We performed molecular dynamics (MD) simulations to investigate equilibrium behavior of urea-water binary droplets on flat (graphitic) and pillared surfaces. The contact angles as a function of urea concentration on the flat surface are computed. It is found that the contact angle decreases as the urea concentration increases. At the equilibrium state, the urea molecules in the droplet tend to be located near the hydrophobic graphite surface. This behavior is consistent with the denaturing effects of urea in protein solutions. We also performed MD simulations of collision between a urea-water droplet and the pillared surface to examine the tendency for the droplet being in the Cassie state (droplet staying on top of the pillared surface) or in the Wenzel state (droplet staying at the bottom of the pillared surface), at various urea concentrations.
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