Contact line dynamics on heterogeneous surfaces

Contact line dynamics on rough or chemically heterogeneous surfaces has been a subject of great interest. Most previous work focused on the issue of contact angle hysteresis in the static limit. This paper is devoted to the study of contact line dynamics on a chemically patterned surface over a wide range of contact line speed. Numerical simulations are carried out for two immiscible fluids confined in a channel and driven by either the shear motion of the two confining walls or an external force. It is found that in the low-speed regime when the averaged contact line speed U≪γ/β*, with γ being the surface tension of the fluid interface and β* the friction coefficient at the contact line, the behavior of the contact line dynamics is very similar to that of the static limit, namely it undergoes a stick-slip motion and the contact angle exhibits hysteretic behavior. At finite speed, the stick-slip behavior gradually diminishes, and the contact line motion becomes more smooth. The effect of these microscale dynamics on the averaged force between the fluid and the solid is investigated. It is found that while the friction force increases linearly with the averaged contact line speed, the force at the contact line due to the defect decreases with U. It is non-zero in the static limit and this is the cause of the contact angle hysteresis. As a result, the total force at the contact line may become non-monotone as a function of the contact line speed. This gives rise to an unstable regime for the contact line dynamics, which is indeed observed in the simulation when the dynamics is driven by an external force.