Influence of Liquid-Surface Vibration on Sonochemiluminescence

J. Phys. Chem. A 2010, 114, 7321–7325

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Influence of Liquid-Surface Vibration on Sonochemiluminescence Intensity Toru Tuziuti,* Kyuichi Yasui, Teruyuki Kozuka, and Atsuya Towata National Institute of AdVanced Industrial Science and Technology (AIST), 2266-98 Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan ReceiVed: February 23, 2010; ReVised Manuscript ReceiVed: May 27, 2010

The influence of surface vibrations on the intensity of sonochemiluminescence (SCL) produced by pulsed ultrasound with a frequency of 151 kHz is investigated through optical measurement of the vibration amplitude. Pulsed ultrasound inhibits the generation of large degassing bubbles that scatter sound waves, thereby restricting the effective spatial region for sonochemical reactions. The vibration amplitude of the liquid surface becomes gradually significant with pulsed ultrasound as the power applied to the transducer increases. At this time, the SCL intensity increases and then decreases after displaying a peak because the resonance of standing wave cannot be maintained stably by the surface vibration. The SCL intensity during high-amplitude pulsing becomes almost zero if the distance between the highest position of the liquid surface and its position in the absence of ultrasound becomes close to larger than one-quarter of the ultrasound wavelength. This condition for the liquid surface vibration provides a limit for establishment of a resonant standing wave that is effective for sonochemical reaction. Adding particles provides nucleation sites for cavitation bubbles, thereby inhibiting the vibration of the surface of the upper liquid level. The SCL intensity is enhanced by the addition of alumina particles under pulsed excitation at relatively low applied powers. 1. Introduction An ultrasonic cavitation bubble can provide extreme conditions within the interior of a bubble, including temperatures of several thousand degrees Kelvin, pressures of several hundred atmospheres, and heating and cooling rates greater than 109 K/s.1,2 Under such conditions, water is easily decomposed and oxidants such as hydroxyl radicals, hydrogen peroxide, and ozone are generated.3 At the interface of the bubbles, these oxidants react with chemicals such as luminol, and light is emitted in a process known as sonochemiluminescence (SCL).4 Chemical reactions involving ultrasonic cavitation bubbles are referred to as sonochemical reactions.2,5 Particle addition to such a sonochemical system has the potential to enhance the efficiency of sonochemical reaction as the particles provide nucleation sites for the initiation of cavitation bubbles that are effective for sonochemical reaction.6-9 The mechanism is as follows. When external pressure (e.g., ultrasound) is not applied, a gas pocket in a crevice on the particle surface is stable because of the surface tension at the concave gas-liquid interface. The volume of gas increases under reduced pressure at a cycle of rarefaction phase when an ultrasound is applied. The expanded gas becomes convex to release a free and tiny cavitation bubble from the crevice. Such a tiny cavitation bubble is not only provided repeatedly but also has a potential to pulsate. In potassium iodide oxidation with 42 kHz ultrasound, we found that the addition of an appropriate amount of alumina particles of a suitable size enhances the sonochemical reaction efficiency when the acoustic cavitation noise increases; the rise in the liquid temperature due to cavitation bubbles also increases.9 In that experiment, the KI solution was sonicated for 60 s, and the absorbance of I3- was measured. The sonochemical enhancement by two times at most compared with the control in the absence of particles was obtained under the condition where the concentration and the * Corresponding author. E-mail: [email protected].

size of the particles were 20 g/L and 40 µm, respectively. Pulsating bubbles generate sound known as cavitation noise. Increasing the number of active (pulsating) bubbles increases the cavitation noise. The addition of an appropriate amount of particles is expected to increase the number of active bubbles for sonochemical reactions. Accordingly, the addition of particles to a dilute solution could increase the number of active bubbles for sonochemical reactions, which would enhance the reaction efficiency. If spatial distribution of bubbles active for sonochemical reaction becomes wide by the addition of particles, then the wide spatial distribution will also be responsible for an increase in the sonochemical reaction efficiency. Which is more effective for the efficiency increase, the spatial effect or the effect of an increase in the number of bubbles? This can be checked by checking the volume fraction occupied by the cavitation bubbles. If the result is only due to spatial distribution, then no increase in the volume fraction is expected. Particle addition has another role in that it suppresses vibration of the liquid surface, which prevents high efficiency of sonochemical reactions. At high acoustic amplitude, liquid surface vibration occurs frequently by the action of acoustic radiation force, at which time the establishment of the resonant standing wave field is disturbed and the sonochemical reaction efficiency is decreased. We previously reported that hydrophobic particles cover the upper surface, which dampens surface vibrations.10 The liquid surface at relatively high acoustic amplitude, SCL intensity higher than that in the absence of the covering could be obtained. Pulsing ultrasound operation is useful to obtain highly efficient sonochemical reaction11-13 because it inhibits the generation of large degassing bubbles that restrict the efficient spatial region for sonochemical reaction. The restriction comes from scattering of sound wave by the degassing large-sized bubbles. We have previously demonstrated that the residual sound pressure during the pulse-off time, in addition to the

