Numerical study of droplet motion on discontinuous wetting gradient surface with rough strip

The self-transport of a droplet on a wetting gradient surface is of great importance in understanding the mechanism of surface coating and the design of numerous functional surfaces. Although it is known that the wetting gradient and surface condition are the main factors that influence the droplet transport, the effect of roughness on the motion on a discontinuous wetting gradient surface is worth further study. In this work, a numerical model based on the front tracking method was utilized to investigate the droplet’s motion on such surfaces. The capillary number Ca and the mass center xc were recorded to scale the transient speed and trace the motion, respectively. The self-transport under two regimes of driving forces for different smooth strip lengths is analyzed, and it is found that the roughness has a significant influence on the transport velocity and stability of the motion. Regimes of droplet crossing states are plotted for the roughness η and the wettability difference Δθ between two adjacent regions. The regime plot shows that the transport modes for droplets on discontinuous wetting gradient surfaces depend on the surface roughness.