Force based dynamic contact angles and wetting kinetics on a Wilhelmy plate

Abstract

A Wilhelmy plate method has been used here to measure the dynamic advancing and receding contact angles using a force balance technique. The liquid phase here was silicone oils of different viscosities and the substrate was a cover glass coated with a perfluorinated polymer. A model for such a system for comparison against the data was based on the de Gennes’ theory that the rate of viscous dissipation is equal to the rate of surface work. In this analysis the region in the immediate vicinity of the contact line is cutoff and the dynamic contact line actually refers to a slope of the wedge-like bulk liquid. de Gennes’ original model has been extended here to cover larger contact angles. The comparison with the experimental data shows good agreement for both advancing and receding contact angles. Data over an extensive range have been provided for the first time that such detailed agreement can be established. In addition, when the speed of the plate is increased, the receding contact angle decreases with increasing capillary numbers but does not show any receding contact angle less than 30°. This sudden stop conforms to de Gennes’ result that there is a minimum receding contact angle below which entrainment takes place. In the entrained systems, the force method shows the dynamic contact angles to be zero. Photographs were taken of the dynamic meniscus to illustrate the onset of entrainment. The experimental results are supportive of de Gennes’ model, and disagree with many conjectures on where the receding contact lines are entrained, or whether the dynamic contact angle is due to the failure of the equilibrium, etc.