Contact line mobility in liquid droplet spreading on rough surface

We quantitatively estimate the effect of the substrate roughness on the liquid droplet spreading. Since the droplet size is in the order of millimeters, the surface energy becomes the dominant factor. A nonequilibrium thermodynamics framework [Y.X. Gao, H. Fan, Z. Xiao, Acta Mater. 48 (2000) 863-874] seems feasible for describing the millimeter size droplet spreading on a solid substrate. Within the framework, there are two system constants, namely the mobilities of liquid/air surface and the triple joint contact line that need to be determined from experimental testing. In the present paper, we demonstrate the experimental process of determining the mobility of the contact line via a droplet spreading on a steel substrate. Particularly, we obtained the contact line mobility on a steel surface with various roughness values. It is shown that the mobility value is lower for a rougher surface.