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Characterization of Acoustic Cavitation Bubbles in Different Sound Fields Adam Brotchie, Franz Grieser,* and Muthupandian Ashokkumar* Particulate Fluids Processing Centre, School of Chemistry, UniVersity of Melbourne, VIC 3010, Australia ReceiVed: June 18, 2010; ReVised Manuscript ReceiVed: July 22, 2010
Various fundamental properties of acoustic cavitation bubbles have been investigated in single- and dualfrequency sound fields. It was found that the relative extent of bubble coalescence in the dual-frequency field correlated strongly with the synergistic enhancement of the sonochemical reaction rates. Both the relative extent of coalescence and the sonochemical synergy observed were enhanced through the addition of coalescence-inhibiting solutes. This was attributed to greater nucleation in the dual-frequency mode compared with the single-frequency modes, producing a very localized and high-density bubble field. The acoustic bubble size, compared with that measured at 355 kHz alone, was found to increase upon the application of synchronous 20 kHz pulses but was reduced dramatically when the low frequency was applied as a continuous wave. This trend is consistent with previous reports indicating that the bubble density and cavitation activity are relatively higher in the pulsed system and that the continuous wave application exerts a strong cancellation effect. The changes in bubble density and coalescence rates are proposed to govern the acoustic bubble size. The bubble lifetime was found to be longer in the dual-frequency field (>0.30 ms; >6 low-frequency oscillations, >100 high-frequency oscillations) compared with both single-frequency fields (0.26 ms and 5 oscillations for the low frequency; 0.22 ms and 75 oscillations for the high frequency). The confluence of a longer bubble lifetime and more asymmetric collapse conditions, the latter inferred from a more pronounced sodium atom emission in the sonoluminescence spectrum, resulted in a lower bubble collapse temperature measured in the dual-frequency system. Introduction The growth of adventitious gas nuclei and their subsequent violent implosion in ultrasound-irradiated fluids, a process referred to as acoustic cavitation, is responsible for many of the vast range of important applications of ultrasound.1-3 The rapid, inertial collapse of cavitation bubbles is quasi-adiabatic, rendering each individual bubble a microreactor, inside which temperatures of the order of thousands of degrees and pressures of hundreds of atmospheres have been shown to exist.4 The exploitation of such conditions has led to ultrasound being developed as a useful technology, for example, in the synthesis of bio- and nanomaterials, often with superior properties and formed via more facile synthesis routes compared with conventional methods.3,5 Acoustic cavitation is also employed in environmental remediation, i.e., pollutant degradation, and in industrially important processes such as cleaning and emulsification.3,6 As the dynamics of gas bubbles govern these and other applications, it is imperative that a fundamental understanding of the different facets of acoustic cavitation is acquired such that ultrasonic reactors and processes can be optimally designed. Until the development of relatively new experimental techniques, ascertaining information relating to properties of cavitation bubbles such as collapse temperature,7-10 bubble lifetime,11 and size distribution12-15 was not possible. Additionally, many approaches have been undertaken to optimize cavitation efficiency through optimization of the ultrasound parameters (e.g., intensity, frequency, pulse modulation, etc.) and the insonated solution (e.g., temperature, gas type and concentration, solute additives, etc.). We have recently reported on the use of dual* To whom correspondence should be addressed. E-mail: franz@ unimelb.edu.au (F.G.),
[email protected] (M.A.).