10.1021/jp101638c  2010 American Chemical Society Published on Web 06/16/2010

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Figure 1. Schematic illustration of the experimental apparatus.

spatial enlargement, contribute to the efficiency of the sonochemical reaction.14 To the best of our knowledge, a quantitative study on the effect of the liquid-surface vibration amplitude on the sonochemical-reaction efficiency has not yet been reported. In addition, the sonochemical effects of pulsed ultrasound under particle addition have not been investigated. In this study, the influence of liquid surface vibration on the SCL intensity by pulsed ultrasound with particle addition was investigated through optical measurements of the vibration amplitude. 2. Experimental Details Figure 1 shows the experimental setup used in this study. A continuous (CW) or pulsed wave sinusoidal signal of 151 kHz was generated by a function generator (NF Electronic Instruments, 1942), and amplified with a power amplifier (ENI, 1140LA) to drive a Langevin-type transducer (Kouwa, 45 mm OD). The transducer was attached to a circular stainless steel plate with a diameter of 110 mm (1 mm thick) set at the bottom of a rectangular glass vessel. The inner dimensions of the glass vessel were 56 × 56 × 80 mm, with a 2-mm-thick side wall. Pulsed sonication involved the repetition of 1000 acoustic cycles-ON and 1000 cycles-OFF. Luminol (3-aminophthalhydrazide) solution was used for measurement of the SCL intensity. Luminol reacts with OH radicals generated in the cavitation bubbles to yield aminophthalate anions and a blue fluorescence when intense ultrasound passes through the luminol solution.15 A solution consisting of 0.33 M NaOH (Wako) and 1.9 mM luminol (Wako) was prepared using distilled water and was saturated with air. The air-saturated liquid was poured in the vessel to a liquid height of 64 mm (the liquid volume was 200 mL). The liquid temperature was 25 °C. The intensity of SCL from the air-saturated liquid in the vessel was measured with a photomultiplier tube (Hamamatsu, R928). Data were recorded using a computer (NEC, PC-9821 Xc16) through a digital multimeter (Advantest, TR6847) that read the output voltage from the photomultiplier tube. The sonication time for each set of data was 1 min. Alumina particles (Adomafine; Adomatechs, AO809) of 10 µm in mean diameter were added to the solution. Alumina particle (density: 3.7 g/cm3) concentrations of 0.6 and 1.2 g/L were used in the experiment. Figure 2 shows images of the liquid surface captured from an inclined angle under conditions of ultrasound off (left) or on (right). To evaluate the amplitude of liquid surface vibration, we measured the time variation in the longitudinal position of the liquid with a laser displacement sensor (OPTEX-FA, CD4), where a laser beam was introduced to a plastic thin film reflector

Figure 2. Change in the liquid surface due to ultrasound in the absence of particles. (a) Ultrasound-off. (b) Ultrasound-on.

