Oscillation Of Cavitation Bubbles Research Articles

Optimization of nano-emulsion production in ultrasonic microreactors: The effects of pre-homogenization, circulating ratio and hybrid frequency application

The study first investigated the effects of individual ultrasound and high-speed shearing on emulsification, both obtaining emulsions with average sizes of approximately 1 μm. Subsequently, a combined strategy to utilize shearing instrument producing coarse emulsion for ultrasonic microreactor was proposed. Immiscible liquids were drawn into the stirring chamber by rotating paddles, where they underwent strong shearing and thorough mixing. The pre-mixed liquids were introduced into microchannel and subjected to further processing by oscillating cavitation bubbles. Through optimization of experimental parameters such as residence time and recycle ratio, the average emulsion size was significantly reduced to 522 nm. Finally, a dual-frequency strategy using ultrasound frequencies of 20 and 40 kHz consecutively further reduced the emulsion size to 307 nm. In comparison with conventional sonoreactors and high-pressure homogenizers, this dual-frequency approach demonstrated superior emulsification performance with less surfactant addition and energy input, indicating its high applicability on continuous emulsification process with proper design and optimization.

Effect of ultrasonication on rising characteristics of a single gas bubble

In recent years, ultrasonication has been used to enhance flotation process. Gas bubble is the unique carrier during mineral flotation, and thus its characteristics play an important role. However, the bubble rising characteristics under ultrasound action are not clear. Therefore, this paper aims to investigate the effect of ultrasonication on bubble rising behaviors in deionized water and MIBC (Methyl Isobutyl Carbinol) solution. A laboratory visualization system of bubble motion with ultrasonication was designed based on the high-speed dynamic video technology to record the bubble rising process and analyze bubble rising characteristics. Bubble aspect ratio was found to increase with the surfactant concentration increasing, but the rising velocity was in opposite state. In each solution, a significantly higher bubble sphericity was observed with ultrasonication than that without ultrasonication, conversely, the bubble rising velocity was smaller under the action of ultrasonication. A roughly negative linear relationship was found between the bubble shape and rising velocity, and the ultrasonication diminished this relationship. Marangoni effect was considered as the cause of MIBC affecting the bubble motion, and possible explanations of ultrasonication affecting the bubble motion were given from the perspective of bubble surface wave, cavitation bubble oscillation and sound radiation force.

Cleaning effects due to shape oscillation of bubbles over a rigid boundary

Recent experiments have revealed the interesting cleaning effects that take place due to the shape mode oscillation of bubbles over a rigid boundary. While a microbubble was undertaking shape oscillation moving over a bacterial biofilm, it removed the contaminants from the boundary and created a clean path through the biofilm. This demonstrated much higher cleaning efficiency than that associated with the volume oscillation of cavitation bubbles; however, the mechanism is unknown. Here, we study this phenomenon using the boundary integral method with the viscous effects modeled using the viscous potential flow theory and the compressible effects using the weakly compressible theory. The viscous stress at the rigid boundary is approximated using the boundary layer theory. We observed that the natural frequencies of shape mode oscillation decrease significantly due to the presence of the boundary. The shear stress at the boundary due to the shape oscillation of a nearby bubble is at least 20 times higher than that due to volume oscillation with the same energy and is significant only within the area directly beneath the bubble. This is explained by the notably faster decay for higher shape modes of the kinetic energy in the fluid as the distance to the center of the bubble r increases with the induced velocity of mode k decaying at a rate of O(r−(k+2)) away from the bubble. These results achieve excellent agreement with the intriguing cleaning effects first observed in the experiment and explain the mechanism behind this new highly efficient method of cleaning.

