On the origin of contact angle hysteresis

In this study, we consider a simple distribution of defects on a substrate, or a sinusoidal surface, and show explicitly how the pinning and depinning occur for a two dimensional liquid drop on such non-ideal surfaces as the volume of the drop is increased or decreased. We show that the contact angle hysteresis (CAH) emerges from this simple model even though we do not take any effect of viscous dissipation into account, which is in contrast with the conventional theory where the CAH originates from the viscous dissipation inside the liquid around the contact line just after depinning.