Interaction of two differently sized oscillating bubbles in a free field

1/2 order subharmonic waves of two cavitation bubbles

In the work, the 1/2 order subharmonic wave of two coupling cavitation bubbles is investigated numerically via Fourier spectrum analysis. By analyzing the dynamics of bubble, we find that the mutual interaction between bubbles can affect the appearance of 1/2 order subharmonic. The results of parameter dependence show that the intensity of 1/2 order subharmonic would be promoted or inhibited with the increase of mutual interaction. The higher the driving amplitude or the smaller the distance between bubbles, the stronger the mutual interaction is, and also the greater the promotion or suppression of the 1/2 order subharmonic is. Moreover, while the 1/2 order subharmonic occurs, the energy of bubble would alternate between two different peaks, and the temperature inside bubble has a similar fluctuation while the bubble collapses. This qualitative analysis suggests that the bubble's dynamics for multi-bubble case is complex. Understanding the generation of subharmonic of bubble's dynamics is of great significance for helpful applying of cavitation bubble.

Interaction between cavitation bubbles and plastrons on superhydrophobic surfaces

The interaction between cavitation bubbles and plastrons on superhydrophobic surfaces was investigated using a low-voltage discharge device and high-speed photography techniques. The plastron adhered to the superhydrophobic surface acts as a liquid-gas interface, giving the boundary the ability to repel cavitation bubbles. The direction of bubble collapse is determined by the vector synthesis of the Bjerknes repulsive force from the plastron and the Bjerknes attractive force from the rigid wall when the bubble collapses for the first time. Various collapse behaviors were observed, including bubbles moving away from the plastron, bubbles orienting towards the plastron, and bubbles splitting into sub-bubbles in opposite directions. During the subsequent evolution of the bubbles, the expansion of the plastron led to the reversal of the downward jet or reduced the impact velocity of the jet. Seven jet patterns were identified based on the evolution of the cavitation bubble. Starting from the impact velocity of the jet, three jet patterns, namely, the jet away from the plastron (JA), the funnel-shaped jet away from the plastron (JAF), and the funnel-shaped jet away from the plastron with vortex shedding (JAFV), were found to have a weaker effect on the boundary. Three criteria for the design of plastrons on superhydrophobic surfaces were established: VP>0.25Vmax, HP>0.55Rmax, DP>1.2Rmax. Passive pulsation of the plastron in response to the cavitation bubble exhibited similar behaviors across seven jet patterns except for the JAF pattern: torus-shaped, dish-shaped, and skirt-shaped. The dimensionless wall distance, volume ratio, and plastron morphology parameters were identified as significant factors influencing the interaction between cavitation bubbles and the plastron.

Amplification of Supersonic Microjets by Resonant Inertial Cavitation-Bubble Pair

We reveal for the first time by experiments that within a narrow parameter regime, two cavitation bubbles with identical energy generated in antiphase develop a supersonic jet. High-resolution numerical simulation shows a mechanism for jet amplification based on toroidal shock wave and bubble necking interaction. The microjet reaches velocities in excess of 1000  m s^{-1}. We demonstrate that potential flow theory established for Worthington jets accurately predicts the evolution of the bubble gas-liquid interfaces unifying compressible and incompressible jet amplification.

Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy

Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultra-high-speed shadowgraph imaging, hydrophone measurements, three-dimensional passive cavitation mapping (3D-PCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated "pear-shaped" bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laser-induced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A non-circular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and self-intensified collapse of an inverted "triangle-shaped" or "horseshoe-shaped" bubble. Third, high-speed shadowgraph imaging and 3D-PCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a "smiling-face" shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pear-shaped bubble are decisive for the solid damage.

Numerical investigation of two-microbubble collapse and cell deformation in an ultrasonic field

Two-microbubble collapse near a spherical cell in an ultrasonic field is numerically analyzed by extending a level-set method for compressible multiphase flows with bubble and cell multiple interfaces. Computations performed with different bubble-bubble distances and size ratios demonstrate various bubble-bubble interactions, such as bubble coalescence, bubble repulsion and attraction, jet penetration into the bubble, and jet collision. The interactions between collapsing bubbles are found to produce strong liquid jet formation and result in significant cell deformation compared to single-bubble collapse. The optimal bubble-bubble distance and size ratio for cell deformation are presented via contour maps based on extensive computations. The influences of the ultrasonic amplitude and frequency on cell deformation are further investigated.