frequency sound fields and the effect of the aforementioned parameters on cavitation activity.16,17 In fact, work in dualfrequency systems has been ongoing for several decades,18-23 yet our knowledge of how cavitation is affected under dualfrequency sonication is minimal. For instance, theoretical predictions24,25 and indirect experimental results22,26-29 for the special case of an isolated single bubble suggest that bubble temperatures can be elevated under dual-frequency driving (due to changes in the radial dynamics). However, such studies have not been reproduced experimentally in a typical multibubble system. The present study reports on acoustic cavitation bubbles through an investigation of the effective bubble lifetimes, coalescence rates, size and collapse temperatures in typical single-frequency and dual-frequency fields, comprising one lowand one high-frequency transducer. Experimental Details Materials. All chemicals were used as received and solutions prepared using Milli-Q purified water. 3-Aminophthalhydrazide (luminol) (>97%), ethanol (100%), tert-butanol (>99%), gold(III) chloride trihydrate (>99%), and sodium chloride (>99.5%) were obtained from Sigma-Aldrich. Ammonium molybdate (>99%), sodium hydroxide (>99%), and sodium dodecyl sulfate (SDS) were purchased from BDH. Potassium hydrogen phthalate (>99.8%) was procured from Ajax Finechem and potassium iodide (>99%) was obtained from Chem. Supply. High-purity grade argon and acetylene were purchased from BOC gases. Ethylene and ethane were obtained from Aldrich. Bubble Size Determination. All solutions were prepared with Milli-Q water that was left to stand overnight under atmospheric conditions. The ultrasound arrangement consisted of a Hameg function generator (model HM8131-2) triggered by a Datapulse (100A) pulse generator. The pulsed electrical output was
10.1021/jp105618q 2010 American Chemical Society Published on Web 08/10/2010
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amplified by a T&C Power Conversion, Inc. amplifier (AG series) and delivered to a 355 kHz Allied Signal barium titanate transducer (55 mm diameter base). A Branson B-30 Cell Disruptor was used to drive a 20 kHz horn (45 mm diameter horn tip) which was inserted vertically into a 200 mL Pyrex cell. Sonochemical luminescence30 (from pH 12 NaOH-luminol solution) measurements were taken by measuring the output of a Hamamatsu photomultiplier tube on a LeCroy digital oscilloscope. The method used to correlate the ultrasound pulse separation with the cavitation bubble size has been described in detail elsewhere.12 Briefly, cavitation bubbles will experience growth, via rectified diffusion and coalescence during ultrasound pulses, and will dissolve (or rise through buoyancy if sufficiently large) during the pulse off-time. Under the conditions of the present study, many pulses (∼20) are required to establish a steadystate level of bubbles producing sonochemical luminescence. As the time interval between pulses is increased, a larger range of bubble sizes undergo total dissolution, reducing the number population of bubbles that can survive until the successive pulses. This, in turn, leads to a reduction in cavitation activity, and sonoluminescence (SL) and sonochemical luminescence intensity. The time of bubble dissolution (taken to be equal to the pulse separation) can be correlated with the bubble radius through the Epstein-Plesset equation:
( ) ( DCS
FggR0
2
)
)
1 RTFgR0 +1 3 2Mγ
(1)
where D is the diffusion coefficient (m2 s-1),31,32 CS is the saturated dissolved gas concentration (kg m-3), Fg is the gas density in the bubble (kg m-3),33 R0 is the initial bubble radius (m), t is the dissolution time (s), R is the universal gas constant (J kmol-1 K-1), T is the absolute temperature of the liquid (K), M is the molecular weight of dissolved gas (kg kmol-1), and γ is the surface tension (N m-1).33 Bubble Coalescence Measurements and Lifetime Determination. Bubble coalescence was quantified following a method developed by Lee et al.34 A customized 770 mL reaction cell fitted with a thin capillary tube (internal diameter: 0.9 mm) was attached to the 355 kHz transducer (Allied Signal). An ELAC rf generator/power amplifier was used to supply the electrical waveform. A 20 kHz horn (29 mm tip diameter) was inserted orthogonally, and the solution height was adjusted with a syringe such that prior to sonication the fluid level was close to the base of the capillary. The solution was sonicated for a nominal period of 30 s, sufficient for the fluid level in the capillary to rise an appreciable distance, from which the total volume change was calculated. As experiments were carried out close to 293 K, the thermal expansion coefficient at this temperature of 2.07 × 10-4 K-1 was used to calculate the thermal expansion of the liquid under sonication. For example, in the dual-frequency mode for water, a thermal expansion of 68 µL was calculated, which is approximately 15% of the total volume change. The same experimental apparatus was also used to estimate acoustic bubble lifetimes. These measurements were carried out using a method developed by Sunartio et al.11 where the nonequilibrium adsorption of a surfactant at the cavitation bubble interface was investigated by correlating the degree of coalescence inhibition by surfactant with that of aliphatic alcohols (which effectively achieve equilibrium adsorption during a typical bubble lifetime). This enables a determination of the
