  Droplet motion over a surface with wettability gradient has been simulated
using molecular dynamics (MD) simulation to highlight the underlying physics.
GROMACS and Visual Molecular Dynamics (VMD) were used for simulation and
intermittent visualization of the droplet configuration respectively. The
simulations mimic experiments in a comprehensive manner wherein micro-sized
droplets are propelled by surface wettability gradient against a number of
retarding forces. The liquid-wall Lennard-Jones interaction parameter and the
substrate temperature were varied to explore their effects on the three-phase
contact line friction coefficient. The contact line friction was observed to be
a strong function of temperature at atomistic scales, confirming the
experimentally observed inverse functionality between the coefficient of
contact line friction and increase in temperatures. These MD simulation results
were successfully compared with the results from a model for self-propelled
droplet motion on gradient surfaces.
