Taking a closer look: A molecular-dynamics investigation of microscopic and apparent dynamic contact angles

Hypothesis: Molecular dynamics (MD) may be used to investigate the velocity dependence of both the microscopic and apparent dynamic contact angles (theta(m) and theta(app)). Methods: We use large-scale MD to explore the steady displacement of a water-like liquid bridge between two molecularly-smooth solid plates under the influence of an external force F-0. A coarse-grained model of water reduces the computational demand and the solid-liquid affinity is varied to adjust the equilibrium contact angle theta(0). Protocols are devised to measure theta(m) and theta(app) as a function of contact-line velocity U-cl. Findings: For all theta(0), theta(m) is velocity-dependent and consistent with the molecular-kinetic theory of dynamic wetting (MKT). However, theta(app) diverges from theta(m) as F-0 is increased, especially at the receding meniscus. The behavior of theta(app) follows that predicted by Voinov: (theta(app))(3) = (theta(m))(3) + 9Ca center dot ln(L/L-m), where Ca is the capillary number and L and L-m are suitably-chosen macroscopic and microscopic length scales. For each theta(0), there is a critical velocity U-crit and contact angle theta(crit) at which theta(app) -> 0 and the receding meniscus deposits a liquid film. Setting theta(app) = 0, theta(m) = theta(crit) and U-cl = U-crit in the Voinov equation yields the value of L/L-m. The predicted values of theta(app) then agree well with those measured from the simulations. Since theta(m) obeys the MKT, we have, therefore, demonstrated the utility of the combined model of dynamic wetting proposed by Petrov and Petrov. (C) 2020 Elsevier Inc. All rights reserved.