nonequilibrium surface excess of surfactant, which can then be easily converted into a bubble lifetime with recourse to adsorption kinetics data available in the literature.11 In this experiment, the reported ∆VT values have all been normalized with respect to pure water. Sonochemical Gold Reduction and Hydrogen Peroxide Formation. Gold chloride trihydrate (0.2 mM) in 0.1% (w/vol) poly(vinylpyrrolidone) (PVP) and 20 mM 1-propanol were sonicated under an argon atmosphere to form colloidal gold. Immediately following sonication, saturated NaBr was added to the sample in order to exchange the chloride ligand for bromide, as described by Okitsu et al.35 The gold bromide concentration was subsequently measured spectrophotometrically after allowing 5 min for the reaction to reach completion. Absorbance spectra were taken at 5 min intervals and the experiment repeated at least three times. In order to maintain an argon atmosphere, a customized seal was placed around the low-frequency sonotrode. The solution was sparged for 15-20 min prior to sonication and a positive argon pressure maintained above the liquid surface during the experiment. The yield of hydrogen peroxide formed during sonication was determined spectrophotometrically using a Varian (Cary) UV-visible spectrophotometer according to an analytical technique developed by Hochanadel.36 Sonication was performed under identical conditions to those described above for the coalescence experiments. Freshly sonicated samples (2.5 mL) were mixed with 1.25 mL of solution A (0.4 M KI, 0.1 M NaOH, 0.2 mM (NH4)6Mo7O24 · 4H2O) and solution B (0.1 M KHC8H4O4). After mixing, solutions were left to stand for 5 min prior to spectroscopic analysis. Fresh reagent solutions were prepared for each experiment and each experiment was repeated three times. This technique is based on the oxidation of iodide by hydrogen peroxide to I3- (λmax 353 nm). A molar extinction coefficient of triiodide of 26 400 M-1 cm-1 was used, as determined by Alegria et al.37 Bubble Temperature Determination. A method based on the gas-phase reactions of methyl radicals, the methyl radical recombination (MRR) method,8,38 was employed to probe the gas-phase temperature in the cavitation bubble during collapse. A sealed vial containing an argon-saturated aqueous solution of a methyl radical source (50 mM tert-butanol) was sonicated under an argon atmosphere at both 355 and 20 kHz, each applied at a power of 10 W. Pyrolysis of the volatile solute inside the cavitation bubbles produces methyl radicals, which recombine to form different hydrocarbon products according to Scheme 1 in ref 38. The headspace of the sonicated solution was analyzed using gas chromatography (Shimadzu GC-17A) to determine the yields of the hydrocarbon products, ethane, ethylene, and acetylene. Using the Henry’s law constants from the literature,38 the total concentration of each hydrocarbon product was calculated and used together with the known Arrhenius parameters of the different reaction pathways8 to calculate the bubble temperature. It is important to mention that the measured bubble temperature is an effective “chemical” temperature, which is averaged both spatially and temporally. Despite many assumptions made in the calculations as detailed in our previous reports,38-40 this method yields values comparable to those obtained in the literature through different methods.4,41 Results and Discussion We have recently reported on bulk processes such as sonoluminescence and sonochemistry in different dual-frequency systems.16,17,42,43 Absent from these studies is a characterization of several important properties of the cavitation bubbles. One
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Figure 1. Sonochemical luminescence from alkaline luminol solution as a function of pulse separation time for a pulsed 355 kHz field in the absence and presence of a 20 kHz source. The sonication conditions of the different systems are detailed in Table 1.
TABLE 1: Ultrasound Power and Pulse Conditions Corresponding to the Sonochemical Luminescence Intensity-Pulse Separation Data Presented in Figure 1, and the Critical Pulse Separations (τcrit at Which the Intensity Reaches Zero or a Limiting Minimum Value and Corresponding Radii at This Critical Separation, rcrit)a system I II III IV V
355 kHz pulsed; pulsed; pulsed; pulsed; pulsed;
10 10 10 10 10
W W W W W
20 kHz
τcrit (ms)
rcrit (µm)
off pulsed; 5 W pulsed; 12 W pulsed; 18 W continuous; 5 W
580 ( 80 740 ( 100 900 ( 120 810 ( 110 230 ( 35
4.7 ( 0.3 5.2 ( 0.3 5.5 ( 0.3 5.3 ( 0.3 3.4 ( 0.2
a It should be noted that these radii values refer to the upper bound of a size distribution.
