Cavitation cloud formation and surface damage of a model stone in a high-intensity focused ultrasound field.

This work investigates the fundamental role of cavitation bubble clouds in stone comminution by focused ultrasound. The fragmentation of stones by ultrasound has applications in medical lithotripsy for the comminution of kidney stones or gall stones, where their fragmentation is believed to result from the high acoustic wave energy as well as the formation of cavitation. Cavitation is known to contribute to erosion and to cause damage away from the target, yet the exact contribution and mechanisms of cavitation remain currently unclear. Based on in situ experimental observations, post-exposure microtomography and acoustic simulations, the present work sheds light on the fundamental role of cavitation bubbles in the stone surface fragmentation by correlating the detected damage to the observed bubble activity. Our results show that not all clouds erode the stone, but only those located in preferential nucleation sites whose locations are herein examined. Furthermore, quantitative characterizations of the bubble clouds and their trajectories within the ultrasonic field are discussed. These include experiments with and without the presence of a model stone in the acoustic path length. Finally, the optimal stone-to-source distance maximizing the cavitation-induced surface damage area has been determined. Assuming the pressure magnitude within the focal region to exceed the cavitation pressure threshold, this location does not correspond to the acoustic focus, where the pressure is maximal, but rather to the region where the acoustic beam and thereby the acoustic cavitation activity near the stone surface is the widest.

Synchrotron X-ray imaging of the onset of ultrasonic horn cavitation.

High-power ultrasonic horns operating at low frequency are known to generate a cone-shaped cavitation bubble cloud beneath them. The exact physical processes resulting in the conical structure are still unclear mainly due to challenges associated with their visualization. Herein, we address the onset of the cavitation cloud by exploiting high-speed X-ray phase contrast imaging. It reveals that the cone formation is not immediate but results from a three-step phenomenology: (i) inception and oscillation of single bubbles, (ii) individual cloud formation under splitting or lens effects, and (iii) cloud merging leading to the formation of a bubble layer and, eventually, to the cone structure due to the radial pressure gradient on the horn tip.

Multimodal imaging for intra-droplet gas-cavity observation during droplet fragmentation.

Herein, planar laser-induced fluorescence (PLIF) is used in combination with shadowgraphy to study water-droplet aerobreakup. The acquired shadowgraph data are in agreement with previous visualization studies but differ from the PLIF results, yielding new insights into the fragmentation process. In particular, the PLIF data reveal changes in droplet topology during fragmentation that result from the entrapment or formation of gas cavities inside the liquid phase. In some instances, topological modification can be observed to arise from the presence of these cavities. In addition, the cavities may act as weak spots, facilitating droplet split-off.

Data on eosin Y solutions for laser-induced fluorescence in water flows.

Dye tracing techniques involve the tagging of a sample of water with dye, providing important qualitative and quantitative information. This article presents physical and fluorescence properties of dye solutions obtained by diluting a pharmaceutical aqueous solution of eosin Y with distilled water. Sample solutions with eosin concentrations ranging from 0 to 20 g/L were examined under various temperatures and laser powers. The data include measurements of dynamic viscosity, surface tension and pH. Fluorescence emission spectra as well as laser beam attenuation and photobleaching measurements are also reported. The datasets provide guidelines for obtaining optimal dye mixtures and suitable optical configurations to implement eosin fluorescence techniques.

High-magnification shadowgraphy for the study of drop breakup in a high-speed gas flow.

Direct observation of the droplet breakup process in high-speed gas flows is a critical challenge that needs to be addressed to elucidate the physical mechanisms underlying the fragmentation phenomenon. Here, we present a high-magnification and high-speed shadowgraph technique that allows the visualization of this process over its whole evolution and resolves detailed features of the breakup zone. The developed experimental method uses a high-speed camera equipped with a long-distance microscope. The backlight illumination source is provided by the laser-induced fluorescence of a dye solution that delivers short pulses at a high-repetition rate. Artifacts resulting from the laser coherence are therefore reduced.