Dynamic wetting: hydrodynamic or molecular-kinetic?

The dynamic wetting behavior of simple liquids (water, glycerin, formamide, ethylene glycol, and a mixture of water and ethylene glycol) and polydimethylsiloxane (PDMS) oils with different viscosities has been investigated. The hydrodynamic, molecular-kinetic, and combined molecular-hydrodynamic models have been applied to the experimental results to evaluate each model's adequacy. The hydrodynamic model does not explain the experimentally observed dependence of dynamic contact angles on the contact velocity of simple liquids, and the fitted parameter (Ls) value is not physically reasonable; all are in the sub-atomic dimension. The molecular-kinetic model fits reasonably well with the experimental data of simple liquids, but the fitted parameters (k and Kw) are significantly different in advancing and receding cases. Finally, for simple liquids investigated in this work, the combined molecular-hydrodynamic model is not superior when compared to the molecular-kinetic model, which contradicts the literatures. These observations lead to a conclusion that the molecular displacement (adsorption and desoprtion) process seems to be dominant for the simple liquids investigated in this work. However, the inconsistency in the advancing and receding cases question the assumption in the molecular-kinetic model that "the same amount of work is done on each site, irrespective of whether the adsorption site is occupied by the advancing phase or the receding phase."

For the dependence of dynamic contact angles on the contact velocity for PDMS oils, the hydrodynamic model fits well with experimental results in terms of a linear relationship, but the fitted equilibrium contact angles are significantly different from the measured values. The molecular-kinetic model provides a relatively poor fitting to the experimental data of PDMS oils. Although the combined model fits reasonably well with the experimental data in terms of curve fitting, the fitted parameter Kw is practically less meaningful. This work provides the first, to the best of knowledge, extensive comparison of the three models with experimental data using a wide range viscosity. Remarkably, the work suggests that none of the evaluated models is sufficient to explain the dependence of dynamic contact angles of the investigated PDMS oils on contact velocity.

The contact angle hysteresis of the simple fluids and PDMS oils has also been investigated. The work supports the hypothesis that the hysteresis strongly depends on the interactions between the contacting fluids and the solid substrate and reports the dependence of hysteresis on fluid viscosity for the first time.