Interaction between shock wave and single inertial bubbles near an elastic boundary

Dynamics of a single cavitation bubble near an oscillating boundary

Cavitation and its effects are well investigated, especially single bubble cavitation and its collapse near rigid and elastic boundaries. In our current article, we investigated novel experiments of a single cavitation bubble near an oscillatory boundary. We generated the cavitation bubble by laser focusing in water. A flat glass plate was fixed to the shaft of the magnetostriction oscillator coil. We investigated the dynamics of bubbles at two relative wall distances (ratio of the distance between the bubble center and plate surface to the maximum radius of the bubble) of the bubble from the glass plate in combination with four modes of oscillation. Each mode has specific frequency and amplitude of oscillation. The high-speed camera captured the dynamics of the bubble using the back-illumination method with a framing rate of 120Kfps and simultaneously we used an optical CMOS sensor to measure the oscillation of the glass plate. We presented a clear comparison among the bubble dynamics near stationary and oscillating plates with parameters such as oscillating modes and direction. We correlated the dynamics of the bubble with the motion of the plate. In addition, we highlighted the differences including the characteristics of bubble shape and jetting that occurred during the collapse phase. The comparison of the time histories of the bubble's equivalent size postulated that the bubble's collapse times vary significantly in some cases compared to the bubble's dynamics near the stationary plate. In all cases, we noticed the shortening of the bubble's collapsing time, i.e. accelerated collapses. In our findings, we noticed a collapse times reduction of about 4-15%. Our finding signifies the importance of introducing the oscillation of the boundaries to obtain effective energy concentration over the time during the collapse. Our study also suggests that forced oscillation of boundaries is undesirable for destructive cavitation effects. The method we suggested for the manipulation of bubble dynamics holds potential for enhancing the efficiency of applications such as lithotripsy in biomedical devices, actuation and micro pumping in microfluidic devices, and effective semiconductor surface cleaning. Not but least, obtained results can be used as benchmark in future for validating numerical methods.

Experiment investigation on effects of elastic modulus on cavitation erosion of silicone rubber

Research into cavitation phenomena in various fields shows that the elastic modulus of a boundary has a potential impact on cavitation erosion. To obtain the direct relationship between the elastic modulus of the boundary and cavitiation erosion, single-layer samples with different chemical composition and moduli, and double-layer samples with different elastic moduli and the same surface layer material, were prepared with silicone rubber. The results of cavitation experiments on single-layer samples, show that the coating chemical composition and mechanical properties together affect the cavitation morphology of the coating, and dominant factors vary with erosion stage. Through the cavitation test of double-layer samples, it was found that there is a positive correlation between the elastic modulus of the coating and the degree of cavitation. This study helps us to understand the relationship between coating elastic modulus and cavitation more directly, and provides theoretical and technical guidance for the application of anti-cavitation for elastic coating in engineering.

Collapse dynamics of spherical cavities in a solid under shock loading

Extraordinary states of highly localised pressure and temperature can be generated upon the collapse of impulsively driven cavities. Direct observation of this phenomenon in solids has proved challenging, but recent advances in high-speed synchrotron radiography now permit the study of highly transient, subsurface events in real time. We present a study on the shock-induced collapse of spherical cavities in a solid polymethyl methacrylate medium, driven to shock states between 0.49 and 16.60 GPa. Utilising multi-MHz phase contrast radiography, extended sequences of the collapse process have been captured, revealing new details of interface motion, material failure and jet instability formation. Results reveal a rich array of collapse characteristics dominated by strength effects at low shock pressures and leading to a hydrodynamic response at the highest loading conditions.

Comparisons of spark-charge bubble dynamics near the elastic and rigid boundaries

The objective of this paper is to apply experimental methods to investigate the dynamics of spark-induced bubbles in the vicinity of the elastic and rigid boundary. In the experiment, the temporal evolution of the bubble is recorded by the high-speed camera at the 25,000 frames per second, as well as corresponding data such as normalized collapse position, the time of bubble collapse, and the velocity of the high-speed liquid jet. Results are presented for a single bubble generated over the elastic and rigid plates, under a wide range of normalized standoff distance from 0.5 to 3.0. The results show that the high-speed jet emitted by non-spherical bubble collapse near the boundary is one of the important factors to cause the destructive erosion pit. With the increase of the standoff distance, the expansion, shrink, jet formation, and rebound of the bubbles vary evidently adjacent to the different boundary conditions. Compared with the rigid boundary cases, the normalized first collapsed position and the time of bubble collapse are much smaller near the elastic boundary. The formation of the high-speed liquid jet in the neighborhood of the elastic/rigid boundary is founded in two different mechanisms. Furthermore, the normalized maximum velocity near the rigid plate is always larger than that near the elastic plate.

