Studies of the wetting kinetics of liquid drops on solid surfaces

Abstract

The viscosityη _L and the surface tensionγ _L of the liquid as well as the equilibrium contact angleθ _e are essential parameters governing the wetting kinetics of liquids on solids. By means of a contact angle apparatus with video image digitization, the dynamic contact angle θ and the radiusr of the contact area of sessile drops on solid surfaces have simultaneously been determined in dependence on time after drop application between about 3·10⁻² s and long times.

The measurements were performed with series of liquids: polydimethylsiloxanes with different molecular masses and solutions of polyisobutylene in decalin and polyacrylic acid in water, covering a wide range of concentrations. The liquids in each series have a constant surface tension, but viscosities ranging over about four orders of magnitude, allowing the influence ofη _L andγ _L to be studied independently. Solids such as glass, polyethylene and polytetrafluoroethylene were chosen so that the cases of complete wetting (spreading) and partial wetting (θ _e) could be studied.

The curves of cosθ andr/R ₀ vs. time for the different liquids of a series can be superimposed to a master curve by plotting them againstγ _L·tη _L·R ₀, whereR ₀ is the radius of the original drop. All these master curves coincide at small wetting times, with exception of the data for the polysiloxanes. That means that the early stage of the wetting process is determined only by the properties of the wetting liquid. The influence of the solid surface, characterized by the equilibrium contact angleθ _e becomes significant only at the end of the wetting process.