Incorporating molecular scale effects in the description of contact lines is difficult but necessary in order to accurately account for all sources of energy dissipation in wetting dynamics. This  holds particularly true in the cases where contact line friction determines wetting dynamics and hydrodynamics models struggle to find a regularisation due to the negligible slip of the wetting liquid over the solid surface. We perform Molecular Dynamics simulations of water/hexane biphasic systems, in the two-phase Couette flow configuration. Wetting occurs over a no-slip silica-like surface with variable wettability. The simulation results are reproduced by a Cahn-Hilliard Navier-Stokes model, which includes localised contact line slip and contact angle dynamics. The continuous equations are directly parametrized from Molecular Dynamics simulation results, under the assumption of the numerical sharp interface limit. The reconfiguration of the liquid/liquid interface and the flow structure are found to be in good quantitative agreement. In particular, interface bending due to viscous flow and contact line friction is fully reproduced. Navier slip is calibrated to ensure numerical stability. The viable combinations of Navier slip and Cahn-Hilliard mobility parameters that agree with Molecular Dynamics simulations in the sharp interface limit are reported and discussed. The results presented in this article indicate that Phase Field modeling can capture the effects of molecular processes on the mobility of contact lines and that an accurate determination of contact line friction is key to fully reproduce Molecular Dynamics simulations.

The main causes of energy dissipation in micro- and nanoscale wetting are viscosity and liquid-solid friction localized in the three-phase contact line region. Theoretical models predict the contact line friction coefficient to correlate with the shear viscosity of the wetting fluid. Experiments conducted to investigate such correlation have not singled out a unique scaling law between the two coefficients. We perform molecular dynamics simulations of liquid water-glycerol droplets wetting silicalike surfaces, aimed to demystify the effect of viscosity on contact line friction. The viscosity of the fluid is tuned by changing the relative mass fraction of glycerol in the mixture and it is estimated both via equilibrium and nonequilibrium molecular dynamics simulations. Contact line friction is measured directly by inspecting the velocity of the moving contact line and the microscopic contact angle. It is found that the scaling between contact line friction and viscosity is sublinear, contrary to the prediction of molecular kinetic theory. The disagreement is explained by accounting for the depletion of glycerol in the near-wall region. A correction is proposed, based on multicomponent molecular kinetic theory and the definition of a rescaled interfacial friction coefficient.

Motion of three-phase contact lines is one of the most relevant research topics of micro- and nano-fluidics. According to many hydrodynamic and molecular models, the dynamics of contact lines is assumed overdamped and dominated by localized liquid-solid friction, entailing the existence of a mobility relation between contact line speed and microscopic contact angle. We present and discuss a set of non-equilibrium atomistic molecular dynamics simulations of water nanodroplets spreading on or confined between silica-like walls, showing the existence of the aforementioned relation and its invariance under wetting modes ( "spontaneous " or "forced "). Upon changing the wettability of the walls, it has been noticed that more hydrophilic substrates are easier to wet rather than de-wet; we show how this asymmetry can be automatically captured by a contact line friction model that accounts for the molecular transport between liquid layers. A simple examination of the order and orientation of near-contact-line water molecules corroborates the physical foundation of the model. Furthermore, we present a way to utilize the framework of multicomponent molecular kinetic theory to analyze molecular contributions to the motion of contact lines. Finally, we propose an approach to discriminate between contact line friction models which overcomes the limitations of experimental resolution. This work constitutes a stepping stone toward demystifying wetting dynamics on high-friction hydrophilic substrates and underlines the relevance of contact line friction in modeling the motion of three-phase contact lines.

The motion of the three-phase contact line between two immiscible fluids and a solid surface arises in a variety of wetting phenomena and technological applications. One challenge in continuum theory is the effective representation of molecular motion close to the contact line. Here, we characterize the molecular processes of the moving contact line to assess the accuracy of two different continuum two-phase models. Specifically, molecular dynamics simulations of a two-dimensional droplet between two moving plates are used to create reference data for different capillary numbers and contact angles. We use a simple-point-charge/extended water model. This model provides a very small slip and a more realistic representation of the molecular physics than Lennard-Jones models. The Cahn–Hilliard phase-field model and the volume-of-fluid model are calibrated against the drop displacement from molecular dynamics reference data. It is shown that the calibrated continuum models can accurately capture droplet displacement and droplet break-up for different capillary numbers and contact angles. However, we also observe differences between continuum and atomistic simulations in describing the transient and unsteady droplet behaviour, in particular, close to dynamical wetting transitions. The molecular dynamics of the sheared droplet provide insight into the line friction experienced by the advancing and receding contact lines. The presented results will serve as a stepping stone towards developing accurate continuum models for nanoscale hydrodynamics.