Quantification of motion characteristics of vertically ascending bubbles in NaCl solution via image processing

In this study, the motion characteristics of bubbles in a vertical rectangular glass groove were investigated using image-processing techniques. A new metric (bubble offset) to evaluate the level of solution disturbance was proposed. The motion characteristics, such as the trajectory, bubble diameter, rise velocity, and coalescence, during the bubble rise were studied using different NaCl concentrations and airflow rates. The functional relationship of the bubble diameter and velocity with height was obtained by least-squares fitting (correlation coefficient R2 ≥ 95%), and fits the power-law distribution. The results showed that the bubble motion characteristics were positively correlated with the airflow rate and negatively correlated with the NaCl concentration in the solution. The solution disturbance was closely related to the bubble offset, and the size of the bubble offset can be used to characterize the degree of the solution disturbance. The disturbance of the solution affects the bubble trajectory, rising velocity and coalescence. The bubble offset changed periodically with height, and its functional relationship was obtained. The accuracy of the functional relationship was verified using experimental data. The study of bubble motion characteristics is crucial for understanding phenomena such as heat and mass transfer and multiphase cooling.

Interaction of two bubbles with distortion in an acoustic field

Expression of the secondary Bjerknes force of two bubbles is obtained by considering the distrotion of two bubbles. The secondary Bjerknes forces in different acoustic fields are simulated, and the influence factors are analyzed and discussed. It is shown that the distortion of a bubble has an important influence on the interaction of two bubbles. The strength and even the directions of the secondary Bjerknes force of two bubbles with distortion differ considerably from the predictions of the sherical symmetry theory. The results show that when two bubbles oscillated stably in an acoustic field, the secondary Bjerknes force of two bubbles with distortion is several times more than that of two spherical bubbles in the same condition. The secondary Bjerknes force of two bubble with distortion has more interaction distance than that of two spherical bubbles. The secondary Bjerknes force of two bubbles with distortion depends on the distance of two bubbles, the shape mode of two bubbles, the equilibrium radii of two bubbles and the driving acoustic filed. The nonspherical distortion effects of the secondary Bjerknes has an importance on understanding the structure formation of bubbles and evolution process of bubble group in an acoustic field.

Acoustic cavitation-induced shear: a mini-review

Acoustic cavitation (or the formation of bubbles using acoustic or ultrasound-based devices) has been extensively exploited for biological applications in the form of bioprocessing and drug delivery/uptake. However, the governing parameters behind the several physical effects induced by cavitation are generally lacking in quantity in terms of suitable operating parameters of ultrasonic units. This review elaborates the current gaps in this realm and summarizes suitable investigative tools to explore the shear generated during cavitation. The underlying physics behind these events are also discussed. Furthermore, current advances of acoustic shear on biological specimens as well as future prospects of this cavitation-induced shear are also described.

Experimental investigations of three laser-induced synchronized bubbles

Herein, we investigated experimentally the dynamics of three laser-induced, same-sized, symmetrically aligned, and synchronized bubbles. Three synchronized laser beams split from the same beam using a Diffractive Optical Element splitter were focused on water, and then we obtained three bubbles. Another nanosecond laser pulse was used to probe the bubbles to obtain shadowgraphs. The exact delay of the excited and detected light was controlled using a delay generator. The results revealed that the maximum volumes of bubbles in arrays decrease as the normalized distance falls, while the lifetimes and translation increase. It was explained by the interaction between the acoustic radiation of bubbles and the surrounding bubbles. The shrinkage of linear bubble arrays exists an anomaly. The center bubbles were stretched, to ellipsoid, stick, even fractured, by the peripheral bubbles. The closer they are, the more distinct is the above phenomenon. However, when the normalized distance was sufficiently small, instead of being stretched, the center bubbles were compressed to disk shape and thus shrank with the whole array. Finally, the dependence of the distance on the energy transfer of the bubble system is also discussed.

