Jiangtao Cheng
Virginia Polytechnic Institute and State University, USA
Title: Contact line dynamic of cassie-state wetting on ultrahydrophobic nano-structured surfaces
Biography
Biography: Jiangtao Cheng
Abstract
We report a molecular dynamics (MD) study on the wetting dynamics of Cassie-state water droplets on ultrahydrophobic nano-structured surfaces. The surface materials were selected to be the amorphous polytetrafluoroethylene (PTFE). Our analysis in the framework of molecular kinetic theory (MKT) indicates that nano-droplets of water exhibit a constant unit displacement length of ~6.05±0.48Å regardless of the surface topography. The contact line friction (CLF) originates from the solid-liquid retarding Gw and viscous damping Gvis, and is also influenced by the fraction of solid-liquid contact. Gw is related to the work of adhesion and is independent of the surface structure. The effects of Gw become manifest in the orderly packing of water molecules at the droplet base. As a result of the solid-liquid retarding, a thin depletion layer of ~2.852 Å thick is formed at the droplet base on smooth PTFE surfaces. However, such depletion phenomenon is mitigated on nanostructured surfaces owing to the sagging of the droplet base. The potential of mean force analysis ascribes Gvis to the fluctuations of relationship of ~sin 20 (θ0 is the static contact angle) is derived In liquid density in the vicinity of solidliquid interface. A heuristic essence, the non-sticking feature of ultrahydrophobic structured surfaces (smaller CLF and larger θ0) indeed roots in the reduced solid-liquid contact. On a smooth PTFE surface, the static friction coefficient, which characterizes the static frictional force exerted on the contact line, was found to be on the same order of magnitude as the dynamic viscosity and increase with the droplet size. A non-dimensional number, which signifies the strength of the inherent contact line fluctuation, was put forward to unveil the mechanism of enhanced energy dissipation in nanoscale, whereas such effects would become unapparent in micro scale. Moreover, regarding a liquid droplet on hydrophobic/super hydrophobic surfaces, an approximate solution to the base radius development was derived by an asymptotic expansion approach.