Mesoscale simulation of a molten droplet impacting and solidifying on a cold rough substrate

摘要

A molten droplet impacting and solidifying on a cold substrate with micro-pillars on the top is studied numerically in this paper using a newly developed multi-component, triple-phase solidification 3D lattice Boltzmann method (LBM). Simulated results confirm that droplets on a rough surface can exist in three states, depending on the static contact angle of the substrate theta: (i) Hemi-wicking Cassie-Baxter state for theta theta(c1), (ii) Wenzel state for theta(c1) theta theta(c2), and (iii) Cassie-Baxter state for theta theta(c2), where theta(c1) and theta(c2) are two critical contact angles whose values depending on the geometry of the micro-pillars. It is shown that the air between the droplet and micro-pillars is compressed by the falling droplet before impact, and the compressed air is pushed into grooves of the surface microstructure after impact. As a result, no air bubble is formed under the droplet bottom surface after its impact and solidification on the rough surface. The solidified droplet's bottom surface is smooth for a substrate with contact angle theta theta(c2) and the bottom surface becomes rougher for theta theta(c2). Two rough substrates with wetting and non-wetting interlaced wettabilities are presented for different applications. Simulated results show that fingers are formed at the triple-phase contact line along the rim of the droplet on the rough surface with non-wetting interlaced base (with theta theta(c2)).