Influence of Degree of Gas Saturation on Multibubble

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Influence of Degree of Gas Saturation on Multibubble Sonoluminescence Intensity Toru Tuziuti,* Kyuichi Yasui, and Kazumi Kato National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ABSTRACT: The influence of the degree of saturation (DOS) of a gas in a solution on the intensity of multibubble sonoluminescence (MBSL) excited by ultrasound with a frequency of 261 kHz is investigated at various ultrasonic powers and with different concentrations of ethanol, which is added as a volatile solute. At relatively low powers and a high DOS, low ethanol concentrations give higher sonoluminescence (SL) intensities than those obtained with pure water. This intensity enhancement decreases as sonication proceeds because the SL intensity for pure water increases with time, whereas it remains almost constant or decreases slightly in solutions containing ethanol. At relatively low powers, a partially degassed solution has a higher SL intensity than a solution with a high DOS for both pure water and solutions containing ethanol. The reason why the DOS decreases more when ethanol is added is considered mainly to be the accumulation of hydrocarbon products and the promotion of rectified diffusion. Adding an alcohol to a solution enhances ultrasonic degassing.

1. INTRODUCTION The violent inertial collapse of an acoustic cavitation bubble can produce extreme conditions of high temperature and pressure within the bubble core.1,2 When a bubble collapses it emits light; this phenomenon is known as sonoluminescence (SL).3 Bubbles in aqueous solutions generate oxidants such as OH radicals, H2O2, and O3 when they collapse and cause decomposition of the water.4 Sonochemical reactions5 use these oxidants generated by the collapse of ultrasonic cavitation bubbles. Sonochemiluminescence (SCL) is light produced by reactions of these oxidants with a solute such as luminol in a bulk solution.6 Investigations of SL and SCL under various conditions are useful for determining the effect of ultrasonic cavitation bubbles on chemical reactions. The addition of volatile solutes to an aqueous solution greatly reduces the SL intensity relative to that in pure water by lowering the cavitation bubble temperature.7 10 Alcohol molecules adsorb on the surface of bubbles and then enter the bubbles during expansion caused by an ultrasonic standing wave that has the potential to realize stable pulsation of bubbles.7,11,12 Hydrocarbon products9,13 are then created inside the bubbles. Decomposition of hydrocarbon products that occurs when the bubble collapses reduces the temperature inside the bubble and quenches the SL. Sunartio et al.9 have showed that at relatively low ultrasonic powers, adding a certain concentration of an alcohol increases the SL intensity relative to that for pure water. They suggested that this enhancement is due to the suppression of bubble bubble coalescence,14 which increases the number of active bubbles. The degree of saturation (DOS) of a gas dissolved in a solution is one of the main parameters that determines the sonochemical reaction efficiency. The DOS generally decreases as sonication proceeds. This is due to a release of bubbles from a r 2011 American Chemical Society

sonicated system through the growth of a bubble by both rectified diffusion and coalescence between bubbles by the action of secondary Bjerknes force. Rectified diffusion increases the bubble volume since more gas diffuses into a bubble during expansion than diffuses out of the bubble during compression since the bubble has a larger surface area during expansion. The secondary Bjerknes force is an attractive force between bubbles that are smaller than the resonant size at an antinode in an ultrasonic standing wave. We showed that the SCL intensity from a supersaturated gas solution at below atmospheric pressure is higher than that at atmospheric pressure and that a DOS of more than unity (= DOS at saturation) is effective for obtaining high SCL intensity under a reduced ambient pressure.15 By controlling the dissolved gas concentration, Dezhkunov et al.16 showed that a degassed aqueous solution gives a high SL intensity. We measured the absorbance of triiodide ions generated by sonochemical oxidation of potassium iodide and light scattered by pulsating bubbles.17 The relative sonochemical efficiency was correlated with the scattered light intensity. It was found that there is an optimum dissolved gas concentration that maximizes the sonochemical reaction efficiency. It was possible to increase the number of active bubbles by using a partially degassed solution. To the best of our knowledge, the effect of adding alcohol on the DOS dependence of the SL intensity has not been reported. The present study investigates the influence of adding ethanol on the SL intensity for 261-kHz ultrasound irradiation at different ultrasonic powers and DOS of air in solution. Received: February 14, 2011 Revised: April 17, 2011 Published: April 29, 2011 5089

