Do Partially Wettable Surfaces Influence the Drying Kinetics of Microliter Water Drops Beyond the Evaporation Models?

The surface free energy of solids is often determined from magnitudes of the advancing contact angle of drops of test liquids resting on the surface in open-air environment. One of the problems connected with such a determination is the adsorption of vapor molecules released from drops on the solid surface from the first moment of their deposition, leading to the formation of adsorption or wetting films ahead of the stationary drop's edge (pinned three-phase contact line). This causes a decrease in the surface energy, affecting its proper evaluation. The question arose (Fang, X.; Pimentel, M.; Sokolov, J.; Rafailovich, M. Langmuir 2010, 26, 7682-7685) whether the adsorption on partially wettable surfaces may be disclosed from the overall evaporation rate of droplets. Experimental time-dependent changes in the volume of microliter water droplets deposited on a series of model poly(ethylene terephthalate) (PET) surfaces, whose initial contact angle decreased continuously from ca. 80 degrees to 40 degrees due to the alkaline hydrolysis hydrophilization pretreatment of the surfaces, are found to be well-describable by a linear model restricted to the pinning stage of evaporation. A thickness of adsorbed films in equilibrium with the droplets was calculated from the augmented Young-Laplace equation in which isotherms of surface forces were inserted with reasonable values of parameters, namely, the effective Hamaker constant, electrostatic potentials, and the pre-exponential structural factor. The thickness varies within a range of few angstroms, prescribing the films to be of molecular dimensions. This indicates that the adsorption of water vapor from drops on partially wettable surfaces is feasible and possibly immediate. Nevertheless, since the volume of these so-called alpha-films is several orders of magnitude smaller than that of the water drops themselves, a growth of the former cannot affect the evaporation kinetics of the latter.