Profile patterns and stability of evaporating liquid sessile drops

An optical method named as a drop-refracting shadowgraphy was developed to determine the profiles of sessile drops and to visualize capillary flow in the drop. The rays are refracted by the drop to form a shadow-graphic image on a screen far from the drop, with or without most bright thick ring and a set of fringes, when a parallel laser beam passes through a sessile drop on a slide glass. Five basic profile patterns were revealed through the analysis of caustics and caustic diffractions, based on wave theory. It was observed that when the drop starts spreading, a precursor film immediately forms on the solid surface. It was found that the three precursor film C-P type, and spheroid cap with smooth protruding and the P-C type profile only occurs at the beginning of spreading.