Collective interactions in the nucleation and growth of surface droplets.

In the process of solvent exchange, oil droplets nucleate and grow on a solid substrate in response to the oversaturation generated through the displacement of a good oil solvent by a poor one. The mean size of the droplets depends on flow rate, flow geometry and solution conditions. In this work, we investigate the surface coverage of the droplets and the correlation between the base area of the droplets and of the bare zone surrounding the droplets for various flow and solution conditions during the solvent exchange. The surface coverage increases with the increase in the flow rate, channel height and the oil concentration, and reaches a plateau between 35% and 50%. The spatial correlation is analysed with the help of the radial distribution function g(r) and a Voronoi tessellation. When the surface coverage reaches ∼25-30%, the number density of the droplets starts to drop, reflecting the mutual interaction and merging of the droplets. With further decrease in the droplet spacing and increase in surface coverage, the Voronoi analysis shows that the base area of the droplets increases linearly with the area size of the depleted zone. The collective interaction in the growth of surface nanodroplets is universal, independent of the specific conditions that control the droplet growth.