Retraction Dynamics of Droplets Impacting on Mask Surfaces*

Phenomena of droplet impact on solid surfaces are ubiquitous in nature and have been widely explored for industrial applications, such as inkjet printing systems, anti-icing, and spray cooling. Recently, droplet impact on a face mask has been considered since COVID-19 pandemic. In this study, we experimentally studied the retraction dynamics of droplets impact on mask surfaces with different roughness. We firstly focused on understanding the relationship between retraction phase and the final impingement mode. During inertia-dominant retraction regime, the kinetic energy was investigated to analyze the retraction dynamics. We observed that the retraction kinetic energy determined the final impingement mode. Secondly, in order to investigate the effect of surface roughness on the droplet behavior, we monitored the retraction dynamics of the droplet upon each surface by high-speed imaging. The results showed that the rough rim pattern formed upon the rough surface decreased the retraction kinetic energy. This study provides a better understanding of retraction dynamics by developing a theoretical energy model to predict the final behavior of the droplet. We also discussed the mask roughness for minimizing disease transmission with a good breathable performance.