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2. EXPERIMENTAL DETAILS A continuous sinusoidal signal with a frequency of 261 kHz was generated by a function generator (NF Electronic Instruments, 1942) and amplified by a power amplifier (Thamway, T145 5516B) to drive a plane transducer (Honda Electronics; outer diameter: 50 mm). The transducer was attached to a circular stainless-steel plate (diameter: 100 mm; thickness: 1 mm) set at the bottom of a rectangular glass vessel (inner dimensions: 56  56  80 mm; side wall thickness: 2 mm). The ultrasonic power absorbed by the liquid was determined by calorimetry. The present experiment was conducted at 0.34 and 0.86 W/cm2. The ethanol used had a purity of g99.5% (Kanto Chemical), and the solutions were made up using distilled water. Two hundred milliliters of an air-saturated solution or a partially degassed solution was poured into the glass vessel. The solution temperature was initially 23 °C. Temperature rise was within a few degrees on ultrasonic irradiation. The SL intensity from the solution in the vessel was measured using a photomultiplier tube (Hamamatsu, R928). The output voltage from the photomultiplier tube was measured using a digital multimeter (Advantest, TR6847) and recorded on a computer (NEC, PC-9821 Xc16). Sonication was performed for 2 min, stopped for 2 min, and then performed for a further 2 min. Thus, the total sonication time was 4 min. The present experiment was conducted using ethanol concentrations of 12, 25, 37, 50, 75, 100, and 130 mM. The dissolved oxygen (DO) concentration of each solution was measured relative to the dissolved air concentration using a DO meter (Horiba, D-25). The DOS is defined as the relative DO concentration divided by the saturated DO concentration at the same temperature. Solutions were degassed prior to adding ethanol. The extent of degassing was adjusted according to the amount of ethanol added after degassing since ethanol contains a certain concentration of dissolved air. 3. RESULTS AND DISCUSSION 3.1. Variation in SL Intensity with Sonication Time for Various Ethanol Concentrations. Figure 1 shows the variation

in the SL intensity with sonication time for various ethanol concentrations. Each set of data was averaged over the sonication time and normalized by the SL intensity obtained when no ethanol was added. Two DOS were used: a higher DOS for which the initial solution prior to sonication was almost saturated and a lower DOS of 0.90. Figure 1a shows the results obtained at a low power of 0.34 W/cm2. It shows that, at the higher DOS and for short sonication times (up to 60 s), the SL intensity is enhanced at relatively low ethanol concentrations. The range of ethanol concentrations that enhances the SL intensity becomes narrower with increasing sonication time. At the lower DOS, no enhancement was observed for any sonication time or ethanol concentration. Figure 1b shows the results obtained at a high power of 0.86 W/cm2. It shows that the higher DOS yields less SL intensity enhancement than the lower power for all sonication times. Furthermore, the ethanol concentration that exhibits the enhancement is narrower than that at the low power. At the lower DOS, no enhancement was observed for any sonication time or ethanol concentration. Sunartio et al.9 measured the SL intensity as a function of the ethanol concentration relative to the SL intensity obtained using pure water in aqueous solutions sonicated at 358 kHz and at

Figure 1. Comparison of the time dependences of the SL intensity relative to pure water between almost air saturated (higher DOS) and partially degassed (lower DOS) solutions with various ethanol concentrations. The higher and lower DOSs were 1.0 and 0.9 prior to sonication, respectively.

