Analysis of dynamic contact angle on discoidal samples measured by the Wilhelmy method

Abstract

Contact angle measurements have been performed using the Wilhelmy microbalance on disc-shaped samples of different thicknesses and these are compared with the theoretical analysis proposed by Dryden and Andrade (D-A). In disagreement with the original D-A results, our conclusion is that not only are the force values well reproducible, but also the determination of the contact angles, by inverting the force equations, provides acceptable estimates. This conclusion is supported by the analysis of error propagation for the various physical and geometrical parameters of the model. By taking into account the thickness of the samples a correction is introduced, the significance of which is analysed with respect to some of the experimental parameters. Moreover, a second method of analysis based on an iterative procedure is proposed; it uses only the force data collected during the immersion of the diameter zone of the disc samples and can be applied to both discoidal (e.g. lenses) and plane disc-shaped samples. Finally, an accurate analysis of the meniscus shape during the immersion leads to a third method for the estimation of contact angles on discs. It requires the detection of the immersion level at which the lateral meniscus becomes perfectly planar. Its use is restricted to the case of thick discs and the precision achieved is not high.