(0.4 mm thickness, 9 mm OD) floating on the liquid surface. The horizontal position of the film was limited loosely within a region surrounded by six thin wires to not disturb the motion of the film and the liquid. The waveform was observed with an oscilloscope (Yokogawa, DL1540C) to determine the vibration amplitude and the position of the liquid surface. Calorimetric studies9,16,17 have been conducted for both CW and pulsed ultrasound in the absence and presence of particles in air-saturated water. On the basis of the results of these studies, we estimated the effective energy of ultrasound absorbed by the sonicated liquid by dividing the calorimetric power by the electric power measured with a power meter (Towa, TAW-60A). 3. Results and Discussion 3.1. Change of SCL Intensity Due to Different Sonication in the Presence/Absence of Particles. Figure 3 shows the dependence of the measured SCL intensity on the power applied to the transducer. Note that each set of data is normalized by the maximum at CW in the absence of particles. At relatively low power up to 17 W, pulsing in the presence of particles (for both 0.6 and 1.2 g/L) provides higher intensity than that in the absence of particles at that power. It is significant that even at relatively low powers particle addition enhances the sonochemical reaction efficiency relative to that with CW excitation. The maximum SCL intensity in the presence of particles obtained with pulsing was 1.9 times higher than that without the addition of particles ((5) compared with (4) in Table 1). It is interesting that the increase (1.9 times) in the sonochemical reaction efficiency with the addition of particles for pulsing is close to that (2 times at most) previously obtained for CW, where the increase in efficiency was investigated for various particle sizes and amounts.9 The intensity peak with pulsing appeared at a lower power than that with CW, which may be due to the increase in the acoustic amplitude with pulsing by the suppression of large degassing bubble generation due to the pulse-off time, compared with that for CW. Therefore, the liquid surface vibration that disturbs the establishment of a resonant standing wave is more evident with pulsing. As the power increased, the intensity with CW increased; the intensity with CW in the

Liquid-Surface Vibration Influence on SCL Intensity

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Figure 4. Example of the time variation in the longitudinal position of the liquid surface under sonication.

Figure 3. Dependence of the measured SCL intensity on the power applied to the transducer.

TABLE 1: Comparison of the Maximum SCL Intensitya

(1) (2) (3) (4) (5) (6)

sonication method

maximum SCL intensity

power to the transducer (W) at the maximum SCL intensity

CW, no particles CW + particles (0.6 g/L) CW + particles (1.2 g/L) pulsed, no particles pulsed + particles (0.6 g/L) pulsed + particles (1.2 g/L)

1 1.46 1.35 0.65 1.22 1.10

44 43 43 10 13 13

a Each intensity is normalized by the SCL intensity for CW excitation when no particles are added.

presence of particles (for both 0.6 and 1.2 g/L) at 43 W was higher than the maximum with pulsing in the presence of 0.6 g/L particles at 13 W (both (2) and (3) compared with (5) in Table 1). Therefore, it should be noted that at higher power with particle addition, the sonochemical reaction with CW can be more effective than that with pulsing. At 28 to 30 W, the intensity with pulsing in the absence of particles approached almost zero, whereas the intensity with pulsing in the presence of particles and for all cases of CW did not change. It is expected that the lowest intensity during pulsing in the absence of particles is due to the highest amplitude of liquid surface vibration, which is discussed later. 3.2. Time Variation in the Longitudinal Position of the Liquid Surface under Ultrasound. Figure 4 shows an example of the time variation in the longitudinal position of the liquid surface under sonication. Data are plotted as the displacement from the position when ultrasound is not applied. Half of the average peak-to-trough distance corresponds to the mean vibration amplitude of the liquid surface. Note that a periodic variation appears every 100 ms, although the period is not necessarily constant and is much lower than that of the ultrasound. The averaged value of the time variation corresponds to the time-averaged displacement of the liquid surface against that without application of ultrasound. Liquid surface vibration18 can exhibit modal motion by the superposition of the surface wave, which propagates from the

Figure 5. Dependence of the liquid surface vibration amplitude on the applied power.

center of the liquid surface to the side wall, and the reflective wave. Hashimoto and Sudo18 have shown that subharmonic waves are excited from observation of ripple stripes on a liquid surface through a longitudinal vibration experiment with a cylindrical vessel in which water is filled up to a certain height and with a driving frequency in the range of 1 to 3500 Hz, although no ultrasound was applied. Modal analysis for liquid surface vibration in the present study is complex because of the presence of ultrasound or particles and has accordingly been left for future study. 3.3. Vibration Amplitude of Liquid Surface. Figure 5 shows the dependence of the liquid surface vibration amplitude on the power applied to the transducer. The plotted data were averaged over three times, and each bar indicates the range of the three values. The vibration amplitude gradually increases as the applied power increases, except for the cases with CW in the presence of particles (0.6 and 1.2 g/L). The vibration amplitude at 1000 cycles ON-1000 cycles OFF in the absence of particles increases rapidly, which is due to lower numbers of generated bubbles than other cases with particles that provide nucleation sites for bubbles or than CW, which has sufficient time to cause coalescence of bubbles leading to large sized bubbles. Note here that the large bubbles in the latter can

