Birth of a bubble: Drop impact onto a thin liquid film for an immiscible three-fluid system

Abstract

When a drop impacts a solid substrate or a thin liquid film, a thin gas disc is entrapped due to surface tension, the gas disc retracts into one or several bubbles. While the evolution of the gas disc for impact on solid substrate or film of the same fluid as the drop have been largely studied, little is known on how it varies when the liquid of the film is different that of the drop. We study numerically the latter unexplored area, focussing on the contact between the drop and the film, leading to the formation of the air bubble. The volume of fluid method was adapted to three fluids in the framework of Basilisk solver. The numerical simulations show that the deformation of the liquid film due to the air cushioning plays a crucial role in the bubble entrapment. A new model for the contact time and the entrapment geometry was deduced from the case of the impact on a solid substrate. This was done by considering the deformation of the thin immiscible liquid layer during impact depending mainly on its thickness and viscosity. The lubrication of the gas layer was found to be the major effect governing the bubble entrapment. However the film viscosity was also identified as having a critical role in bubble formation and evolution; the magnitude of its influence was also quantified.