Dynamics of an oscillating cavitation bubble within a narrow gap

The oscillation characteristics of a bubble in a confined space have important implications for various applications, including liquid pumping and mixing and particle conveyance in microfluidic devices. In this study, analytical solution with second-order accuracy and numerical solution are derived for the free oscillation of a single bubble in a narrow gap between parallel plates, and the applicability to dimensionless initial values of the analytical solutions is clarified. Moreover, the free-oscillation characteristics of the bubble within the gap are explored and described and are compared to those of a bubble in an infinite liquid. The primary conclusions are as follows: (1) The inherent nature of bubble oscillation in a gap is significantly influenced by the bubble equilibrium radius, and the oscillation amplitude of different orders of the analytical solution is significantly influenced by the dimensionless initial radius. (2) The difference between the natural frequency and acoustic damping constant during bubble oscillation in a gap and those in an infinite liquid decreases with increasing equilibrium radius, and the value of the difference is not less than 50%. (3) Within the gap, the bubble radius, wall velocity, and wall acceleration of a bubble in a narrow gap predicted by the bubble equation dramatically differ from those of a bubble in an infinite liquid, with the differences increasing with the dimensionless initial radius, where the values of the differences in the acceleration can be as high as the order of 104%.

A method for consistent cavitation bubble generation at different voltages

A study of the dynamics of a single cavitation bubble is fundamental for understanding a wide range of applications in science and engineering. Underwater electrical discharge is a widely used method for generating cavitation bubbles to study their inception, subsequent dynamics, and collapse. In this work, an existing underwater low-voltage discharge circuit for generating cavitation bubbles is improved further to get a wider range of maximum bubble radius. In this novel electric circuit design, the operating voltage can be varied (up to 420 V in steps of 60 V) by connecting a network of capacitors in different series-parallel combinations with the help of relay-based control. Therefore, this device can generate oscillating cavitation bubbles up to a maximum radius of 14 mm by adjusting the available discharge energy. A voltage sensor circuit is included in this design to measure the drop in voltage during the sparking event, and a correlation between the delivered energy and the potential energy of the bubble is established. The dependence of bubble radius on circuit resistance, electrode resistance, and electrode material is studied for the entire voltage range. A suitably rated semiconductor field effect transistor is used as a switch that enables the generation of bubbles of a consistent maximum radius and ensures the repeatability of the experiment. A high-speed imaging system is used to estimate the bubble radius and nucleation period, which are compared with the existing theoretical models based on empty cavity collapse. Results show that delaying the oxidation of electrodes with a protective layer influences the collapse phase and the average pressure inside the spark-generated bubble.

Analysis of suppressive effect of large bubbles on oscillation of cavitation bubble in cavitation field

In this work, the interaction among multiple bubbles in a cavitation field is investigated by combining the experimental observation of small bubbles hovering around large bubbles. A model composed of three bubbles is developed, and the dynamic behavior of cavitation bubble is analyzed. By considering the time delay effect of the interaction among bubbles and the nonspherical oscillation of large bubbles, the modified bubble dynamic equations are obtained. Numerical results show that the nonspherical effect of large bubbles has little effect on the oscillation of cavitation bubble. The suppressive effect of large bubble on cavitation bubble is closely related to the radius of the large bubble. The larger the size of the large bubble, the stronger the suppression is. When the size of large bubble approaches to the resonant radius, the oscillation of cavitation bubble presents coupled resonance response, and the maximum expansion radius of bubble shows a resonance peak. The distribution of the secondary Bjerknes force versus bubble radius and the separation distance is strongly influenced by driving frequencies or sound pressure. When the large bubble is on the order of submillimeter, the intensity of the secondary Bjerknes force and the acoustic response mode are different due to the different intensity of the nonlinear response of the cavitation bubble. As the distance decreases, when the acoustic pressure increases to a certain value, the secondary Bjerknes force on the cavitation bubble decreases due to abnormal acoustic absorption. The secondary Bjerknes force on cavitation bubble is likely to be repulsive at different separation distances. The theoretical results accord well with experimental phenomenon.

Ultrasound-Assisted Synthesis of Potentially Food-Grade Nano-Zinc Oxide in Ionic Liquids: A Safe, Green, Efficient Approach and Its Acoustics Mechanism.

