Davide Bernardo Preso

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Cavitation bubble dynamics and microjet atomization near tissue-mimicking materials

Mohamed Farhat, Davide Bernardo Preso, Armand Baptiste Sieber

In recent years, considerable interest has been devoted to the interactions between cavitation bubbles and tissue-mimicking materials due to their promising applications in medicine and biomedical sciences. The strong fluid-structure interaction between a cavitation bubble and these elastic surfaces triggers unique collapse dynamics, characterized by bubble splitting and subsequent microjetting phenomena that can damage adjacent boundaries. In this work, we investigate how the elasticity of the boundary and the distance between the bubble and the elastic surface affect the bubble dynamics and the velocity of its microjet. To this end, we generate single laser-induced cavitation bubbles in the vicinity of agarose hydrogels with different degrees of elasticity and follow the bubble dynamics using high-speed imaging techniques, with a special focus on the formation and evolution of the microjets. We provide a time-resolved evidence of the atomization of the liquid microjet within the bubble, which precedes the establishment of a fully liquid microjet. The atomized portion of the microjet can reach supersonic velocities of up to 2000 ms( -1), while the ensuing fully developed liquid microjet travels at averaged speeds of up to 1000 ms (-1). To gain further insight into the bubble dynamics leading to the formation of these very fast microjets, we also propose a numerical model based on the boundary integral method and observe a remarkable agreement between the numerical simulations and the experimental observations.(c) 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http:// creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0136577

AIP Publishing2023

Perfluorocarbon emulsion enhances MR-ARFI displacement and temperature in vitro: Evaluating the response with MRI, NMR, and hydrophone

Mohamed Farhat, Ryan Holman, Davide Bernardo Preso

Sonosensitive perfluorocarbon F(8)TAC(18)-PFOB emulsion is under development to enhance heating, increase thermal contrast, and reduce treatment times during focused ultrasound tumor ablation of highly perfused tissue. The emulsion previously showed enhanced heating during ex vivo and in vitro studies. Experiments were designed to observe the response in additional scenarios by varying focused ultrasound conditions, emulsion concentrations, and surfactants. Most notably, changes in acoustic absorption were assessed with MR-ARFI. Phantoms were developed to have thermal, elastic, and relaxometry properties similar to those of ex vivo pig tissue. The phantoms were embedded with varying amounts of F(8)TAC(18)-PFOB emulsion or lecithin-PFOB emulsion, between about 0.0-0.3% v:w, in 0.05% v:w increments. MR-ARFI measurements were performed using a FLASH-ARFI-MRT sequence to obtain simultaneous displacement and temperature measurements. A Fabry-Perot hydrophone was utilized to observe the acoustic emissions. Susceptibility-weighted imaging and relaxometry mapping were performed to observe concentration-dependent effects. F-19 diffusion-ordered spectroscopy NMR was used to measure the diffusion coefficient of perfluorocarbon droplets in a water emulsion. Increased displacement and temperature were observed with higher emulsion concentration. In semi-rigid MR-ARFI phantoms, a linear response was observed with low-duty cycle MR-ARFI sonications and a mono-exponential saturating response was observed with sustained sonications. The emulsifiers did not have a significant effect on acoustic absorption in semi-rigid gels. Stable cavitation might also contribute to enhanced heating.

FRONTIERS MEDIA SA2023

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We study the dynamics of a cavitation bubble near beds of sand of different grain sizes. We use high-speed imaging to observe the motion of the bubble and the sand for different values of the stand-off parameter gamma (dimensionless bubble-boundary distance) between 0.3 and 5.3. Compared with a rigid boundary, we find that a granular boundary leads to bubbles with shorter lifetimes and reduced centroid displacements. Above gamma approximate to 1.3, the behaviour of the bubble is almost independent of the granularity of the sand. When the stand-off parameter lies between 0.6 and 1.3, a mound of sand develops beneath the bubble, which can force the latter to assume a conical shape as it collapses. For gamma less than or similar to 0.6, the bubble develops a bell-shaped form, leading to the formation of thin and surprisingly fast micro-jets (v(jet) > 1000 m s(-1)). Moreover, between gamma approximate to 1.3 and gamma approximate to 0.3, granular jets erupt from the sand surface following the bubble collapse. We additionally develop a simple numerical model, based on the boundary integral method, to predict the dynamics of a cavitation bubble near a bed of sand, which we replace by an equivalent liquid. The simulations are remarkably consistent with experimental observations for values of gamma down to 1.3. We also show how the anisotropy parameter a dimensionless version of the Kelvin impulse, can be adapted for the case of a nearby bed of sand and predict the displacement of the bubble centroid for zeta less than or similar to 0.08.

CAMBRIDGE UNIV PRESS2022