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

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 θ: (i) Hemi-wicking Cassie-Baxter state for θ < θc1, (ii) Wenzel state for θc1 < θ < θc2, and (iii) Cassie-Baxter state for θ > θc2, where θc1 and θc2 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 θ > θc2 and the bottom surface becomes rougher for θ < θc2. 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 θ > θc2).