The role of compound droplet size on transition from jetting to bubble entrapment during its impact on liquid

Abstract

Inertia has always proven to be a key parameter in controlling regime transitions when simple drops impact a liquid surface. However, the scenario of compound drops impacting a liquid surface has received the least attention, and poses the question of whether any factor besides inertia can act as a switching criterion for regime transition. Through axisymmetric two-dimensional volume-of-fluid based computations of a compound drop falling with a certain velocity in a liquid pool, we demonstrate a non-trivial switching from jetting to large bubble entrapment phenomenon by decreasing the radius ratio of the compound drop, under identical inertial condition. Six different regimes that can be categorized into fundamental regimes of pre-jetting, jetting, transition, and bubble entrapment are mapped on the radius ratio–Weber number plane. Hence, with a suitable combination of radius ratio and impact velocity, the interplay of inertia and buoyancy forces can be exploited to achieve the final outcome of a secondary drop or an entrapped bubble. Our results reveal that the strength of buoyancy force decreases with decrease in the radius ratio of compound drops and, as a result, the intervening physics changes from crater expansion to wave swell retraction and finally to roll jet formation with decrease in radius ratio. These results are further explained in light of capillary wave propagation and vortex formation and may turn out to be of immense consequence in providing insight into the underlying complex physical mechanisms dictating intricate control on compound drop impact events.