Effect of Nano-Scale Steps on the Microscopic Dynamic Contact Angle

Abstract

The motion of the contact line on a solid surface is strongly related with the contact angle there. In these days the relation between them has been revealed with the aid of molecular dynamics techniques; however, most of the studies on such phenomena have been made by assuming the solid surface to be molecularly planer and homogeneous, which condition is not necessarily satisfied in general industries. In this study, we conducted molecular dynamics simulation to investigate the behavior of dynamic contact line with a solid surface accompanied by steps parallel to the contact line. We applied a Couette flow geometry, where two immiscible fluids are confined between two parallel walls, one of which has the steps and is forced to move in the direction normal to the contact line. The behavior of the contact line and the variation in the contact angle were investigated by changing the height of the step from α0 to 4α0 where α0 is the lattice constant of the wall structure. It is shown that in case of the step height identical to α0, the contact line is temporary captured on the edge of the step. On the other hand with higher step the contact line is almost completely pinned on the step edge. The change in the contact angle by the pinning of the contact line on the step edge can be estimated by the macroscopic model as far as the step height is larger than the interaction range between molecules.