Influence of roughness and coating on the rebound of droplets on fabrics

We investigate the behavior of small water droplets impacting fabrics of different roughness and coating type in a range of impacting conditions, and compare it to measurements of static and roll-off angles. Fabrics with low roughness display lower static contact angle, no roll-off, a slightly slower retraction rate for impacting droplets, a narrower Weber number (We) range in which rebound occurs, and a limited maximum rebound at intermediate We. Rougher fabrics exhibit slightly higher static contact angles, lower roll-off angles, a slightly faster retraction rate for impacting droplets, and a larger We range for rebound; additionally, the restitution of kinetic energy upon rebound is proportional to 1/We, indicating a near-to-constant rebound height independent of the impact velocity. As regards coating type, roll-off angle, We range for rebound, and to a lesser extent restitution coefficient, depend on coating type, with the more environmentally-friendly wax-based coating being superior to silicone and perfluorobutyl-based ones in both drop impact and roll-off experiments. Our results thus provide hope that perfluorinated coatings can be eliminated in standard textiles, and provide an extensive view of how roughness, coating and impact conditions control the fate of small droplets falling on textiles.