In food application, nano-zinc oxide (nano-ZnO) is a very important nano metal material; thus, it is necessary to prepare potentially food-grade nano-ZnO. Nano-ZnO synthesized by the ultrasound-assisted method can reach a safe level because of its import physical processing characteristics. Firstly, the micromorphology and microstructure of nano-ZnO synthesized by the ultrasonic method were compared with that by the mechanical stirring method through atomic force microscopy, X-ray diffraction, and Fourier-transform infrared. Secondly, the on-line monitoring of different ultrasonic fields in real-time was studied during the whole synthesis process of nano-ZnO by polyvinylidene fluoride sensor, and two control groups (water medium) were set. The results showed that nano-ZnO obtained by the ultrasonic method were smaller in size and had less surface roughness compared with the mechanical stirring method. The nucleation and crystallization process of nano-ZnO was controlled by the ultrasonic method with sharp diffraction peaks of higher intensities. Moreover, for the ultrasonic mechanism, it was found that the oscillation behavior of bubbles varied from liquid to liquid, and variation was also found in the same liquid under different restraint of interfaces. Based on voltage waveforms monitored in the three liquid media, differences in the life cycle of cavitation bubble oscillation, cycle of collapse stage, maximum voltage amplitude, and acoustic intensity were observed. The physical mechanism of ultrasound-assisted synthesis of nano-ZnO was revealed through voltage fluctuations of the acoustics signal, which can lay a theoretical foundation for the controllability of food ultrasonic physical processing.

Microbubble dynamics and jetting near tissue-phantom biointerfaces.

Precise excitation of cavitation is a promising mechanism for microsurgery procedures and targeted drug delivery enhancement. The underlying phenomenon of interest, jetting behaviour of oscillating cavitation bubbles, occurs due to near-surface interactions between the boundary, liquid, and bubble. Within this study we measured boundary effects on the cavitation bubble dynamics and morphology, with an emphasis on observation and measurement of jetting behaviour near tissue-phantom biointerfaces. An important mechanism of boundary poration has been observed using time-resolved optical microscopy and explained for different tissue-phantom surface densities and Young's modulus. Below a critical distance to the boundary, around γ = 1.0, the resulting jets penetrated the tissue-phantom, resulting in highly localized few micrometer diameter jets.

The control effect of dual-frequency ultrasonic excitation on the chaotic oscillation of cavitation bubbles

Abstract Dual-frequency ultrasonic excitation is not only an important mean of inducing ultrasonic cavitation, but also a key approach to control the oscillation characteristics of cavitation bubbles. In this paper, the control effect of dual-frequency excitation on the oscillation characteristics of cavitation bubbles is studied by means of nonlinear dynamic analyses. The results show that both suppression and promotion of the chaotic oscillation of cavitation bubbles can be achieved by dual-frequency excitation, which deeply depends on the parameters setting (second frequency value, pressure amplitude ratio and phase difference). By increasing the second frequency value, reducing the pressure amplitude ratio and keeping the phase difference within a certain range, the transformation of cavitation bubbles from periodic to chaotic oscillation can be promoted. The results of this paper are of great significance for promoting the highly efficient sonochemical reactions.

Mechanism of effect of stable cavitation on dendrite growth in ultrasonic field

Ultrasonic waves used in liquid alloys can produce refined grain structures, which mainly contributes to ultrasonic cavitation and acoustic streaming. According to the bubble lifetime and whether they are fragmented into “daughter” bubbles, acoustic cavitation can be divided into transient cavitation and stable cavitation. Compared with the transient cavitation, the interaction between stable cavitation bubbles and solidifying alloys have been rarely investigated previously . In this work, the effect of stable cavitation on the dendritic growth of succinonitrile (SCN)-8.3% (mole fraction) water organic transparent alloy is systematically investigated by high-speed digital image technique and numerical simulation. It is found that when the bubble migration direction is consistent with that of dendritic growth, the periodic high pressure generated in bubble oscillation process increases the local undercooling, speeding up the dendrites growth effectively. Meanwhile, the concentrated stress inside dendrites induced by the linearly oscillation of cavitation bubble can break up dendrites into fragments. Specifically, if there exist stable cavitation bubbles suspended around the liquid-solid interface, periodically alternating flow field and high shear force in their surrounding liquid phase is produced. As a result, the nearby dendritic fragments will be attracted to those bubbles and then transformed into spherical grains.