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Figure 7. Dependence of the highest longitudinal position of the liquid surface on the applied power. Figure 6. Dependence of the longitudinal position of the liquid surface on the applied power.

contribute to the reflection of sound. Iida et al.19 have shown with a laser diffraction method that when the pulse-on time increases with a fixed pulse-off time and becomes close to the CW condition, not only total bubble volume increases but also the size of the bubbles also increases. The bubble volume decreases and the acoustic amplitude increases with pulsing, and as a result, the amplitude of the surface vibration increases. The addition of particles inhibits the increase in the surface vibration amplitude both for CW and pulsed sonication. Tiny bubbles, due to the addition of particles, absorb the sound energy and pulsate. Accordingly, more sound energy is consumed inside the liquid in the presence of particles, and the addition of particles suppresses the liquid surface vibration. Part of the sound energy contributes to the violent collapse of bubbles, which leads to sonochemical reaction via the formation of oxidants; therefore, the SCL intensity in the presence of particles can be higher than that in the absence of particles, as shown in Figure 3. 3.4. Longitudinal Displacement of the Liquid Surface. Figure 6 shows the dependence of the longitudinal position of the liquid surface on the applied power. The data show the timeaveraged longitudinal displacement from the position under no application of ultrasound. As the applied power is increased, the displacement appears to increase up to 30 W, except for the two cases of CW in the presence of particles, although the data were scattered. Pulsing with 1000 cycles ON and 1000 cycles OFF in the absence of particles causes a decrease in the displacement for applied power greater than 30 W. This is because at this time, the vibration of liquid surface occurs violently not only at around the sound beam axis but also over the entire surface, which means that the sound energy is distributed rather homogeneously over the entire liquid surface. It is reasonable that both of the cases for CW in the presence of particles (0.6 and 1.2 g/L) exhibit lower displacement than under other conditions because the SCL intensities at applied power greater than 23 W are correspondingly higher, as shown in Figure 3. 3.5. Condition for Decrease in SCL Intensity at High Acoustic Power. In this section, we discuss the remarkable decrease in the SCL intensity to around zero for the case of pulsing 1000 cycles ON and 1000 cycles OFF with no particle addition, as shown in Figure 3. Summing the vibration amplitude

shown in Figure 5 and the displacement shown in Figure 6 gives the highest longitudinal position of the liquid surface. Figure 7 shows the highest position normalized by the ultrasound wavelength. The broken line represents one-quarter of the ultrasound wavelength. At powers greater than ∼30 W, the liquid-surface position is over one-quarter of the ultrasound wavelength from its position when ultrasound is not applied. This condition for the liquid surface vibration provides a limit for the establishment of a resonant standing wave that is effective for sonochemical reaction. Bubbles grow and contribute to sonochemical reaction near a pressure antinode in a resonant standing wave. A pressure node is present in the air-liquid interface near the side wall; however, the above condition provides a pressure antinode at the central region in the same horizontal plane. This means that it is impossible to establish a stable horizontal stripe specific to the resonant standing wave. Note here that the contribution from the vibration amplitude to the highest position is dominant in comparison with that from the liquid surface displacement. 3.6. Change of Calorimetric Power Due to Different Sonication in the Presence/Absence of Particles. In this section, the measured temperature rise between CW and pulsing of 1000 cycles ON-1000 cycles OFF in the presence/absence of particles is compared under the same signal amplitude from the function generator when ultrasound is applied. The calorimetric power Pc (W) is calculated using the following equation9,16,17

Pc ) (dT/dt)CpM

(1)

where dT/dt is the temperature rise per second at time zero, Cp is the heat capacity of the liquid, and M is the mass of the liquid. The energy efficiency of ultrasound consumed in the sonicated liquid, Eeff, is determined from the calorimetric power (Pc) divided by the electric power (Pel). Table 2 shows the calculated results. Pc increases by the addition of particles for both entries (1) and (2) with CW and for entries (3) and (4) with pulsing, and Eeff increases accordingly. This indicates that the addition of particles increases the number of collapsing bubbles that play the role of a heat source. The heat is conducted from the interior of the collapsing bubble into the water bulk, with a corresponding increase in the liquid temperature. Consequently, Eeff is almost the same for CW and

Liquid-Surface Vibration Influence on SCL Intensity

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TABLE 2: Calorimetric Power and Energy Efficiency for Different Sonication Conditions in the Presence/Absence of Particles sonication method (1) CW, no particles (2) CW + particles (1.2 g/L) (3) pulsed, no particles (4) pulsed + particles (1.2 g/L)

