonication time, attenuation of the acoustic pressure increases. This subsequently led to the development of acoustic streaming and radiation force from the traveling wave field which disrupts the standing wave field, and drives bubbles to the liquid surface. The disruptive forces were weaker for 168 kHz compared to 448 kHz and 726 kHz. This was found to be due to lower void volume generated and that higher frequencies attenuate to a greater extent. It is demonstrated in this study that at 448 kHz, the streaming of the bubbles observed is predominately due to the increase in the traveling wave proportion and weakening of the standing wave field. At 726 kHz, the streaming of bubbles is pre-
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