Visualization of dynamic wetting by means of critical light reflection from curved liquid surface

Abstract

A technique for dynamic wetting visualization, based on the critical light reflection, is developed. The technique is capable of real-time charactering the shape and contact angle of a liquid surface curved by a moving rod. A particular reflection pattern with oval or diamond shape resulted from critical reflection of the curved surface is observed. Study reveals the dependence of the characteristic width of the pattern on the critical reflective angle decided by the surface shape. The correlations of the width to advancing or receding angle are also experimentally established. The rang of advancing contact angle change is experimentally found over 50° so that the curved water surface turns from upward to downward. Theoretical descriptions based on dynamic wetting and geometric optics are used to analyze the formation, the change and the optical fringes of the special reflection pattern, and which account for most of the observations.