Role of droplet viscosity on the formation of residual droplets on grooved hydrophobic surfaces

Abstract

Surfaces with groove structures, such as butterfly wings and rice leaves, are frequently observed in nature, and the anisotropic nature of grooved structures is known to control fluid transport. Although the receding contact-line dynamics of the droplets on the grooved hydrophobic surfaces affect the behavior of droplets in motion, their depinning mechanism has not been sufficiently addressed in the literature. In this study, the receding contact-line dynamics of viscous droplets moving on inclined grooved hydrophobic surfaces were investigated using high-speed imaging. The droplet viscosity and surface-inclination angle were systematically varied to observe changes in the receding motion of droplets. The receding contact lines of each droplet contracted along the top of the groove structure and then ruptured due to discontinuity in the structure, leaving small volumes of droplets on top of the structure. Various morphological changes in the droplet were observed when it retracted along the grooves, which depended on the surface-inclination angle and viscosity of the droplet. A Rayleigh-like instability induced additional breakup of the tail of the droplet, resulting in satellite droplets being deposited on top of the grooves. The lateral size of the residual droplets deposited on the grooves increased as both the droplet viscosity and surface-inclination angle increased. The sizes of the residual droplets on tested surfaces collapsed into a single curve through a simple scaling equation developed by dimensionless analysis.