such property is the bubble size distribution. Figure 1 contains sonochemical luminescence data for five different systems, the details of which are listed in Table 1. The luminescence is presented as a function of pulse separation time of a 355 kHz field, operated in the absence (system I) and presence (systems II-V) of a 20 kHz source. The low frequency was applied in a pulsed mode at different acoustic powers (systems II-IV) and also as a continuous wave (system V). When pulsed, the two sound sources were synchronized. The intensity decays as the pulse separation is increased, eventually either reaching zero or reaching a minimum plateau. The critical separation times at which the intensity reaches this minimum value are reported in Table 1 with the corresponding bubble sizes, calculated using eq 1. For the high-frequency field operated alone, a bubble size of 4.7 ( 0.3 µm is calculated. When low-frequency pulses are synchronously applied, the size is increased to about 5.5 ( 0.3 µm; however, when the low frequency is applied as a continuous wave, the size is considerably reduced to 3.4 ( 0.2 µm. In a single-frequency system, we have shown that, for a given acoustic power, the bubble size distribution becomes smaller and narrower as the frequency increases.44 The fact that in the present study the size decreases by running the low-frequency source in continuous mode might at first glance appear counterintuitive. However, we have recently demonstrated that, for given acoustic power, frequency, and pulse parameters, the main determinant of the bubble size is the extent of bubble-bubble coalescence occurring in the acoustic system.45 It can therefore
be inferred from the results in Table 1 that the pulsed dualfrequency system, which yields a slightly larger bubble size, increases the rate of bubble coalescence, whereas the continuouswave system leads to a reduction in bubble coalescence. There are two main mechanisms by which ultrasound-induced coalescence rates can be affected in a solution of fixed composition and temperature. The first is through bubbles being driven at higher velocities, increasing the collision frequency and probability of coalescence. The second is by a change in the spatial distribution and density of cavitation bubbles (e.g., low bubble density will reduce the collision frequency and coalescence rates). As the former mechanism would be expected to affect the pulsed and continuous-mode systems in a similar fashion, it is deduced that the latter mechanism is responsible for the observed trends. Hence, it can be summarized that pulsed operation increases the bubble density, elevating coalescence rates and increasing the bubble size, whereas continuous operation has the opposite effect. The way in which the interaction between the two sound fields is proposed to influence the movement and distribution of cavitation bubbles is illustrated in Figure 2. We note the need, however, for further verification to confirm this interpretation. The effect of bubble population and clustering density on the sonochemical activity is complex. At high acoustic power, acoustic shielding and impedance effects serve to reduce the cavitation and sonochemical efficiency. The power regime over which this is significant is far higher than that used in the present study. It is likely that, in our low-power system, the bubble population and density relate directly to the cavitation activity and that this is reflected in the rates of coalescence. Indeed, it has been reported that only in the pulsed dual-frequency system can a synergism with respect to SL and sonochemistry be observed,16,46 and that the continuous-mode application exhibits a strong cancellation effect.21 The reason that the cavitation activity (and therefore the coalescence rates and bubble size) is reduced in the continuous mode is thought to be due to a strong disturbance of the high-frequency standing wave, an issue which is partially alleviated in the pulsed mode operation. Direct measurement of bubble coalescence in an acoustic field can provide insight into the cavitation activity. If all other experimental parameters are fixed, and the solution is doped with coalescence-inhibiting solutes, there will exist an inverse relationship between the amount of measurable coalescence and
Acoustic Cavitation Bubbles in Sound Fields
Figure 2. Representation of the influence of the two sound fields on the movement and distribution of cavitation bubbles. The highfrequency source produces a standing wave field, indicated by the white bubbles, which align at pressure antinodes. The low frequency is transmitted as a traveling wave and cavitation (shown in gray) is localized near the horn tip. Many fragments are produced as a result of low-frequency-driven bubble collapse. In this depiction, highfrequency antinodal bubbles feed into the low-frequency cavitation zone and hence have a stimulating effect. However, the high-frequency standing wave is partially disturbed, which can, under certain conditions, lead to a net cancellation effect of cavitation activity.
