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Acoustic cavitation bubbles are considered to be generated due to heterogeneous nucleation because a smaller amount of energy is required in this case compared with homogeneous nucleation. A typical example of such nucleation sites is tiny cracks filled with gas, commonly termed as "gas pocket." Such microcracks can exist in abundance on the surface of reactor walls or sonotrodes used for the cavitation treatment. Eventually, the heterogeneously nucleated cavitation bubble is detached from the solid surface; however, the detachment mechanism has been unclear so far. Therefore, the present study numerically investigated the dynamic behavior of the cavitation bubble originated from the heterogeneous nucleation at wall cracks. During propagation of an ultrasound wave through a liquid, gas pockets in cracks oscillate sinusoidally when the sound pressure amplitude and crack radius are small. In this case, the bubble cannot detach from the surface. On the other hand, as the sound pressure amplitude, crack size, and/or contact angle become larger, the bubble begins to oscillate non-linearly that causes liquid jetting. The liquid jetting can cause the bubble detachment from the surface due to the interaction between the liquid jet and the bottom wall. Stronger liquid jetting makes many small detached bubbles. [ABSTRACT FROM AUTHOR]

• Mass transfer across a gas–liquid interface of acoustic cavitation is investigated. • Time-averaged Sherwood number is decreased with sound pressure amplitude. • Time-averaged Sherwood number can be negative value due to area and shell effect. • The instantaneous Sherwood number is varied in the range of O (101) ∼ O (102). In the present study, mass transfer across a gas–liquid interface of acoustic cavitation bubble is numerically investigated. The volume of fluid (VOF) method coupled with a continuous species transfer model is used to simulate the bubble dynamic behavior and mass transfer across the gas–liquid interface. Chemical species are transferred repeatedly between the liquid and gas phases in accordance with the bubble oscillation, and the chemical species are transferred into the bubble during an expansion phase, and to the outside of bubble during a compression phase. Time-averaged Sherwood number is decreased with an amplitude of sound pressure due to area effect and shell effect , and its value becomes negative at high pressure amplitude. Meanwhile, the instantaneous Sherwood number is largely varied in accordance with the bubble oscillation, and its magnitude becomes O (101) ∼ O (102). [ABSTRACT FROM AUTHOR]

Aiming at improving the efficiency of cavitation treatment, this work investigates characteristics of acoustic streaming and cavitation generated in water by dumbbell-shaped sonotrodes with plane, truncated and conical tips. The main emphasis was placed on elucidating the effects of tip shape and vibration amplitude ranged from 40 to 60  μ m . The PIV technique and Weissler reaction were used to measure flow pattern and velocity of acoustic streaming, and cavitation efficiency, respectively. To provide a theoretical explanation to the experimental results, a self-developed mathematical model was used to simulate the acoustic streaming and predict the size of cavitation zone numerically. Both the experimental and numerical results revealed that the sonotrode tip shape affects the acoustic streaming significantly, altering the flow magnitude and direction from fast and downward under the plane and truncated tips to relatively slow and upward near the conical tip. Besides, the conical tip provides a more efficient cavitation treatment in comparison with the plane and truncated tips. The simulation results showed that widening of cavitation zone and altering of acoustic streaming velocity and direction near the sonotrode tip are responsible for the enhancement of cavitation treatment efficiency. [ABSTRACT FROM AUTHOR]

Acoustic cavitation occurs in ultrasonic treatment causing various phenomena such as chemical synthesis, chemical decomposition, and emulsification. Nonlinear oscillations of cavitation bubbles are assumed to be responsible for these phenomena, and the neighboring bubbles may interact each other. In the present study, we numerically investigated the dynamic behavior of cavitation bubbles in multi-bubble systems. The results reveal that the oscillation amplitude of a cavitation bubble surrounded by other bubbles in a multi-bubble system becomes larger compared with that in the single-bubble case. It is found that this is caused by an acoustic wake effect, which reduces the pressure near a bubble surrounded by other bubbles and increases the time delay between the bubble contraction/expansion cycles and sound pressure oscillations. A new parameter, called "cover ratio" is introduced to quantitatively evaluate the variation in the bubble oscillation amplitude, the time delay, and the maximum bubble radius. [ABSTRACT FROM AUTHOR]