Spatiotemporal control of micromechanics and microstructure in acoustically-responsive scaffolds using acoustic droplet vaporization

Acoustically-responsive scaffolds (ARSs), which are composite fibrin hydrogels, have been used to deliver regenerative molecules. ARSs respond to ultrasound in an on-demand, spatiotemporally-controlled manner via a mechanism termed acoustic droplet vaporization (ADV). Here, we study the ADV-induced, time-dependent micromechanical and microstructural changes to the fibrin matrix in ARSs using confocal fluorescence microscopy as well as atomic force microscopy. ARSs, containing phase-shift double emulsion (PSDE, mean diameter: 6.3 μm), were exposed to focused ultrasound to generate ADV - the phase transitioning of the PSDE into gas bubbles. As a result of ADV-induced mechanical strain, localized restructuring of fibrin occurred at the bubble-fibrin interface, leading to formation of locally denser regions. ADV-generated bubbles significantly reduced fibrin pore size and quantity within the ARS. Two types of ADV-generated bubble responses were observed in ARSs: super-shelled spherical bubbles, with a growth rate of 31 μm per day in diameter, as well as fluid-filled macropores, possibly as a result of acoustically-driven microjetting. Due to the strain stiffening behavior of fibrin, ADV induced a 4-fold increase in stiffness in regions of the ARS proximal to the ADV-generated bubble versus distal regions. These results highlight that the mechanical and structural microenvironment within an ARS can be spatiotemporally modulated using ultrasound, which could be used to control cellular processes and further the understanding of ADV-triggered drug delivery for regenerative applications.

Numerical analysis of the effect of bubble distribution on multiple-bubble behavior

Understanding multiple-bubble behavior in a megasonic field is essential for efficient megasonic nanodevice cleaning without pattern damage. In this study, we numerically studied the effects of equilibrium radius and initial void fraction on multiple-bubble behavior and induced pressure. We analyzed the nonspherical collapse, coalescence, and breakup of bubbles in a megasonic field using a compressible, locally homogeneous model of a gas-liquid two-phase medium. Bubbles were simulated with a uniform equilibrium radius or with a bubble size distribution. Our results indicate that the bubble behavior and induced pressure depended mainly on the initial void fraction. For the case of bubbles with uniform equilibrium radius, small bubbles generated high wall pressure at large initial void fractions. When there was a size distribution, bubbles with small equilibrium radii contributed little to the wall pressure because of the damping effect of the oscillation of larger bubbles. Furthermore, the addition of a large bubble suppressed the resonant behavior of the bubbles that induced high wall pressure.

Numerical investigation on the collapse of a bubble cluster near a solid wall

This paper studies numerically the collapse of a cluster of cavitation bubbles (as a primitive model for a bubble cloud) near a solid wall. The homogeneous two-phase mixture model is used, with the liquid-vapor interface resolved by volume of fluid method. The liquid is treated as compressible, allowing the propagation of pressure waves at the speeds determined by a state equation. This cluster consists of 27 identical bubbles, evenly distributed in a cubic region, with various bubble-wall and bubble-bubble distances considered. Our simulations suggest that the bubble-wall distance plays a more significant role. The maximum impulsive pressure of 41MPa is achieved when the cluster is very close to the wall. The inward progress of collapse is observed by examining the evolutions of bubble shapes and flow fields, with two distinctly different sequences of collapse identified between the small and large bubble-wall distances. At a large bubble distance, the centermost bubble is the last to collapse, while at a small bubble distance, it is the central bubble nearest to the wall which collapses lastly. This difference can also explain the more intensive impulsive pressure for the smaller bubble-wall distances. The proposed numerical approach is of special interest because it can resolve the details of bubble-bubble and bubble-wall interactions, which are significant to the study of the collapse of a cavitation cloud, and its potential damage to hydraulic systems.