Eeff (dT/dt) × () Pc/Pel) × 10-2 (K/s) Pc (W) Pel (W) 100 (%) 1.76

14.8

19.2

77

2.07

17.4

19.8

88

0.942

7.91

10.3

77

1.05

8.82

10.2

86

pulsing for both entries (1) and (3) in the absence of particles and for both entries (2) and (4) in the presence of particles. In the present experiment, the maximum SCL intensity increases much more (1.9 times) than Eeff (1.1 times). There is not necessarily a linear correspondence between the energy efficiency and the SCL intensity (or sonochemical yield). In a previous study,9 we found that when particles were added the sonochemical yield increased by at most a factor of 2, whereas the energy efficiency increased by more than a factor of 2 (2.4 ) 1.7 W/0.72 W), which is larger than the increase in the energy efficiency obtained in the present study. 4. Conclusions Quantitative measurement of the amplitude and displacement of liquid surface vibration clarified that under the condition where the surface of the liquid level becomes farther from its position in the absence of ultrasound by a distance of more than one-quarter of the ultrasound wavelength, a significant decrease in the sonochemical reaction efficiency occurs. A stable standing wave at resonance does not form when the amplitude of the

liquid surface vibration exceeds a quarter of the ultrasound wavelength. Particle addition can lead to an enhancement of the sonochemical-reaction efficiency under both pulsing operation and CW. Acknowledgment. This work was partially supported by a Grant-in-Aid (Kakenhi) from the Ministry of Education, Culture, Sports, Science and Technology of Japan. References and Notes (1) Leighton, T. G. 5. Effects and Mechanisms. In The Acoustic Bubble; Academic: London, 1996. (2) Suslick, K. S. Science 1990, 247, 1439. (3) Yasui, K.; Tuziuti, T.; Sivakumar, M.; Iida, Y. J. Chem. Phys. 2005, 122, 224706. (4) Walton, A. J.; Reynolds, G. T. AdV. Phys. 1984, 33, 595. (5) Mason, T. J. 1. An Introduction to the Uses of Power Ultrasound in Chemistry. In Sonochemistry; Oxford University Press: New York, 1999. (6) Madanshetty, S. I.; Apfel, R. E. J. Acoust. Soc. Am. 1991, 90, 1508. (7) Sekiguchi, H.; Saita, Y. J. Chem. Eng. Jpn. 2001, 34, 1045. (8) Keck, A.; Gilbert, E.; Ko¨ster, R. Ultrasonics 2002, 40, 661. (9) Tuziuti, T.; Yasui, K.; Sivakumar, M.; Iida, Y.; Miyoshi, N. J. Phys. Chem. A 2005, 109, 4869. (10) Tuziuti, T.; Yasui, K.; Kozuka, T.; Towata, A.; Iida, Y. J. Phys. Chem. A 2007, 111, 12093. (11) Flynn, H. G.; Church, C. C. J. Acoust. Soc. Am. 1984, 76, 505. (12) Henglein, A.; Ulrich, R.; Lilie, J. J. Am. Chem. Soc. 1989, 111, 1974. (13) Casadonte, D. J.; Flores, M.; Pe´trier, C. Ultrason. Sonochem. 2005, 12, 147. (14) Tuziuti, T.; Yasui, K.; Lee, J.; Kozuka, T.; Towata, A.; Iida, Y. J. Phys. Chem. A 2008, 112, 4875. (15) Pe´trier, C.; Lamy, M.-F.; Francony, A.; Benahcence, A.; David, B.; Renaudin, V.; Gondrexon, N. J. Phys. Chem. A 1994, 98, 10514. (16) Lorimer, J. P.; Mason, T. J.; Fiddy, K. Ultrasonics 1991, 29, 338. (17) Koda, S.; Kimura, T.; Kondo, T.; Mitome, H. Ultrason. Sonochem. 2003, 10, 149. (18) Hashimoto, H.; Sudo, S. Trans. Jpn. Soc. Mech. Eng., B 1983, 49, 1841. (19) Iida, Y.; Ashokkumar, M.; Tuziuti, T.; Kozuka, T.; Yasui, K.; Towata, A.; Lee, J. Ultrason. Sonochem. 2010, 17, 473.

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