Abstract
We use molecular dynamics (MD) simulations to investigate the dynamic wetting of nanoscale water droplets on moving surfaces. The density and hydrogen bonding profiles along the direction normal to the surface are reported, and the width of the water depletion layer is evaluated first for droplets on three different static surfaces: silicon, graphite, and a fictitious superhydrophobic surface. The advancing and receding contact angles, and contact angle hysteresis, are then measured as a function of capillary number on smooth moving silicon and graphite surfaces. Our results for the silicon surface show that molecular displacements at the contact line are influenced greatly by interactions with the solid surface and partly by viscous dissipation effects induced through the movement of the surface. For the graphite surface, however, both the advancing and receding contact angles values are close to the static contact angle value and are independent of the capillary number; i.e., viscous dissipation effects are negligible. This finding is in contrast with the wetting dynamics of macroscale water droplets, which show significant dependence on the capillary number.
Highlights
A large depletion layer is formed near the hydrophobic surface, primarily because the structure of water molecules next to a hydrophobic surface is less ordered than in the bulk phase, which significantly reduces the cohesive strength of the water
In addition to the solid-liquid interaction effects, viscous dissipation effects were investigated by moving the silicon and graphite surfaces
It was found that for nanoscale droplets the solid-liquid interactions play a vital role in determining the wetting dynamics, while viscous dissipation effects induced by the moving surface were found to be only slightly important for the silicon surface and negligible for the graphite surface
Summary
Wetting phenomena play an important role in diverse processes across physics, chemistry, and biology.[1,2,3] The physics of wetting phenomena for water droplets is of fundamental importance in the design of surfaces that can mimic natural surfaces.[4,5,6] A thorough understanding of solid-liquid interactions at a molecular level is crucial to technological applications, including surface coating, emulsions, oil recovery, and in microfluidic and nanofluidic applications.[7,8,9,10]. The key experimental parameter describing the degree of wetting is the static contact angle θs, measured through the liquid L placed in contact with a solid S, at the contact line. The wettability determines the equilibrium configuration of the system: if θs is zero, the liquid is said to wet the solid completely and the solid surface is fully hydrophilic; if it is 180◦, the system is said to non-wet the solid and the surface is fully hydrophobic
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