Coalescence of suspended and sessile isotropic and radial nematic liquid crystal drops.

Abstract

We investigate the coalescence of suspended and sessile liquid crystal (LC) drops with homeotropic anchoring in both isotropic and nematic phases. Previous theoretical studies have suggested that coalescence of two radial nematic liquid crystal (NLC) drops could be prevented by an energy barrier due to formation of a topological defect (ring defect) between the two drops. We employ polarized optical microscopy (POM) to observe droplet merging in different configurations and confirm the predicted defect dynamics and director field evolution. The experimental work did not find significant differences in the coalescence of isotropic <i>versus</i> nematic drops. However, we observe a large difference in the merging rates of sessile <i>versus</i> suspended drops. Nearby sessile drops (nematic and isotropic) merge rapidly upon contact. POM, surface tension measurements, simulation, and simple analysis help explain why LC phase is unimportant for merging and suggest that compression forces between drops are significant factor for merging of the sessile drops; these forces facilitate coalescence by enhancing fluid drainage from the intervening thin film, by increasing contact area, and potentially <i>via</i> surfactant expulsion. Numerical simulations corroborate some of the features we observe and suggest that elastic energy costs due to defect formation are less important than surface energy gains during coalescence.