Molecular Dynamics on the Contact Line Behavior at a Nanoscale Step

Abstract

Microscopic contact angle at a contact line traveling over each backward-facing step with heights of 0.3-1.2 nm was investigated with molecular dynamics (MD). The simulations were performed for a Couette-flow-like geometry which is composed of an upper flat plate and a lower plate with a step. Two kinds of immiscible liquids between the plates were sheared by the lower plate moving with a velocity of 1 m/s. When the contact line reaches the backward-facing step, it tends to be strongly captured (pinned) at the top edge of the step. The minimum contact angle of the receding fluid, which was observed at the moment of the depinning of the contact line, is decreased with the increment in the step height. In order to elucidate the mechanism relating the minimum contact angle to the step height, a description on the stress balance along the step surface was derived under the assumption of the static condition. It has been found that the static contact angle is decreased by the stress on the vertical surface of the step, the stress which associates with the anisotropic pressure in the liquid-liquid interface region. In addition, such decrement in the contact angle is enhanced by the increase in the step height through the intensification of the total shear stress over the vertical surface of the step.