Lithotripter shock wave interaction with a bubble near various biomaterials

Following previous work on the dynamics of an oscillating bubble near a bio-material (Ohl et al 2009 Phys. Med. Biol. 54 6313-36) and the interaction of a bubble with a shockwave (Klaseboer et al 2007 J. Fluid Mech. 593 33-56), the present work concerns the interaction of a gas bubble with a traveling shock wave (such as from a lithotripter) in the vicinity of bio-materials such as fat, skin, muscle, cornea, cartilage, and bone. The bubble is situated in water (to represent a water-like biofluid). The bubble collapses are not spherically symmetric, but tend to feature a high speed jet. A few simulations are performed and compared with available experimental observations from Sankin and Zhong (2006 Phys. Rev. E 74 046304). The collapses of cavitation bubbles (created by laser in the experiment) near an elastic membrane when hit by a lithotripter shock wave are correctly captured by the simulation. This is followed by a more systematic study of the effects involved concerning shockwave bubble biomaterial interactions. If a subsequent rarefaction wave hits the collapsed bubble, it will re-expand to a very large size straining the bio-materials nearby before collapsing once again. It is noted that, for hard bio-material like bone, reflection of the shock wave at the bone-water interface can affect the bubble dynamics. Also the initial size of the bubble has a significant effect. Large bubbles (∼1 mm) will split into smaller bubbles, while small bubbles collapse with a high speed jet in the travel direction of the shock wave. The numerical model offers a computationally efficient way of understanding the complex phenomena involving the interplay of a bubble, a shock wave, and a nearby bio-material.

Bubbles with shock waves and ultrasound: a review

The study of the interaction of bubbles with shock waves and ultrasound is sometimes termed 'acoustic cavitation'. It is of importance in many biomedical applications where sound waves are applied. The use of shock waves and ultrasound in medical treatments is appealing because of their non-invasiveness. In this review, we present a variety of acoustics-bubble interactions, with a focus on shock wave-bubble interaction and bubble cloud phenomena. The dynamics of a single spherically oscillating bubble is rather well understood. However, when there is a nearby surface, the bubble often collapses non-spherically with a high-speed jet. The direction of the jet depends on the 'resistance' of the boundary: the bubble jets towards a rigid boundary, splits up near an elastic boundary, and jets away from a free surface. The presence of a shock wave complicates the bubble dynamics further. We shall discuss both experimental studies using high-speed photography and numerical simulations involving shock wave-bubble interaction. In biomedical applications, instead of a single bubble, often clouds of bubbles appear (consisting of many individual bubbles). The dynamics of such a bubble cloud is even more complex. We shall show some of the phenomena observed in a high-intensity focused ultrasound (HIFU) field. The nonlinear nature of the sound field and the complex inter-bubble interaction in a cloud present challenges to a comprehensive understanding of the physics of the bubble cloud in HIFU. We conclude the article with some comments on the challenges ahead.

Efficacy and spatial distribution of ultrasound-mediated clot lysis in the absence of thrombolytics

Ultrasound and microbubble (MB) contrast agents accelerate clot lysis, yet clinical trials have been performed without defining optimal acoustic conditions. Our aim was to assess the effect of acoustic pressure and frequency on the extent and spatial location of clot lysis. Clots from porcine blood were created with a 2-mm central lumen for infusion of lipid-shelled perfluorocarbon MBs (1×10(7) ml(-1)) or saline. Therapeutic ultrasound at 0.04, 0.25, 1.05, or 2.00 MHz was delivered at a wide range of peak rarefactional acoustic pressure amplitudes (PRAPAs). Ultrasound was administered over 20 minutes grouped on-off cycles to allow replenishment of MBs. The region of lysis was quantified using contrast-enhanced ultrasound imaging. In the absence of MBs, sonothrombolysis did not occur at any frequency. Sonothrombolysis was also absent in the presence of MBs despite their destruction at 0.04 and 2.00 MHz. It occurred at 0.25 and 1.05 MHz in the presence of MBs for PRAPAs > 1.2 MPa and increased with PRAPA. At 0.25 MHz the clot lysis was located in the far wall. At 1.05 MHz, however, there was a transition from far to near wall as PRAPA was increased. The area of clot lysis measured by ultrasound imaging correlated with that by micro-CT and quantification of debris in the effluent. In conclusion, sonothrombolysis with MBs was most efficient at 0.25 MHz. The spatial location of sonothrombolysis varies with pressure and frequency indicating that the geometric relation between therapeutic probe and vascular thrombosis is an important variable for successful lysis clinically.

Acoustically driven cavitation cluster collapse in planar geometry

We demonstrate the dynamics of arrays of transient cavitation bubbles exposed to a sound field in a planar geometry. Single, double, and complex configurations of cavitation bubbles are obtained by shaping a pulsed laser beam with a digital hologram and focusing it into a thin gap of liquid. The liquid is driven with an oscillating pressure field of variable phase and amplitude. We compare the dynamics of a single bubble recorded with high-speed photography with a two-dimensional Rayleigh model. For multibubble configurations we observe bubble-bubble interaction and coalescence which depends on the phase of the acoustic field. Larger clusters demonstrate drastically enhanced collapse for high-amplitude driving, enabling the study of artificial cavitation clusters under strong driving.

