  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 phenomena 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 (MD)
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 (SPC/E) water model with particle-mesh Ewald
electrostatics treatment. This model provides a very small slip and a more
realistic representation of the molecular physics than Lennards-Jones models.
The Cahn-Hilliard phase-field model and the Volume-of-Fluid model are
calibrated against the drop displacement from MD reference data. It is
demonstrated that the calibrated continuum models can accurately capture
droplet displacement and droplet breakup for different capillary numbers and
contact angles. However, we also observe differences between continuum and
atomistic simulations in describing the transient and unsteady droplet
behavior, in particular, close to dynamical wetting transitions. The molecular
dynamics of the sheared droplet provide insight of the line friction
experienced by the advancing and receding contact lines and evidence of
large-scale temporal "stick-slip" like oscillations. The presented results will
serve as a stepping stone towards developing accurate continuum models for
nanoscale hydrodynamics.
