The impact and spreading dynamics of internal gel-phase emulsion drops on solid surfaces

The dynamics of drop impact, spreading, and evaporation on solid surfaces is fundamental in the design of many processes such as 3D printing, combustion, spray cooling, and coating. Such dynamics for an emulsion drop, i.e. a continuous phase containing many droplets of a second and immiscible phase is less known. Here, we formulate drops of heptane and sorbitan monooleate (Span80) as the continuous phase containing a diluted microemulsion phase made of either DI-water or silica-gel. In the latter, the microemulsion phase undergoes a gelation process, so the heptane drops host gelled droplets. We examine the heptane drop impacts and spreading on a glass surface. We employ an automated dosing needle and a side-view camera to conduct and visualize the experiments in room conditions. The results manifest a two-stage process: the drop rebounds after the impact until spreading becomes dominant. It is found that the impact-rebound stage is identical for both DI-water and gel microemulsion phases. However, the presence of gel microemulsion slows the spreading dynamics and significantly reduces the final wetted area. Microscopic images of dried drops, after heptane evaporation, indicate the uniform deposition of gel droplets.