Sliding motion of highly deformed droplets on smooth and rough surfaces: Shape oscillation, chaotic breakage, corner shape, and pearling

The sliding behavior of droplets on smooth and rough surfaces with various surface wettabilities is investigated by many-body dissipative particle dynamics simulations. On a smooth surface, as the driving force ( B o ) increases, the droplet shape and velocity ( C a c ) before breakage can be classified into four distinct regimes: (I) nearly spherical cap with C a c ∝ B o ; (II) oval shape with negative deviation from the linear relation; (III) elongated shape without a neck, where C a c decreases with increasing B o ; and (IV) oscillation of an elongated shape with fluctuating sliding velocity. On rough surfaces, corner-shaped droplets, which are absent on a smooth surface, can be observed. A further increase in B o leads to the formation of cusp and pearling. Different from pinching-off on rough surfaces, which produces a cascade of smaller droplets through groove-induced shedding, chaotic breakage of a droplet on a smooth surface is caused by an unsteady flow field. Finally, a universal linear relationship between the sliding velocity based on the surface velocity ( C a s ) and the modified driving force ( B o * * ) is derived to take into account the effects of surface wettability and roughness.