A possible way to extract the dynamic contact angle at the molecular scale from that measured experimentally.

HYPOTHESIS

The maximum velocity of dewetting encodes sufficient information on the hydrodynamics of the wetting process to enable the local dynamic contact angle at the molecular scale, θ, to be determined from the apparent contact angle measured experimentally at much larger scales, θ.

METHODS

Effective models of wetting dynamics need to account for differing channels of dissipation. One such model was recently verified by large-scale molecular dynamics (MD). It combines the 2-parameter molecular-kinetic theory of dynamic wetting (MKT), which attributes the velocity-dependence of θ to dissipation at the contact line, with the Cox-Voinov hydrodynamic (HD) model. The latter attributes the difference between θ and θ to viscous bending of the interface and contains an additional, non-predictable, logarithmic parameter. Crucially, the MD simulations indicated that viscous bending may play a minor role during wetting, but dominates dewetting. This observation suggested that by applying the MKT to the advancing contact angle only and combining the results with the maximum velocity of dewetting, it might be possible to extract the value of the logarithmic parameter and so determine θ and, hence, the relative significance of the two channels of dissipation. A simple iterative procedure has been developed to achieve this.

FINDINGS

Data available to test the procedure are sparce, but comparisons with the MD results and those from three experimental studies are encouraging. Near perfect agreement is achieved with the simulations, where both θ and θ are known, and plausible results are obtained for the experimental systems. Moreover, the procedure appears to be more effective than simply fitting θ to the 3-parameter model.