Oil-on-water droplets faceted and stabilized by vortex halos in the subphase

Abstract

Oil droplets on water test the principle of surface free energy minimization when volatility or dissolution produce flow-induced viscous shear stresses that take the system out of equilibrium. We find that single droplets adopt polyhedral shapes in shallow water, droplet pairs pulsate in their spacing and shape, multiple oil droplet arrays display active motion, and these droplets are protected from coalescence. Combined experiments, scaling arguments, and linear stability analysis trace these effects to rapid Marangoni flow (“vortex halos”) that shears individual droplets while maintaining them separate from other droplets. The analogy that active emulsion droplets are like an iceberg with submerged halos formed by high-Reynolds-number flow thus presents an approach to control and manipulate this common system.