• Cavitation bubbles implode causing fine droplets of bismuth to form emulsion. • Acoustic streaming transfers bismuth droplets throughout the melt. • After solidification, 90% of bismuth particles have size under 10 m. • Most bismuth particles remain embedded inside aluminum grains. The aim of this study is to develop an ultrasound-assisted process for fabrication of frozen emulsion alloys, which are a type of miscibility gap alloys (MGAs). Al-Bi system was used to produce 2-kg ingots of Al-Bi frozen emulsion alloys in the following two steps: (1) emulsification treatment of Al-Bi melt by introducing high-intense ultrasound vibrations into it in a temperature range of Al-Bi binodal decomposition, and (2) casting of the melt into a book mold. The ingot samples were analyzed with SEM/EDX for the alloy microstructure and bismuth particle distribution. Besides, cold model experiments, thermodynamic and CFD numerical simulations were performed to elucidate the underlying mechanisms of emulsification including droplet fragmentation and acoustic streaming. The results reveal that the alloys with Bi concentration of 4%~9.4% can be fabricated by the proposed technique. Within this range, 90% of Bi particles have a diameter under 10 µm, and the majority of them are incorporated into aluminum grains. Especially, a uniform microstructure is obtained when concentration of Bi is less than 6%. Emulsification was found to occur at the interface between two liquids when cavitation bubbles collapse in the interface vicinity followed by high-velocity micro-jetting which is responsible for the emulsion droplet formation. Another positive effect of ultrasound is acoustic streaming which is responsible for the effective agitation of molten bath preventing the sedimentation of Bi droplets during the ultrasound treatment and casting operation. [ABSTRACT FROM AUTHOR]

• An emulsification process of water-gallium system during ultrasound irradiation was investigated. • Formation of ultrafine droplets occurs when a greatly expanded cavitation collapses near the water-gallium interface and the distance between the droplet and bubble is small. • A high-velocity liquid jet makes a cavity in the surface of gallium forcing it to take a crown shape that eventually results in the droplet fragmentation. • The above mechanism is supported by a simplified model of droplet formation considering the balance of energy and pressure. Aiming at elucidating ultrasonic emulsification mechanisms, the interaction between a single or multiple acoustic cavitation bubbles and gallium droplet interface was investigated using an high-speed imaging technique. To our best knowledge, the moment of emulsification and formation of fine droplets during ultrasound irradiation were observed for the first time. It was found that the detachment of fine gallium droplets occurs from the water-gallium interface during collapse of big cavitation bubbles. The results suggest that the maximum size of cavitation bubble before collapsing is of prime importance for emulsification phenomena. Previous numerical simulation revealed that the collapse of big cavitation bubble is followed by generation of high-velocity liquid jet directed toward the water-gallium interface. Such a jet is assumed to be the prime cause of liquid emulsification. The distance between cavitation bubbles and water-gallium interface was found to slightly affect the emulsification onset. The droplet fragmentation conditions are also discussed in terms of the balance between (1) interfacial and kinetic energies and (2) dynamic and Laplace pressure during droplet formation. [ABSTRACT FROM AUTHOR]

• Combination of acoustic cavitation and plasma results in an enhancement of degradation efficiency. • Acoustic cavitation assisted plasma can be created in wide ranges of pH and electrical conductivity. • Higher flow rates of circulating solutions cause a further enhancement in the degradation efficiency. • The process proposed has the potential to extend the application area of underwater plasma. The present study investigates the wastewater treatment performance of an acoustic cavitation assisted plasma (ACAP) process in a circulating reactor using Rhodamine B (RhB) as a model water pollution. The concept of this process was proposed by the authors recently for a batch type rector. The measurements revealed that combining the ultrasound irradiation with pulsed discharge plasma allows the RhB degradation efficiency to be drastically increased as compared with the plasma-alone case. This effect is especially significant at higher values of solution electrical conductivity examined in a range of 20 ~ 400 μS/cm. Acidic conditions and larger flow rates of solution were found to be favorable for the degradation efficiency. The effect of flow rate was also analyzed through numerical simulation. The results indicated that the mass transfer of RhB to the plasma-cavitation zone is one of the controlling parameters influencing the degradation performance. Behavior of bubbles and pulse discharge frequency were examined using a high-speed video camera. Relatively large bubbles were found to favor the plasma pulse generation and propagation when move near the high-voltage electrode. On the whole, the results of this study suggest that the ACAP process has the potential to synergistically extend the application area of underwater plasma in both research and industry. [ABSTRACT FROM AUTHOR]