the cavitation activity. This is largely due to the prevention of bubbles coalescing and exceeding resonance size that results in bubbles leaving the system through buoyancy or moving to acoustic pressure nodes. It has previously been reported that, in a dual-frequency system comprising a low-frequency traveling wave and a high-frequency standing wave emitter, the synergy in SL in the dual-frequency mode can be increased by adding coalescence-inhibiting solutes.16 For a fixed solution composition, however, an increase in coalescence as a result of a change in ultrasound parameters is more likely to correlate directly with the cavitation activity (e.g., an increase in applied power generally increases the sonochemical output as well as the bubble density and coalescence rate). The relationship between coalescence and sonochemistry is examined in Figure 3, for water and solutions of different alkyl alcohols and polymer, PEO. The ratio of the extent of coalescence in the dual-frequency field to that in the single-frequency field is presented as a function of the dual-frequency sonochemical synergy index (SI), k12/(k1 + k2), where k1 and k2 are the sonochemical rate constants under the two single-frequency sources and k12 is the rate constant for the dual-frequency mode. SI values greater than 1 indicate that dual-frequency operation enhances the process compared to the single-frequency modes combined but separately operated. The sonochemical reactions studied were hydrogen peroxide formation in the case of water and polymer solution and gold chloride reduction in the alcohol solutions. The strong correlation between the synergy index and the coalescence ratio reveals two important pieces of information. First, the relative extent of coalescence can be used as a good indicator of the system’s synergy and, second, solutes that inhibit coalescence tend to increase the coalescence ratio compared with pure water. In our previous report,17 we proposed that small bubbles produced in the high-frequency field nucleated cavitation in the low-frequency field. The efficiency of this mechanism was increased through the addition of solutes that serve to
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Figure 3. Dual-frequency sonochemical enhancement factor as a function of the relative dual- to single-frequency coalescence in water and aqueous solutions of different alkyl alcohols and PEO. ∆VT is the thermally corrected total volume change in a capillary tube during sonication.
Figure 4. Illustration of the effects of a coalescence-inhibiting solute (here, a polymer) on the bubble size, number, and extent of coalescence in a high-frequency (standing wave) and a dual-frequency (standing wave + probe) reactor.
increase the nuclei number population. Moreover, when solutes were present, fragmentation from low-frequency-driven cavitation was found to stimulate high-frequency-driven cavitation, which was otherwise suppressed in the dual-frequency mode. Figure 4 illustrates the differences in coalescence in the single(high) and dual-frequency fields when a coalescence-inhibiting solute is added. The addition of solutes increases the bubble density under the horn tip in the dual-frequency mode to a greater extent than in water. Consequently, despite the absolute coalescence levels being lower in the solute solution than in water for all sonication modes, the ratio of dual- to singlefrequency coalescence increases. However, a discussion of sonochemistry is incomplete without consideration of the bubble temperature. To this end, the collapse temperature was determined under our different frequency conditions, shown in Figure 5. The temperatures obtained (determined using a chemical method outlined earlier and described in some detail in the literature8,38) are presented as a
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Figure 5. Bubble temperatures determined using the methyl radical recombination method under sonication at 20, 355, and 20 + 355 kHz as a function of sonication time.
