RESUMEN
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.
RESUMEN
Elastography is a non-invasive technique to detect tissue anomalies via the local elastic modulus using shear waves. Commonly shear waves are produced via acoustic focusing or the use of mechanical external sources, shear waves may result also naturally from cavitation bubbles during medical intervention, for example from thermal ablation. Here, we measure the shear wave emitted from a well-controlled single laser-induced cavitation bubble oscillating near a rigid boundary. The bubbles are generated in a transparent tissue-mimicking hydrogel embedded with tracer particles. High-speed imaging of the tracer particles and the bubble shape allow quantifying the shear wave and relate it to the bubble dynamics. It is found that different stages of the bubble dynamics contribute to the shear wave generation and the mechanism of shear wave emission, its direction and the efficiency of energy converted into the shear wave depend crucially on the bubble to wall stand-off distance.
RESUMEN
The influence of the compressibility effects is discussed, including the time delays on the dynamics of acoustically excited bubbly screens. In the linear regime, it is shown that the proposed model for the infinite bubbly screen recovers the results predicted by the effective medium theory (EMT) up to the second order without introducing any fitting parameter when the wavelength is large compared to the inter-bubble distance. However, the effect of boundaries on the finite bubbly screens is shown to lead to the appearance of multiple local resonances and characteristic periodic structures, which limit the applicability of the EMT. In addition, a local resonance phenomenon in the liquid spacings between the bubbles is observed for both the infinite and finite bubbly screens with crystal structures, and these effects vanish as the crystal structure is perturbed. In the nonlinear regime, the current model is treated with time-delay effects as a delay differential equation, which is directly solved numerically. The appearance of an optimal distance for the subharmonic emission for the crystal structures is shown, and the accuracy of the EMT in the strong nonlinear regime is discussed.
RESUMEN
The subharmonic acoustic emission of a stable oscillating bubble inside a rigid tube is investigated by direct numerical simulation. The mechanisms of bubble-tube interaction on the acoustic wave emitted by the bubble are clarified. When the bubble is small compared to the tube diameter, a critical threshold for the pressure amplitude appears beyond the point which nonspherical effects become important and bubble breaks. For a finite tube diameter, the scattered wave by the bubble is shown to generate a plane wave where the intensity of the subharmonic component becomes maximum for an optimal distance between the bubble and the tube wall. This effect seems to be directly related to the appearance of local resonance phenomena and a bubble resonance shift where liquid's compressibility plays a major role.
RESUMEN
The dynamics of bubble clouds induced by ultrasound field are investigated in a regime where the cloud size is much smaller than the ultrasound wavelength. Two frequently used models describing the dynamics of individual bubbles inside a bubble cluster in an acoustic field are studied, one based on the homogeneity assumption, and the other based on the simultaneous motion assumption. A modified formula of the homogenization-based model is presented, and an inherent distinction in bubble-bubble interaction term is found in comparison to the simultaneous motion model. To gain insight into the mechanisms of such distinction, a reduced model unifying these two models is presented, and such distinction is explained by the spatial dependence of the bubble-bubble interaction in a bubble cluster accordingly. To validate the reduced model, the normalized distance γbb and the cloud interaction parameter B0 are used as two scaling parameters, and the comparison between the present model and the coupled Rayleigh-Plesset type equations is made. A conclusion is that, in the weak bubble-bubble interaction case (γbb>10), the reduced model can well reproduce the radial motion of bubbles in the cluster during the growth stage and the collapse stage in each acoustic cycle; in the strong bubble-bubble interaction case (γbb<10), the growth phase of bubbles in the cluster can be accurately predicted by the reduced model only if B0 or the amplitude of driving field is small.
RESUMEN
The influence of the spatial distributions of bubbles on the propagation of linear acoustic waves in polydisperse bubbly liquids is studied. Using the diagrammatic approach, the effective wavenumber, which includes both spatial information and higher orders of multiple scattering, is presented. The phase speed and attenuation coefficient of acoustic waves in bubbly liquids are calculated from the effective wavenumber. A three-dimensional random model, the Generalized Matérn's hard-core point process, is used to close the model. Numerical simulations reveal that as the bubble volume fraction becomes larger so does the effect of the bubble distributions on the attenuation and phase speed. The irregular discrepancy between previously reported experimental results and the classical theory is attributed to the influence of bubble clustering on the propagation of linear waves. The comparison between the present model and the experimental measurements [Leroy, Strybulevych, Page, and Scanlon. (2011). Phys. Rev. E 83, 046605] reveals that the proposed correction term significantly improves the theoretical predictions.
