Impacts of emulsion droplets on non-wetting surfaces

Oil-in-water emulsion sprays are used in various applications such as pesticide delivery and steel manufacturing. Here, we experimentally study the impact of emulsion droplets on non-wetting surfaces. We observe an unusual behavior where droplets bounce at low Weber numbers, stick at moderate Weber numbers and bounce at high Weber numbers. We also observe that some of the oil from the droplet impregnates the surface texture during the impact. We experimentally and theoretically quantify the coverage of the surface with oil as a function of the emulsion parameters. We then show that because of the impregnated oil, the droplet retracts on a pseudo liquid impregnated surface. The bouncing-sticking-bouncing transition is rationalized by comparing droplet inertia and the force exerted by the newly formed oil layer. We finally show that viscosity plays two conflicting roles: while it increases the dissipation in the lubricating film, it also increases the timescale of impregnation, making high viscosity oils non-effective at the timescale of a droplet impact. Using our models, we make a two-dimensional design map that can guide the choice of droplet size, velocity and oil viscosity to enhance the retention of sprays.