Coalescence-induced droplet jumping on superhydrophobic surfaces: Effects of droplet mismatch

On low-adhesion surfaces, coalescing droplets can spontaneously jump off, known as coalescence-induced droplet jumping. It is observed on a variety of synthetic and natural superhydrophobic surfaces, and gives rise to a range of applications, such as self-cleaning condensers, anti-icing coatings, and thermal diodes. Through three-dimensional simulations, this paper demonstrates the fluid dynamics of droplet jumping upon binary unequal-sized-droplet coalescence. Parametric studies show the influence of droplet mismatch, viscosity, and contact angle on jumping velocities, where liftoff regimes are defined on the basis of Ohnesorge number and droplet size ratio. Because of the strong asymmetric flow behavior, the well-known small conversion efficiency for equal-sized-droplet jumping, where around 6% of the released surface energy is convertible into translational kinetic energy, is further reduced for unequal-sized-droplet jumping. The findings offer insights into their fluid dynamics and give a starting point for further modeling of dropwise condensation on superhydrophobic surfaces.