Cavitation by phase shift of focused shock waves inside a droplet
Localized cavitation in liquids and soft tissues, typically initiated by the rarefaction phase of high-amplitude ultrasound waves, is leveraged in several biomedical applications such as ablation techniques and drug delivery with vaporizing agents. However, safety considerations aimed at avoiding unwanted bubble activity outside the targeted region pose a limit to the maximum allowed peak rarefaction pressure, which on the other hand can hinder the therapeutic efficacy of these techniques. This study shows that a purely compressive shock wave can generate localized, negative pressure and initiate cavitation inside a sub-millimetric perfluorohexane droplet, without requiring any externally applied rarefaction wave. The Gouy phase shift is identified as the physical mechanism responsible for the conversion of positive pressure into tension during shock focusing, and its occurrence is demonstrated through numerical simulations and direct experimental measurements. Comparison of the regions affected by cavitation, visualized \emph{in-situ} by means of high-speed x-ray phase-contrast imaging, with prediction from Classical Nucleation Theory suggests homogeneous nucleation as the underlying mechanism behind bubble formation. The presented findings offer valuable insights into the physics of shock wave propagation which can inspire the development of novel acoustic driving strategies for cavitation generation, facilitating the reduction of negative pressures outside the target region and improving the safety and precision of biomedical treatments.