TABLE 2: Hydrocarbon Product Yields Measured after 30 min Sonication of Argon-Saturated 50 mM tert-Butanol at 20, 355, and 20 + 355 kHz frequency (kHz)
[C2H2] (µm)
[C2H4] (µm)
[C2H6] (µm)
20 355 20 + 355
2.1 13.6 12.6
2.6 25.6 32.6
3.0 18.0 24.6
function of sonication time. The bubble temperature is affected by a range of factors, with the amount and type of material within the bubble prior to collapse being one of the major determinants. This is dependent on the bubble lifetime, dissolved gas type and concentration, solution temperature, the presence of volatile solutes, etc. In addition to the content of the bubble core, the radial dynamics of collapse sensitively influence the temperature profile. Specifically, the temperature is determined by the radial excursion prior to collapse and the speed of collapse, which are related to the parameters of the local pressure field, and also to the symmetry of the collapse. Although our solution to the Rayleigh-Plesset equation does indicate that in the dual-frequency field bubbles will often undergo a larger excursion prior to collapse, it has been shown experimentally, using the same chemical method as employed in the present study, that an increase in acoustic power in a single-frequency field (which will have a similar effect on the radial dynamics) actually leads to lower collapse temperature.47 This was attributed to greater solvent and solute evaporation during expansion. Therefore, in the present system, it is plausible that the larger radial excursion contributes to the observed reduction in bubble temperature. Table 2 contains the hydrocarbon product yields following sonication. It can be seen that there is a superadditive increase in product yield in the dual-frequency mode. Although this is expected based on the preceding argument (i.e., more material inside the bubble leads to a greater amount of degradation products forming), it is also true that there is will be a greater bubble population in the dual-frequency field under these conditions. It is therefore difficult to distinguish between the contributions of these two factors. Due to the much higher bubble density in the dual-frequency field (at least in the vicinity of the low-frequency probe), it can also be expected that the symmetry of collapse in significantly lower. One way to indirectly probe the collapse symmetry is through monitoring the SL emission spectra in the presence of an alkali salt. Although there is a degree of uncertainty with
Figure 6. Sonoluminescence spectra recorded in argon-saturated 1 M NaCl solution sonicated at 20, 575, and 20 + 575 kHz.
respect to the mechanism producing electronically excited metal atoms from metal ions,48-51 it is has been shown that the alkali metal atom emission in the SL spectra requires a chaotic and asymmetric collapse environment.51,52 This has been interpreted in favor of the droplet injection model for metal atom emission. Low-resolution SL spectra recorded in 1 M NaCl are shown in Figure 6. The SL emission from the 20 kHz field is negligible when operated alone, whereas at high frequency (575 kHz) there is a relatively strong but predominantly featureless (the sodium emission at 590 nm is only very faintly observed) emission intensity. The continuum of the dual-frequency spectrum is comparable to that of the high frequency, although a pronounced sodium atom emission band at 590 nm is very prominent. It should be noted that, at high frequency, as has been reported in the literature,53 it is possible to observe a relatively greater sodium emission than is shown in Figure 6, under slightly different conditions of sonication (e.g., at higher power). The conditions of sonication were adjusted to accentuate the difference between the single- and dual-frequency modes. The more pronounced sodium emission supports the assumption that the radial dynamics of bubble collapse are more asymmetric in this dual-frequency system. This can be expected to contribute considerably to the lower temperatures measured in the combined frequency mode operation. A property of both fundamental and practical importance that potentially has a significant bearing on the bubble temperature is that of the effective acoustic bubble lifetime in different acoustic fields. As described in detail elsewhere,11 a coalescence method can be employed to ascertain this information, which is not possible through other techniques. Parts a and b of Figure 7 display the extent of bubble coalescence (∆VT) relative to pure water, measured in the single- and dual-frequency modes, as functions of ethanol bulk concentration and surface excess, respectively. It has been shown that short-chain alcohols effectively reach equilibrium adsorption during a typical bubble lifetime.11 As the mechanism of coalescence inhibition is short range and mainly steric in nature, Figure 7b can be used as a master curve to correlate the extent of coalescence measured for any solute (provided that long-range interactions, such as electrostatics are absent) with a surface-excess value. For solutes such as surfactants that do not equilibrate at the bubble interface during the lifetime of the bubble, this value will be a nonequilibrium surface excess. If the adsorption kinetics of the solute is known, the nonequilibrium surface excess can be used to determine the bubble lifetime.
Acoustic Cavitation Bubbles in Sound Fields
J. Phys. Chem. B, Vol. 114, No. 34, 2010 11015 rectified diffusion and therefore the lifetime. Irrespective of the mechanism, a longer lifetime has important implications. For instance, if a bubble survives for a greater number of acoustic cycles, a greater amount of water vapor and, if present, volatile hydrocarbon material can be expected to evaporate into the bubble core, which will serve to lower the temperature attained upon transient collapse. The similarity in lifetime calculated for the two single frequencies is also worthy of discussion. Sunartio et al.11 recently showed that over the frequency range 213-1062 kHz, there is a relationship between frequency and lifetime. Dramatic differences between the behavior of low- and high-frequencydriven bubbles is often explained in terms of differences in lifetime. For example, the fact that SL quenching is observed at high frequency but not at low frequency has been explained based on the assumption that high-frequency bubbles survive for a significantly longer time, during which accumulation of the quenching species inside the bubble can occur.54,55 As we show in the present study that there is little difference in lifetime, it is clear that other factors must be responsible for the frequency-dependent bubble behavior. Summary and Conclusions
Figure 7. Normalized extent of bubble coalescence (∆VT) as a function of (a) ethanol bulk concentration and (b) ethanol surface excess under 20, 355, and 20 + 355 kHz sonication.
