Boger fluid droplet impact and interaction regimes on anti-wetting spheres

Droplet impact events on curved surfaces are commonly observed in both nature and man-made systems. It is anticipated that the curvature shall significantly affect the post-impact dynamics and regimes. This study explores the dynamic behavior of elastic fluid (Boger fluid) droplets impacting superhydrophobic spheres. In our experiments, we have highlighted the variations in the post-impact dynamics arising from the elastic properties of the fluid and the curvature of the surface. It was found that the secondary droplets, typically observed in the case of Newtonian fluids, transformed into enduring, slender filaments due to the fluid's elastic behavior. Depending on the polymer concentration (used to synthesize the Boger fluids), these filaments either disintegrated into secondary droplets or retracted to merge back into the parent droplet, which ultimately rebounded off the spheres. The rebound suppression phenomenon, observed and reported in elastic fluid droplets on flat superhydrophobic surfaces, does not take place on spherical surfaces due to the curvature effect. Subsequently, a regime map encompassing all the post-impact outcomes is presented as a function of the Weber number, and a non-dimensional parameter that captures the effect of droplet impact velocity, the relaxation time of the fluid, and the sphere diameter.