Coalescence-induced jumping of droplets from superhydrophobic surfaces - The effect of contact-angle hysteresis

Abstract

Droplets coalesce and jump from superhydrophobic surfaces, a result that stems from the dominance of capillary and inertial forces and the presence of high contact angles. This phenomenon has been a subject of intensive numerical research mostly for cases when the degree of hydrophobicity is described by a single contact-angle value (a static contact angle). The introduction of various degrees of contact-angle hysteresis complicates the numerical modeling of the jumping process due to the sensitivity of the results to the effective value of the contact angle. We have developed and validated a comprehensive volume-of-fluid-immersed boundary numerical framework that accounts for the effect of hysteresis by focusing on the representation of actual (i.e., effective) values of contact angles. By comparing the behavior of jumping droplets on superhydrophobic surfaces with several degrees of hysteresis (up to 15°), we quantified the influence of hysteresis on the jumping process and identified various stages of the merged droplet's detachment and re-attachment to the surface. The latter phenomena were observed in all our simulations with droplets of different initial radii. In all the cases with hysteresis, the merged droplet eventually jumps, but we point out the decrease in the jumping velocity as compared to cases with only a static contact angle imposed. Finally, by using the Kistler dynamic contact-angle model, we demonstrate and quantify the importance of accurately capturing the dynamic receding contact angle when droplets jump from superhydrophobic surfaces with various degrees of hysteresis.