Velocity-Dependent Contact Angle and Energy Dissipations of Dynamic Wetting Nanodroplets on Nanopillared Surfaces

Dynamic wetting, described by a dynamic contact angle (DCA), is a fundamental behavior of fluid on surface. With the development of blue energy, the research of droplet nanogenerator is flourishing. There is a growing interest in the dynamic wetting behavior of nanodroplets on surfaces. Molecular dynamics simulations are performed to reveal the influence of the velocity of nanodroplets and the wetting state (Cassie and Wenzel) on the DCA and the energy dissipation on the contact line. The simulation results demonstrate a more complicated scenario of dynamic wetting than the static wetting: The increasing rate of advancing DCA is lower than the decreasing rate of the receding DCA with respect to the nanodroplet velocity. As for the Wenzel state, larger surface roughness increases the dynamic wetting hysteresis, while for Cassie nanodroplets, the larger surface roughness leads to smaller dynamic wetting hysteresis. It is found that a structural force exists on the rough surface. The energy dissipation of the dynamic wetting mainly comes from the motion of the contact line, which is positively correlated to the velocity and can be decomposed to the viscosity and friction dissipations, respectively. The Cassie state causes much lower energy dissipation than the Wenzel state. Furthermore, the quasi-static contact angle is proposed to describe the contact angle on a rough surface. These findings advance the understanding of dynamic wetting behavior and inspire theoretical guidance for the design of novel functional interfaces.