Impact dynamics of polymer and emulsion droplets on a nonwetting substrate

Droplet impact behavior on solid surfaces is crucial in various industrial applications, including spray coating, food production, printing, and agriculture, where controlling impact dynamics is particularly challenging for non-Newtonian liquids with complex rheology. In agriculture, emulsions used for spraying oil-based pesticides often bounce off hydrophobic plant surfaces, leading to environmental contamination of water sources and soil. This study investigates the impact dynamics of emulsions composed of aqueous polymer (polyvinylpyrrolidone) solutions and hexadecane oil on superhydrophobic substrates. We demonstrate that increasing the polymer concentration from 1 to 10 000 ppm reduces droplet retraction rates by approximately 63% - an effect primarily attributed to the increase in the shear viscosity. In the case of emulsion, the reduction in the retraction rate due to the presence of polymers enables oil imbibition into the surface asperities. We show that incorporating oil into the aqueous polymer emulsion decreases the minimum polymer concentration required to suppress droplet rebound on superhydrophobic surfaces from 10 000 to 100 ppm. The impact dynamics of polymer–oil–water emulsion droplets are influenced by an emerging oil layer on the substrate, which substantially slows the retraction rate. Notably, even at a low oil concentration of 3 vol. %, impinging emulsion droplets adhere to the superhydrophobic substrate regardless of the Weber number. These findings provide new insights into controlling droplet impact behavior and suggest an environmentally friendly and effective strategy for reducing unwanted rebound, particularly in agricultural applications.