Catastrophe Optics Method to Determine the Micro-Nano Size Profiles at TPL of Liquid Films on a Solid Surface

As discovered by recent studies, the wetting and spreading is directly affected by the curvature in the micro-region rather than the macroscopic contact angle. Consequently, measuring the liquid profile in the micro-region becomes an important research topic. Recently, catastrophe optics has been applied to these measurements. Optical catastrophe occurring in far field of waves of liquid-refracted laser beam yields a wealth of information about the liquid spreading not only for liquid drops but also for films. When a parallel laser beam passes through a liquid film on a slide glass at the three-phase-line (TPL), very interesting optical image patterns occur on a screen far from the film. An analysis based on catastrophe optics discloses and interprets the formation of these optical image patterns. The analysis reveals that the caustic line manifested as the bright-thick line on the screen implies the lowest hierarchy of optical catastrophes, called fold caustic. This optical catastrophe is produced by the inflexion line on the liquid surface at the liquid foot, which is formed not only in the spreading of drops but also in spreading of films. The generalized catastrophe optics method enables the identification of the edge profiles and determine the edge foot height of liquid films.