Pulsating tandem microbubble for localized and directional single-cell membrane poration

The interaction of laser-generated tandem microbubble (maximum diameter of about 50  μm) with single (rat mammary carcinoma) cells is investigated in a 25-μm liquid layer. Antiphase and coupled oscillation of the tandem microbubble leads to the formation of alternating, directional microjets (with max microstreaming velocity of 10  m/s) and vortices (max vorticity of 350 000  s{-1}) in opposite directions. Localized and directional membrane poration (200 nm to 2  μm in pore size) can be produced by the tandem microbubble in an orientation and proximity-dependent manner, which is absent from a single oscillating microbubble of comparable size and at the same stand-off distance.

Structure and dynamics of shock-induced nanobubble collapse in water

Shock-induced collapse of nanobubbles in water is investigated with molecular dynamics simulations based on a reactive force field. We observe a focused jet at the onset of bubble shrinkage and a secondary shock wave upon bubble collapse. The jet length scales linearly with the nanobubble radius, as observed in experiments on micron-to-millimeter size bubbles. Shock induces dramatic structural changes, including an ice-VII-like structural motif at a particle velocity of 1  km/s. The incipient ice VII formation and the calculated Hugoniot curve are in good agreement with experimental results.

Interaction between supersonic disintegrating liquid jets and their shock waves

We used ultrafast x radiography and developed a novel multiphase numerical simulation to reveal the origin and the unique dynamics of the liquid-jet-generated shock waves and their interactions with the jets. Liquid-jet-generated shock waves are transiently correlated to the structural evolution of the disintegrating jets. The multiphase simulation revealed that the aerodynamic interaction between the liquid jet and the shock waves results in an intriguing ambient gas distribution in the vicinity of the shock front, as validated by the ultrafast x-radiography measurements. The excellent agreement between the data and the simulation suggests the combined experimental and computational approach should find broader applications in predicting and understanding dynamics of highly transient multiphase flows.

Dynamics of impacting a bubble by another pulsed-laser-induced bubble: jetting, fragmentation, and entanglement

We investigate experimentally the detailed dynamics of how an existing microbubble B1 is impacted and shattered by another nearby pulsed-laser-induced microbubble B2, and the backward interaction on B2 in a thin liquid layer. Mediated by the flow field, potential energy can be accumulated or lost through the alternate compression and expansion of the two bubbles. The symmetry breaking induced by the presence of the nearby counterbubble generates push-pull-type alternate forward and backward axial jetting on the compressed bubble associated with the elongated shape or even entrainment of the counterexpanding bubble into the jet-indented boundary. The strong penetrating axial jet through B1, and its interplay with the transverse jets by the flow field surrounding B1 in the first compression stage and the second expanding stage of B1 lead to a complicated fragmentation pattern of B1. Increasing the interbubble interaction by decreasing the interbubble distance causes B2 to become entangled with B1 through its entrainments into the backward axial jet-indented region of B2, in the expansion phase of B2. At the extreme of large laser energy for B2, the leftward reexpansion of B1 is suppressed. The strong shear flow field generates many tiny bubbles around the liquid-gas boundaries of the two axial jet-induced major daughter bubbles from B1. The detailed interaction behaviors over a broad range of the energy of B2, 0.14-0.55 microJ (corresponding to the maximum bubble expansion energy), and of the interbubble distance (170-500 microm) are presented and discussed.

Interaction of lithotripter shockwaves with single inertial cavitation bubbles

The dynamic interaction of a shockwave (modelled as a pressure pulse) with an initially spherically oscillating bubble is investigated. Upon the shockwave impact, the bubble deforms non-spherically and the flow field surrounding the bubble is determined with potential flow theory using the boundary-element method (BEM). The primary advantage of this method is its computational efficiency. The simulation process is repeated until the two opposite sides of the bubble surface collide with each other (i.e. the formation of a jet along the shockwave propagation direction). The collapse time of the bubble, its shape and the velocity of the jet are calculated. Moreover, the impact pressure is estimated based on water-hammer pressure theory. The Kelvin impulse, kinetic energy and bubble displacement (all at the moment of jet impact) are also determined. Overall, the simulated results compare favourably with experimental observations of lithotripter shockwave interaction with single bubbles (using laser-induced bubbles at various oscillation stages). The simulations confirm the experimental observation that the most intense collapse, with the highest jet velocity and impact pressure, occurs for bubbles with intermediate size during the contraction phase when the collapse time of the bubble is approximately equal to the compressive pulse duration of the shock wave. Under this condition, the maximum amount of energy of the incident shockwave is transferred to the collapsing bubble. Further, the effect of the bubble contents (ideal gas with different initial pressures) and the initial conditions of the bubble (initially oscillating vs. non-oscillating) on the dynamics of the shockwave-bubble interaction are discussed.