Bubble collapse and jet formation inside a liquid film

Abstract

The cavitation of a hemispherical bubble nucleated inside a liquid film of microscale thickness and the subsequent formation of liquid jets are investigated numerically, using the compressible volume of fluid method, in particular, in the context of laser-induced forward transfer (LIFT). Because of the presence of both a solid wall and a free surface, the dynamics of the bubble differ notably from those near a solid wall alone or a free surface alone. When the liquid film is sufficiently thin, the shape of the expanded bubble becomes narrow along the axial direction, forging a stretched cone with a spike. The interface of the bubble near the spike opposite to the solid wall contracts much earlier than the other parts, leading to the separation of the spike. As a result of this separation, the collapse of the bubble is stronger, and the outward jet and inward jet along the axial direction are sharper and faster. However, as the film thickness increases, the tip separation of the bubble becomes weaker, and both outward and inward liquid jets become thicker and slower. In addition, an increase in liquid viscosity changes the bubble shape from an axially stretched cone shape to a round shape because of enhanced viscous force along the radial direction inside the thin film, which eventually results in disappearance of the inward jet. The fundamental insights elucidated in this work can serve as a quantitative design guideline for the LIFT.