• A liquid jet of acoustic cavitation near a droplet was investigated numerically. • The liquid-jet strength becomes strong with increase of ultrasonic pressure amplitude. • The liquid-jet direction depends on the material of droplet. • The directionality of liquid jet can be explained by the location of high-pressure spot. The present study numerically investigates liquid-jet characteristics of acoustic cavitation during emulsification in water/gallium/air and water/silicone oil/air systems. It is found that a high-speed liquid jet is generated when acoustic cavitation occurs near a minute droplet of one liquid in another. The velocity of liquid jet significantly depends on the ultrasonic pressure monotonically increasing as the pressure amplitude increases. Also, the initial distance between cavitation bubble and liquid droplet affects the jet velocity significantly. The results revealed that the velocity takes maximum values when the initial distance between the droplet and cavitation bubble is moderate. Surprisingly, the liquid jet direction was found to depend on the droplet properties. Specifically, the direction of liquid jet is toward the droplet in the case of water/gallium/air system, and vice versa the jet is directed from the droplet in the case of water/silicone oil/air system. The jet directionality can be explained by location of the high-pressure spot generated during the bubble contraction. [ABSTRACT FROM AUTHOR]

• Cavitation bubble fragmentation was investigated through a three-dimensional simulation and experiment. • The bubble takes a non-spherical shape resulting from the pressure distribution around the bubble. • The non-spherical oscillations of bubble are amplified during its high-frequency oscillations after Rayleigh collapse. • The amplified oscillations cause the bubble fragmentation. In the present study, dynamic behavior and fragmentation mechanism of acoustic cavitation bubbles are investigated under relatively small pressure amplitudes of ultrasonic wave through a three-dimensional compressive multiphase flow simulation and experimental observations. It is found that the oscillating bubble takes a non-spherical shape soon after occurring the Rayleigh collapse following the sound pressure distribution around the bubble. Then, the amplitude of non-spherical deformation is enhanced during small high-frequent oscillations after the Rayleigh collapse due to the fluid inertial effect. Finally, the oscillating bubble is fragmented into two smaller ones with the Laplace pressure gradient becoming the final trigger of bubble fragmentation. Besides, the results reveal that the temperature of bubble surface is varied when the non-spherical bubble deformation is large, while during spherical bubble oscillations the surface temperature remains almost unchanged. [ABSTRACT FROM AUTHOR]

Highlights • Investigations on acoustic cavitation assisted plasma were conducted with a novel reactor. • Treatable range of solution electric conductivity has been remarkably expanded with acoustic cavitation. • Micro cavitation bubbles behave as "bridges" to help plasma propagate through water. • ACAP process shows good performance on Rhodamine B degradation with synergistic effects. Abstract A novel wastewater treatment process, acoustic cavitation assisted plasma (ACAP) is proposed in this study aiming at expanding the treatable range of water pollutants due to a synergetic effect of ultrasound irradiation and high voltage plasma discharge. In this process, the role of acoustic cavitation is not only to provide generation of chemically active OH radicals, as for example in conventional ultrasonic wastewater treatment techniques, but also to ensure conditions for stable plasma generation in wastewater and, thus, to extend the treatable range of water pollutants. Rhodamine B (RhB) was used as a model pollutant in experiments examining effects of ultrasound amplitude, RhB initial concentration, output voltage, solution pH and electrical conductivity on the RhB degradation efficiency. The results revealed that the ultrasound-assisted plasma generation requires lower output voltages and allows to increase the acceptable range of electrical conductivity of treatable solutions up to 1000 μS/cm, that is about 24 times higher than in the case of conventional plasma discharge treatment. The alkaline and acid medium were found to be favorable for higher degradation efficiency. Additional measurements and results of recent investigations concerning underwater plasma showed that microbubbles presented in cavitation zone could serve as "bridges" making the pulse discharge propagation between the electrodes easier than in the conventional case. Besides, acoustic cavitation assists a faster transition of plasma discharge from ineffective streamer type to more effective spark type that further contributes to the improvement of the treatment performance. [ABSTRACT FROM AUTHOR]