Jet and Shock Wave from Collapse of Two Cavitation Bubbles

As a common hydrodynamic phenomenon, multi-cavitation dynamics is widely found in many industries such as hydraulic engineering, shipping industry and chemical industry. The jet and shock wave phenomenon in the interaction of two cavitation bubbles are the basis of multi-cavitation bubbles interaction research. By respectively inducing two cavitation bubbles through laser and underwater low-voltage discharge, this paper tested the jet and shock wave resulting from the collapse of the two cavitation bubbles, and the following conclusions are obtained: (1) If the two cavitation bubbles are synchronously generated but in different size, as the distance between the two cavitation bubbles increases or the maximum radius of the smaller cavitation bubble increases, the effect of the small cavitation bubble on the larger one gradually changes from the surface wave phenomenon to jet that breaks through the larger bubble. When the two bubble center lines are parallel to the wall surface, this jet suppresses the formation of the jet to the wall surface when the large cavitation bubble collapses; if the two cavitation bubbles are generated at the same time with same size, as the initial distance of the two cavitation bubbles gradually decreases, the two bubbles are more likely to form a face-to-face collapse, and the smaller the distance between the two, the easier it is to fuse. (2) The impact of the initial moment of the cavitation bubble on the structure of the collapse shock wave is as follows: for two bubbles of different sizes formed synchronously, the shock wave propagates to the periphery in the form of a number of consecutive waves appearing in the larger bubble, while for the unsynchronized ones, shock waves appeared in both cavitation collapses, and a number of consecutive waves appear in the late-formed cavitation bubble. And multiple consecutive shock waves may overlap in some areas of the space. These conclusions have obvious implications for preventing cavitation damage and utilization of cavitation.

Experimental investigation on multiple breakdown in water induced by focused nanosecond laser

Multiple breakdowns in liquids still remains obscure for its complex, non-equilibrium and transient dynamic process. We introduced three methods, namely, plasma imaging, light-scattering technique, and acoustic detection, to measure the multiple breakdown in water induced by focused nanosecond laser pulses simultaneously. Our results showed that linear dependence existed among the cavitation-bubble lifetime, the far-field peak pressure of the initial shock wave, and the corresponding plasma volume. Such a relationship can be used to evaluate the ideal size and energy of each bubble during multiple breakdown. The major bubble lifetime was hardly affected by the inevitable coalescence of cavitation bubbles, thereby confirming the availability of light-scattering technique on the estimation of bubble size during multiple breakdown. Whereas, the strength of collapse-shock-wave and the subsequent rebound of bubbles was strongly influenced, i.e., the occurrence of multiple breakdown suppressed the cavitation-bubble energy being converted into collapse-shock-wave energy but enhanced conversion into rebound-bubble energy. This study is a valuable contribution to research on the rapid mixing of microfluidics, damage control of microsurgery, and photoacoustic applications.

Acoustic generation of underwater cavities-Comparing modeled and measured acoustic signals generated by seismic air gun arrays

Underwater vapor cavities can be generated by acoustic stimulation. When the acoustic signals from several air guns are reflected from the sea surface, the pressure drop at some locations is sufficient for cavity growth and subsequent collapse. In this paper the generation of multiple water vapor cavities and their collapses are numerically modeled and the results are validated by comparing with field data from a seismic air gun array test. In a first modeling attempt where cavity interaction is neglected, a correspondence between measured and modeled data is found. Then, this correspondence is improved by assuming that the acoustic signal generated by the other cavities changes the hydrostatic pressure surrounding each cavity. This modeling can be used to estimate the amount and strength of high frequency signals generated by typical marine air gun arrays, given that a calibration step is performed prior to the modeling.

Cavitation microstreaming generated by a bubble pair in an ultrasound field

An analytical theory has been developed to calculate the acoustic streaming velocity inside and outside bubbles for the case of a bubble pair suspended in an unbounded viscous liquid, taking into account the two predominant modes of a bubble pair: The volume and translation modes. It was demonstrated that the interaction between bubbles can affect the magnitude and direction of acoustic streaming, especially for tangential components outside of bubbles. The acoustic streaming intensifies as the radius of the neighboring bubble increases and is weakened as the distance between the bubbles increases.