An experimental methodology for determining solid/liquid interfacial energy for wettable surfaces

The solid/liquid interfacial energy is needed to predict the outcome of interactions between liquids and solids. Although several investigations have been carried out to determine this important quantity, no direct experimental method exists to achieve this goal. We perform contact angle experiments on different hydrophilic substrates with different degrees of roughness, a primary factor affecting the wettability of a surface. As the contact line of a sessile droplet advances, a resisting force acts in a direction opposite to that of the contact-line advancement. This resisting force increases linearly with surface roughness, and the slope of the corresponding curve is equal to the solid/liquid interfacial energy. The resisting force (similar to solid-solid frictional force) finds its origin in the roughness of the surfaces, and is obtained by performing simple sessile-droplet experiments. The present method demonstrates a new and simple approach to predict the solid/liquid surface energy for hydrophilic surfaces. The experimental findings are elucidated using well-known arguments from wetting theory and contact angle hysteresis.