The effect of stable cavitation on dendrite growth within ultrasonic field

Applying ultrasonic waves in liquid alloys produces refined grain structures, which is mainly contributed to ultrasonic cavitation and acoustic streaming. According to the bubbles lifetime and whether are fragmented into "daughter" bubbles, acoustic cavitation can be divided into transient cavitation and stable cavitation. As compared with the transient cavitation, the interaction between stable cavitation bubbles and solidifying alloys have been rarely reported in previous literatures. In this work, the effect of stable cavitation on the dendritic growth of succinonitrile (SCN)-8.3mol.% water organic transparent alloy is systematically investigated by high-speed digital image technique and numerical simulation. It is firstly found that when the bubble migration direction was consistent with that of dendritic growth, the periodic high pressure generated during bubble oscillation process increased the local undercooling, promoting the dendrites growth velocity effectively. Meanwhile, the concentrated stress inside dendrites induced by the linearly oscillation of cavitation bubble could break up dendrites into fragments. Specifically, if there were stable cavitation bubbles suspended around the liquid-solid interface, periodically alternating flow field and high shear force in their surrounding liquid phase was produced. As a result, the nearby dendritic fragments would be attracted to those bubbles and then transformed into spherical grains.

Enhancement of oscillation amplitude of cavitation bubble due to acoustic wake effect in multibubble environment

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.

Effect of ultrasonic intensity and intervals of ultrasonic exposure on efficiency of sonochemiluminescence in gel phantom for sonodynamic therapy

Sonodynamic treatment (SDT) is one of the non-invasive modalities for cancer treatment. In SDT, ultrasound, reactive oxygen species (ROS) generated from cavitation bubbles, and a sonosensitizer are used in combination. In this study, high-intensity focused ultrasound (HIFU) was employed as ultrasound to generate and oscillate cavitation bubbles. When cavitation bubbles oscillate and collapse, the gas inside the bubble is extremely compressed and heated, inducing ROS generation. The disadvantage of SDT is a long treatment time because of its smallness of a treatment region by a shot of HIFU. To overcome this, the effect of the intensity and interval of HIFU for oscillating cavitation bubbles was investigated by using luminol sonochemiluminescence and high-speed imaging. The results showed that a HIFU exposure sequence with an interval of 300 ms and a burst-wave intensity of 0.25 kW cm−2 improve the energy efficiency of ROS generation.

Ultrasound-Assisted Through-Mask Electrochemical Machining of Hole Arrays in ODS Superalloy.

Micro-hole arrays have found wide applications in aerospace, precision instruments, and biomedicine. Among various methods of their production, including mechanical, laser, and electrical discharge, electrochemical machining (ECM) is considered the most lucrative due to its wide processing range, high surface quality, and excellent productivity. In particular, ultrasound-assisted through-mask ECM exhibits an enhanced machining precision due to ultrasonic cavitation, which promotes the removal of the electrolytic products and bubbles. In this study, the equation of cavitation bubble oscillation was derived and numerically solved to study the influence of six different parameters on the ultrasonic cavitation and electrolysis process, and their optimal values were determined. The feasibility of the proposed ultrasound-assisted through-mask ECM technology with the optimized parameters was experimentally corroborated by the fabrication of a high-quality hole array in an oxide dispersion strengthened (ODS) MA956 superalloy.

Dynamics of the passive pulsation of a surface-attached air bubble subjected to a nearby oscillating spark-generated bubble

The dynamics of a spark-generated bubble (a discharge short circuit) generated in proximity to a stationary air bubble attached to a plate is experimentally investigated by high-speed photography. Numerous interesting and complex interactions occur during the two bubble coupling pulsation owing to the deformation properties or “free surface” characteristics supplied to the plate by the attached air bubble. Complex bubble jetting behaviors, such as bubble splitting, jets away from the plate, variable directional jets, and multidirectional jets are observed. Passive pulsation of the air bubble is observed in response to the spark bubble. Moreover, five types of bubble behaviors are summarized: bubble coalescence, the air bubble skirt phenomenon, the “mountain”-shaped bubble, and the “cup cover”-shaped air bubble with or without splitting. To develop a better understanding of the coupling interactions between the two bubbles during their oscillations, four types of bubble volume–time curves are summarized using the image outline identification code established to obtain information regarding the bubble shape. The complex phenomena during the two-bubble interactions, such as the bubble jetting direction, air bubble shapes, and volume–time curves, are summarized as graphs and are highly dependent on the bubble size ratio, dimensionless cavitation bubble oscillation time, and initial displacement parameter.

Numerical study on dual-frequency ultrasonic enhancing cavitation effect based on bubble dynamic evolution.

