Reflection-interference method to determine droplet profiles.

An optical method is developed to determine the instantaneous profile near the contact region of a minute droplet evaporating on a horizontal plate. A parallel coherent beam is split into two rays: one directed on the droplet at an oblique angle to the plate and the other passed through the droplet at a normal angle with the plate. The former produces a set of interference fringes by the light beams reflected from the droplet surface. Simultaneously, the latter creates a shadowgraph of the droplet on a screen. The liquid–vapor interfacial profile can be determined from the fringe patterns utilizing the droplet size on the plate calculated from its image on the shadowgraph. In addition, the contact angles can be uniquely defined and determined from the droplet profile.