(142aj) Mixing in Sessile Drops Merging On a Surface

We investigate the mixing of two sessile drops that merge on a surface. The drops consist of low viscosity glycerol-water mixtures deposited on a silicone elastomer surface with contact angle near 90°. We observe the shape of the drops and the location of their intersection by placing a fluorescent dye in one drop and using a laser light sheet to image a plane perpendicular to the surface. The initial healing of the meniscus bridge between the merging drops, and the damping of capillary waves appearing on their surfaces occur on timescales comparable to the inertio-capillary relaxation time. However, the interface between the two fluids remains sharp, broadening diffusively over several minutes. The shape of the merged drops and the boundary between them also continues to evolve on a timescale of minutes. This later motion is controlled by a gravity current established by the density difference between the liquids in the two initial drops. The gravity current leads to the eventual stratification of the liquids to form two separate layers within the stationary shape of the merged drop. We characterize the long timescale flow arising from the gravity current as a function of the drop sizes and the density and viscosity differences between the two merging fluids. We use dimensional analysis to aid in elucidating the essential physics.