Bubble entrapment during head-on binary collision with large deformation of unequal-sized tetradecane droplets

Abstract

Head-on collision dynamics of unequal-sized binary tetradecane droplets in air is numerically studied for numerous diameter ratios Dr = 1.5–2.5 and larger values of Weber number We = 50–220. Our study is within two collision regimes, permanent coalescence and separation after temporary coalescence, where entrapment of an air bubble is revealed at larger We and intermediate values of Dr. We found two types of bubble entrapment, permanent and temporary, which are demarcated—within the two collision regimes—in a regime map, with the transition from the permanent to temporary entrapment at a larger We and larger Dr. A bubble-entrapment mechanism is proposed in three stages: stage-1 is radial expansion and a thin lamella-layer formation at the center of the coalesced droplets, stage-2 is radial contraction and an advancing lamella layer that results in a horseshoe-like droplet, and stage-3 is radial contraction and a receding lamella layer. Inertia force dominates stage-1, while both inertia and surface tension forces dominate stage-2 and stage-3. Finally, a quantitative analysis is presented for the effect of We and Dr on the unsteady interface dynamics of the initially disk-like and later horseshoe-like coalesced droplets, which leads to the bubble entrapments. The present findings are significant as bubble entrapments inside a fuel droplet facilitate a micro-explosion during combustion. It can also lead to a novel technique for the production of a compound droplet, which is used to achieve a precise control over processes in many applications, such as bio-analysis, pharmaceutical manufacturing, and material synthesis.