various powers. They observed an enhanced SL intensity at an ethanol concentration of 100 mM and a power of 0.78 W/cm2. The frequency used in the present experiment (261 kHz) is lower than that used by Sunartio et al.9 When sonication is conducted at 261 kHz (i.e., the present driving frequency) at a high ethanol concentration and a high power (these conditions are suitable for SL enhancement), bubbles will expand and SL will be quenched. Since ethanol is volatile, more ethanol will enter bubbles during bubble expansion at lower frequencies. However, in the present experiment, the SL intensity was enhanced at both concentration and power lower than those used by Sunartio et al.9 Sunartio et al.9 identified two main parameters that affect SL enhancement and quenching: the number of active bubbles that collapse violently and emit SL and the cavitation bubble temperature. It is important to note that adding alcohol to a solution containing bubbles inhibits coalescence above a certain alcohol 5090

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The Journal of Physical Chemistry A concentration.14 Alcohol molecules in a solution containing bubbles under sonication are adsorbed on the surfaces of the bubbles. They then diffuse into the bubbles and produce hydrocarbon products such as ethylene, acetylene, and ethane.9 Such hydrocarbon products consume thermal energy when they decompose, thereby lowering the bubble temperature and quenching SL. At low powers, adding alcohol inhibits bubble coalescence resulting in a large number of active bubbles that have a low hydrocarbon concentration; this gives rise to a high SL intensity. Even at low powers there is an optimal alcohol concentration for SL enhancement because a high alcohol concentration will promote the production of hydrocarbon products. Bubbles expand more with increasing power, which promotes the production of the above-mentioned hydrocarbons by causing more volatile materials to enter the bubble, leading to SL quenching. 3.2. Conversion of SL Intensity from Time to DOS Dependence. This section describes the procedure used to estimate the DOS dependence of the SL intensity. Figure 2a shows an example of the time dependence of the SL intensity. Figure 2b shows the time dependence of the DO concentration for the same conditions as Figure 2a; the DOS was determined from the DO concentration. Curve fitting was carried out on the DOS data to allow the DOS to be estimated at any sonication time. The DOS dependence of the SL intensity is estimated by eliminating the time parameter from the time dependences of both the SL intensity and the DOS (Figure 2c). Note that the SL and DOS data in Figure 2c represent averages over 20 s. Figure 2a exhibits considerable scatter. This is probably due to intermittent changes in bubble formation, as mentioned by Mettin et al.18 Many bubbles will reduce the sound pressure and thereby reduce the number of bubbles; the sound pressure will subsequently increase due to the reduction in the number of bubbles. 3.3. DOS Dependence of SL Intensity. Figures 3 and 4 show the DOS dependence of the SL intensity at various ethanol concentrations for low and high powers, respectively. The sonication time increases from right to left in both figures since the DOS generally decreases with bubble degassing. In each case, solutions containing ethanol had a lower SL intensity than pure water (0 mM). In Figure 3a (higher DOS; close to saturation), the SL intensity at 0 mM seems to increase as the DOS decreases, whereas the SL intensity from the solution containing ethanol seems to be almost constant or decreases, although the data are rather scattered. At a high DOS (i.e., ∼0.94 0.99), the SL intensity at relatively low ethanol concentrations (12, 25, and 37 mM) is higher than that for pure water. In Figure 3b (lower DOS; partially degassed), pure water gives a higher SL intensity than solutions containing ethanol. Figure 4 exhibits similar trends to Figure 3. However, in Figure 4a, a solution with an ethanol concentration of only 12 mM gives a higher SL intensity than pure water (0 mM) for a DOS of about 0.95 0.99. Figure 4b shows a marked increase in the SL intensity for pure water when the DOS is less than 0.85. From Figures 3 and 4, the DOS for solutions containing ethanol seems to be much lower than that for pure water. This reduction in the DOS is considered to be caused by bubble degassing in the solution due to bubble growth by rectified diffusion, since adding ethanol inhibits bubble coalescence.19 Lee et al.19 suggested that rectified diffusion is responsible for an increase in the total bubble volume at higher alcohol concentrations. A higher bubble volume

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Figure 2. Conversion from time dependence to DOS dependence of SL intensity by curve fitting the time-dependent DOS estimated from measured DO data. (a) Example of time dependence of SL intensity. (b) DOS as a function of time determined from a curve fitted to DOS data, which was estimated from the measured time dependence of the DO concentration. (c) Example of DOS dependence of SL intensity.

is thought to promote degassing from the solution and reduce the DOS. Indeed, the present results reveal that the DOS decreases markedly at relatively high ethanol concentrations and high powers (see Figure 4a,b). For pure water, bubble degassing mainly 5091

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Figure 3. DOS dependence of SL intensity for various ethanol concentrations at 0.34 W/cm2.

