Abstract
In this paper, we report molecular kinetic analyses of water spreading on hydrophobic surfaces via molecular dynamics simulation. The hydrophobic surfaces are composed of amorphous polytetrafluoroethylene (PTFE) with a static contact angle of ~112.4° for water. On the basis of the molecular kinetic theory (MKT), the influences of both viscous damping and solid-liquid retarding were analyzed in evaluating contact line friction, which characterizes the frictional force on the contact line. The unit displacement length on PTFE was estimated to be ~0.621 nm and is ~4 times as long as the bond length of C-C backbone. The static friction coefficient was found to be ~{10}^{-3} Pa·s, which is on the same order of magnitude as the dynamic viscosity of water, and increases with the droplet size. A nondimensional number defined by the ratio of the standard deviation of wetting velocity to the characteristic wetting velocity was put forward to signify the strength of the inherent contact line fluctuation and unveil the mechanism of enhanced energy dissipation in nanoscale, whereas such effect would become insignificant in macroscale. Moreover, regarding a liquid droplet on hydrophobic or superhydrophobic surfaces, an approximate solution to the base radius development was derived by an asymptotic expansion approach.
Highlights
In addition to the viscous dissipation and adhesion energy that are manifest in the macroscopic processes[16], the excess surface free energy can be dissipated in the form of contact line friction, which becomes more prominent in the molecular level or in nanoscale, as a result of the oscillations of contact line molecules from their equilibrium positions[17]
In spite of the tremendous research efforts in studying the dynamic wetting process at the sub-continuum level, there continues to be a lack of understanding on the physical meanings of molecular kinetic parameters and the dominant factors governing the dynamic wetting process still remain elusive. de Ruijter et al.[21] applied the molecular kinetic theory (MKT) to the spontaneous spreading of liquid droplets on various solid materials and found the unit displacement length λ was around 1 nm
The MKT was coupled with molecular dynamics (MD) simulations and was reported to be able to depict the dynamic behaviors of nano-droplets[17, 21, 28,29,30]
Summary
In addition to the viscous dissipation and adhesion energy that are manifest in the macroscopic processes[16], the excess surface free energy can be dissipated in the form of contact line friction, which becomes more prominent in the molecular level or in nanoscale, as a result of the oscillations of contact line molecules from their equilibrium positions[17]. The continuous and macroscopic displacement of a contact line results from the collective manner of discretized forward or backward jumps of fluid molecules within the contact zone on a solid surface. The MKT was applied in processes involving dewetting[22], wetting on chemically heterogeneous surfaces[23], forced reactive wetting[24], and even to solid-liquid-liquid systems[25, 26] containing ionic liquids[27] In these studies, MKT-based predictions of some macroscopic parameters yielded satisfactory agreement with the experimental values. A meticulous analysis of the molecular kinetics in dynamic wetting process is needed to understand the physical meanings of MKT parameters and the fundamental mechanisms governing wetting dynamics Another topic of vast scientific interest is the contact zone formation, a multiscale phenomenon spanning from the molecular scale to the macroscopic scale. The solid material was chosen to be the amorphous polytetrafluorethylene (PTFE)
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