A New Mechanism for the Wetting of a Solid Surface by Emulsion Droplets

The wetting of a solid surface by the drop of an emulsion has traditionally been thought to be mediated by the formation of a liquid bridge that connects the drop and the surface. In this experimental work, an alternative mechanism was observed. When a glycerol drop suspended in a high molecular weight silicone oil approached a mica surface or a plasma treated SU-8 surface under the influence of gravity, islands of glycerol were observed to grow on the substrate. Wetting of the surface then occurred by the merging of the parent drop with one of nucleated sites. Scaling relationships were developed to describe the film height and the instant of time at which the parent drop merges with the islands. This work has revealed new variables that could be potentially used to control wetting: the solubility of the drop in the medium, the diffusivity of the drop phase in the medium and surface modification (to tune nucleation rates). The fundamental effect discovered here will guide the tailoring of emulsion-based coatings or paints to have specific wetting times.