Cavitation bubble dynamics inside a droplet suspended in a different host fluid

TL;DR

This study combines theoretical, experimental, and numerical approaches to understand cavitation bubble behavior inside droplets in different host fluids, revealing distinct jetting and droplet breakup mechanisms relevant for applications like emulsification.

Contribution

It introduces a modified Rayleigh model for bubble dynamics in droplet systems and provides detailed experimental analysis of jet penetration and droplet breakup mechanisms.

Findings

Liquid jet penetrates in O/W droplets during bubble collapse.

Bubble induces droplet motion and pinch-off in W/O droplets.

Boundary integral model accurately reproduces nonspherical bubble dynamics.

Abstract

In this paper, we present a theoretical, experimental, and numerical study of the dynamics of cavitation bubbles inside a droplet suspended in another host fluid. On the theoretical side, we provided a modified Rayleigh collapse time and natural frequency for spherical bubbles in our particular context, characterized by the density ratio between the two liquids and the bubble-to-droplet size ratio. Regarding the experimental aspect, experiments were carried out for laser-induced cavitation bubbles inside oil-in-water (O/W) or water-in-oil (W/O) droplets. Two distinct fluid-mixing mechanisms were unveiled in the two systems, respectively. In the case of O/W droplets, a liquid jet emerges around the end of the bubble collapse phase, effectively penetrating the droplet interface. We offer a detailed analysis of the criteria governing jet penetration, involving the standoff parameter and impact velocity of the bubble jet on the droplet surface. Conversely, in the scenario involving W/O droplets, the bubble traverses the droplet interior, inducing global motion and eventually leading to droplet pinch-off when the local Weber number exceeds a critical value. This phenomenon is elucidated through the equilibrium between interfacial and kinetic energies. Lastly, our boundary integral model faithfully reproduces the essential physics of nonspherical bubble dynamics observed in the experiments. We conduct a parametric study spanning a wide parameter space to investigate bubble-droplet interactions. The insights from this study could serve as a valuable reference for practical applications in the field of ultrasonic emulsification, pharmacy, etc.