(90c) Numerical Study of Contact Line Jumping in Drop Dewetting

In this work we demonstrate the boundary conditions applied at the contact line can profoundly affect the resulting dynamics of a drop dewetting from an unfavorable surface.

In a 2-D simulation, we study the dynamics of a drop whose contact line encounters a change in its surface energy from a well wet region to a poorly wet region. The dynamics of contact line pinning, depinning and hopping is studied as the film thins, forming a ridge which propagates toward the bulk drop as the contact line recedes. We simulate the motion of the drop in the lubrication limit using two commonly adopted mechanisms to prevent the singular behavior at the contact line. In one approach, we assume the existence of an infinite, ultrathin, equilibrium liquid film ahead of the contact line that is stabilized by the balance of surface tension and disjoining pressure. In the other, we allow finite slip at the contact line.

When adopted for wetting studies, these two mechanisms predict largely the same wetting dynamics and free surface shapes outside the cutoff region. Simulations using either cut-off mechanism generate the universal (and experimentally observed) Hoffman's law (θ ³ ∞ Ca) at low to moderate capillary number. Thus, macroscopic wetting dynamics are insensitive to the local displacement mechanism close to the contact line.

In contrast, we find that the contact line boundary conditions give differing dynamics for drop dewetting. We compare the basic features of the evolution the film as given by the two models. Finally, we study how surfactants affect the flow.