TABLE 3: Extent of Coalescence (∆VT) Relative to Pure Water and Nonequilibrium Surface Excess Values in 2 mM SDS/0.1 M NaCl under 20, 355, and 20 + 355 kHz Sonication for a Period of 30 s, and the Corresponding Bubble Lifetimes and Number of Acoustic Cycles ∆VT in lifetime Γ frequency 2.0 mM SDS/ 0.1 M NaCl (molecules cm-2) (ms) (kHz) 20 355 20 + 355
0.24 0.28 0.20
4.3 × 1013 3.7 × 1013 >5.0 × 1013
0.26 0.22 >0.30
acoustic cycles 5 75 >6 (20 kHz) > 110 (355 kHz)
Table 3 contains the ∆VT and nonequilibrium surface excess values, and the acoustic bubble lifetimes (calculated using adsorption kinetics data from the literature11) in the three different frequency modes for the anionic surfactant SDS. Longrange electrostatic repulsions were minimized by using excess NaCl (0.1 M). The outcome of this analysis is that the dualfrequency-driven bubbles exist for a relatively longer time and sustain a greater number of acoustic cycles than those bubbles in either of the separate single-frequency fields. This could be explained if, as previously proposed, bubbles exist for some time in regions of the high-frequency field prior to being drawn, through hydrodynamic forces, to the more confined lowfrequency active zone, nucleating low-frequency cavitation. A disturbance to the high-frequency standing wave and pressure field can also be expected to influence rates of growth via
Investigation into the coalescence and sonochemical activity in a dual-frequency system has revealed that the relative extent of coalescence in the dual-frequency mode correlates strongly with the enhancement in sonochemical reaction rates over the separate single frequencies in combination. Solutions containing coalescence-inhibiting solutes increased both the relative extent of dual-frequency coalescence and also the sonochemical synergy index. The acoustic bubble lifetime was determined to be longer in the dual-frequency system and the collapse temperature found to be significantly reduced relative to both single-frequency modes. The lower bubble temperature of the dual-frequency operation was attributed, in part, to the longer lifetime in the dual-frequency field permitting water vapor and volatile hydrocarbon material to be evaporated into the bubble core. Moreover, the SL spectra measured in sodium chloride solution strongly suggested that the collapse symmetry was lower in the dual-frequency mode. Acknowledgment. Financial support from the Australian Research Council is gratefully acknowledged. A.B. also acknowledges the receipt of an Australian Postgraduate Award and a postgraduate award (David Lachlan Hay Memorial Fund) from The University of Melbourne. References and Notes (1) Leighton, T. G. The Acoustic Bubble; Academic Press Inc.: London, 1994. (2) Ultrasonic physical mechanisms and chemical effects; Webster, J., Ed.; John Wiley & Sons, Inc.: New York, 1999. (3) Ashokkumar, M.; Grieser, F. ReV. Chem. Eng 1999, 15, 41–83. (4) Suslick, K. S.; Flannigan, D. J. Annu. ReV. Phys. Chem. 2008, 59, 659–683. (5) Suslick, K. S.; Price, G. J. Annu. ReV. Mater. Sci. 1999, 29, 295– 326. (6) Mason, T. J. Ultrason. Sonochem. 2007, 14, 476–483. (7) Hart, E. J.; Fischer, C. H.; Henglein, A. Radiat. Phys. Chem. 1990, 36, 511–516. (8) Tauber, A.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 1999, 1129–1136. (9) McNamara, W. B. I.; Didenko, Y. T.; Suslick, K. S. Nature 1999, 401, 772–775. (10) Misik, V.; Miyoshi, N.; Riesz, P. J. Phys. Chem. 1995, 99, 3605– 3611. (11) Sunartio, D.; Ashokkumar, M.; Grieser, F. J. Am. Chem. Soc. 2007, 129, 6031–6036.
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