Shear-driven two colliding motions of binary double emulsion droplets

Abstract

We combine experimental visualization with numerical simulation to explore the hydrodynamic shear-driven binary collision of double emulsion droplets. Two typical colliding motions, passing-over motion and reversing motion, are found and identified by quantitatively characterizing the corresponding detailed motion trajectory and morphology development. Compared with the ordinary single-phase droplets, double emulsion droplets are demonstrated to exhibit similar motion trajectories but different deformation development during the binary collision process, which arise from an additional interaction induced by the inner droplet. Especially, we clarify that two typical colliding motions are determined by the competitions among the drag of passing-flow region and the entrainment from reversing-flow and vortex regions in the matrix fluid, which are significantly affected by the dimensionless shear stress (Ca) and confinement degree of shear flow (Co). With the increasing Co, the colliding motion of binary double emulsion droplets transits from the passing-over to reversing, owing to that the entrainment of the reversing-flow region turns to play a dominant role. The drag of the ambient passing-flow in the matrix fluid is increased by enlarging Ca, resulting in the emergence of passing-over motion of the colliding droplets. Accordingly, a regime diagram is provided to quantitatively recognize the corresponding regime of these two typical colliding motions, as a function of Ca and Co.