The role of surface microstructures in particle-droplet coalescence and jumping from superhydrophobic surfaces

We study here by direct numerical simulations how geometrical and wetting properties of rectangular pillar-shaped microstructured superhydrophobic surfaces affect the process of particle-droplet coalescence and jumping. Such a process represents a passive mechanism of self cleaning where a single droplet coalesces with a particle of micrometer size, spreads on the particle and drives particle-droplet jumping. An in-house volume of fluid-immersed boundary framework is used to study the influence of characteristic dimensions of pillars (width, solid area fraction and height) and contact angles on particle-droplet coalescence and jumping. We relate our results for pillared surfaces to those obtained when using corresponding planar superhydrophobic surfaces. A range of different behaviors are disclosed when it comes to energy dissipation mechanisms and outcome of the coalescence and jumping process for these two types of superhydrophobic surfaces. We quantify and explain how the detachment of the particle-droplet system from a pillared substrate takes place at a later stage than that from a corresponding planar substrate. A reduced effect is demonstrated on the process of having configurations with a larger distance between the pillars and that longer pillars facilitate earlier jumping as compared to that on surfaces with shorter pillars. We also look at the transition between Cassie-Baxter and Wenzel states (determining whether jumping of the particle-droplet system will take place at all) as a function of wetting properties of the surfaces. Our results contribute to optimization of superhydrophobic surfaces aiming to achieve an enhanced self-cleaning efficiency. • DNS of particle-droplet coalescence and jumping from pillared surfaces. • Delayed jumping from pillared compared to planar surfaces. • The pitch between pillars has a limited effect on the jumping energy. • Short pillars cause a reduced jumping velocity as compared to long ones.