Role of substrate surface roughness in the collision of molten Sn droplets with stainless-steel substrates

In engineering applications, including additive manufacturing, metal spraying, microcircuit printing, and intense ultraviolet lithography, regulating the adherence of molten droplets is crucial for decreasing material waste and simplifying cleaning operations. This work studies the impact dynamics of tin molten droplets on stainless-steel surfaces with varied roughness using high-speed microscopy, concentrating on how surface roughness affects wettability and droplet motion. Results reveal that surface roughness strongly impacts spreading, retraction, and bouncing behaviors, particularly during the retraction phase. Notably, droplets bounce exclusively on surfaces with roughness Sa = 0.58 μm, showing distinct dynamics, whereas other roughness levels show no bouncing. A predictive model, based on the Reynolds number, Weber number, and surface roughness coefficient, properly depicts the influence of roughness on the maximum spreading factor, with just 2.48% forecast variation. Energy study demonstrates that surface roughness predominantly regulates droplet behavior by altering adhesive energy, with modest impacts on kinetic and surface energies. By controlling wettability and adhesive energy, surface roughness offers precise control over droplet motion, enabling the fabrication of specialized surface structures to improve spreading, retraction, and bouncing for applications.