Capillary-viscous retraction dynamics of droplets The role of the dynamic contact angle

This study investigates the retraction dynamics of viscous droplets on hydrophobic surfaces in the capillary-viscous regime. Using glycerol-water solutions on various hydrophobic substrates, we show that the observed droplet retraction rates can be described using the dynamic receding contact angle rather than the static one. When incorporating the dynamic receding contact angle into existing models, experimental data from different wettability substrates collapse onto a unified curve proportional to the inverse of the viscous timescale. Thus, we reveal that the retraction dynamics in this regime is governed by a balance between capillary forces and viscous dissipation, with the dynamic receding contact angle serving as the key parameter characterizing surface wettability effects. These findings establish a framework for predicting droplet behavior based on nonequilibrium wetting parameters. They extend previous observations from the inertial-capillary regime and provide insights for applications involving viscous droplet interactions with hydrophobic surfaces, including self-cleaning technologies and coating processes.