Coalescence-induced jumping and condensation of argon nanodroplets in the Cassie or the Wenzel state on nanopillar-arrayed surfaces

Coalescence-induced droplet jumping on pillar-arrayed surfaces has received extensive attention. The pillar size and distribution affect the coalescence and jumping of droplets with unclear mechanisms. For the first time, this study performs molecular dynamics (MD) simulations to reveal the effects of initial wetting states on jumping and associated condensation processes at various intrinsic contact angles (theta Y), solid fractions (f), and roughness factors (r). The results show that although jumping and its velocity strongly depend on theta Y, f, and r, as reported in previous studies, the underlying mechanism to determine whether jumping occurs is the initial wetting state of coalescing droplets on textured surfaces. With the same theta Y and f, when the droplets are initially in the Cassie state, jumping can take place, whereas it may be hindered when the droplets are in the Wenzel state. The jumping velocity shows a decreasing trend for Cassie droplets when increasing f, but the reverse is true for Wenzel droplets. Additionally, it is found that the wetting state of a condensed droplet would transition from the Wenzel state to the Cassie state when the solid fraction increases but the pillar height and intrinsic wettability remain unchanged.