Figure 4. DOS dependence of SL intensity for various ethanol concentrations at 0.86 W/cm2.

occurs as a result of bubbles coalescing. As the DOS of pure water decreases as sonication proceeds, the number of large bubbles decreases and tiny active bubbles are distributed over a wider region.20,21 In this case, little coalescence of bubbles occurs, and the degassing rate of pure water is lower than that of solutions containing ethanol. A relatively high DOS is maintained for pure water, and this results in more bubbles than for solutions containing ethanol. Thus, pure water has a higher SL intensity than solutions containing ethanol in suitably degassed conditions. The following is the proposed mechanism for the reduction in the DOS of gas (air) in a solution containing ethanol. Volatile alcohol (ethanol) molecules evaporate into the bubble and gaseous hydrocarbons are produced when the bubble collapses. Bubbles expand due to the accumulation of the gaseous hydrocarbons that are insoluble in water, thereby increasing their surface areas. This promotes diffusion of gas (air) dissolved in the solution into the bubbles. Bubble growth results in degassing from the solution, which reduces the DOS.

Increasing the alcohol concentration will enhance degassing since more hydrocarbons will be produced in bubbles resulting in greater bubble growth. Using a higher power will cause bubbles to expand more, resulting in more evaporation of alcohol into bubbles, accelerating the accumulation of hydrocarbon products.9 3.4. SL Intensity Enhancement by Lowering DOS. This section discusses the effect of using a partially degassed solution on the SL enhancement. Figure 5 shows SL intensity relative to higher DOS at various concentration of ethanol and different power. Each intensity is averaged over the whole sonication time (0 240 s) at the lower DOS and is normalized by the average SL intensity at the higher DOS. At the lower power (0.34 W/cm2), the SL intensity is enhanced at all ethanol concentrations. This implies that using the lower DOS and the lower power resulted in an appropriate amount of bubbles. Reducing the DOS slightly appropriately reduces the number of bubbles created under sonication and inhibits bubble bubble interactions. This increases the number of active bubbles that can violently collapse 5092

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’ REFERENCES

Figure 5. SL intensity relative to higher DOS for two different powers. Each intensity is averaged over the whole sonication time (0 240 s) at lower DOS and is normalized by the average SL intensity at higher DOS.

and emit SL. At the higher power (0.86 W/cm2), the higher and lower DOS have similar SL intensities. It is conjectured that at the higher power an excessive number of bubbles are created, which inhibits SL enhancement under the same conditions as the lower DOS before sonication at the lower power. To obtain SL enhancement at such a high power, it is necessary to reduce the DOS more prior to sonication. Gas saturation is also controlled by adding salts to water.22,23 Wall et al.22 showed that gas solubility decreases with concentration of added salts while SL intensity increases. Brotchie et al.23 showed that total bubble volume decreases due to a decrease in bubble bubble coalescence under low solubility. They pointed out that lowered gas solubility leads to a decrease in bubble bubble interaction. This provides symmetric and efficient collapse process to attain high temperature inside the bubble core and then a high SL intensity. These studies support the interpretation for SL intensity enhancement by lowering DOS offered in the present study.

4. CONCLUSIONS Measurements of the SL intensity from a solution containing ethanol and the DO concentration of the solution clarified that at a relatively low power and a high DOS the SL intensity is higher in solutions with a low ethanol concentration than in pure water. A partially degassed solution can provide a higher SL intensity than a solution with a high DOS for both pure water and a solution containing ethanol. The greater reduction in the DOS for solutions containing ethanol relative to pure water is considered to be mainly due to the accumulation of hydrocarbon products and the promotion of rectified diffusion.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported in part by the Japan Society for the Promotion of Science. 5093

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