Ultrasonic cavitation is a physical dynamic phenomenon of bubbles inflation, compression, and collapse in liquid. A dual-frequency ultrasonic cavitation dynamics model is established in this paper to investigate dynamic evolution of bubble under single and dual frequency ultrasonic modes. The variation of bubble radius, pressure, energy, temperature, and number of water vapor molecules inside the bubble in single and dual frequency ultrasonic modes are analyzed, respectively. The results show the oscillation of cavitation bubbles is more unstable and easier to collapse in dual-frequency ultrasound field than those in single-frequency ultrasound field. With the increase of the ultrasonic frequency, cavitation effect is weakened due to the shortage of oscillation period. Under the same ultrasonic power, the maximums of bubble radius, pressure, and water vapor molecules number inside the bubble in the dual-frequency mode are larger than those in the single-frequency mode. Under the ultrasonic excited by 50 kHz + 70 kHz, the maximum bubble radius and pressure can reach 36.061 μm and 2285.9 MPa, respectively, which are much larger than 18.183 μm, 730.61 MPa at 50 kHz and 14.576 μm, 332.25 MPa at 70 kHz. The calculation results of three different frequency combinations (30 kHz + 50 kHz, 40 kHz + 60 kHz and 50 kHz + 70 kHz) indicate dual-frequency ultrasound can significantly enhance the cavitation effect.

Uniform Acoustic Cavitation of Liquid in a Disk**Supported by the National Natural Science Foundation of China under Grant Nos 11574150 and 11334005

A dynamical propagation model coupled to the oscillation of cavitation bubbles is applied to describe the imploding acoustic field in a cavitating liquid where the acoustic waves transmit from the outside to the inside of a circle disk. Numerical simulation shows that the imploding ability of a ring source can elevate the sound pressure or partly eliminate the decay due to both the bulk attenuation and the attenuation caused by cavitation. However, the imploding ability is limited and there exists a critical radius. When the radius of the disk is larger than the critical one, the imploding ability is not enough to eliminate the attenuation. Fortunately, the cavitation region can be effectively expanded if a hot plate is attached under the center of the disk because the cavitation threshold is related to the temperature of the liquid, which means that a region with good uniformity of cavitation can be enhanced by adjusting the temperature difference between the central and side liquid.

Low-intensity ultrasound induced cavitation and streaming in oxygen-supersaturated water: Role of cavitation bubbles as physical cleaning agents

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

Letter: Entrapment and interaction of an air bubble with an oscillating cavitation bubble

The mechanism of the formation of an air bubble due to an oscillating cavitation bubble in its vicinity is reported from an experimental study using high-speed imaging. The cavitation bubble is created close to the free surface of water using a low-voltage spark circuit comprising two copper electrodes in contact with each other. Before the bubble is created, a third copper wire is positioned in contact with the free surface of water close to the two crossing electrodes. Due to the surface tension at the triple point (wire-water-air) interface, a small dip is observed in the free surface at the point where the wire is immersed. When the cavitation bubble is created, the bubble pushes at the dip while expanding and pulls at it while collapsing. The collapse phase leads to the entrapment of an air bubble at the wire immersion point. During this phase, the air bubble undergoes a “catapult” effect, i.e., it expands to a maximum size and then collapses with a microjet at the free surface. To the best of our knowledge, this mechanism has not been reported so far. A parametric study is also conducted to understand the effects of wire orientation and bubble distance from the free surface.

Cavitation bubble oscillation period as a process diagnostic during the laser shock peening process

Laser shock peening (LSP) technology is a laser-induced shock process implemented as a surface enhancement technique to introduce beneficial compressive residual stresses into metallic components. The process employs water to confine and enhance the pressure pulse delivered to the target. For thick water layers, or fully water immersed LSP, a cavitation bubble is generated by the surface vaporization of the LSP laser pulse. This research shows that the first bubble oscillation period of the cavitation bubble can be used to characterize effective and repeatable energy delivery to the target. High-speed shadowgraphy is implemented to show that variations in the bubble period occur before visual observations of dielectric breakdown in water. The diagnostic potential of the first bubble oscillation period is used to identify the dielectric breakdown threshold of water, which shows an increase with increasing water quality measured by water conductivity.