Two-perspective fluorescence analysis of droplets creeping down a tilted plate

Many experimental techniques have been developed and applied to investigate hydrodynamics of liquid films and single droplets on solid substrates. A simple but reliable measurement technique has been recently proposed to quantify thickness and apparent contact angles of droplets and rivulets (Hagemeier et al. Exp Fluids 52(2):361–374, 2012). However, this technique leads to ambiguities for any contact angle exceeding 90°. An improved version has thus been derived to solve the most important issues associated with the original method. For this purpose, top and sideways two-perspective images are acquired simultaneously, both relying on fluorescence. Analyzing the data obtained from both views, a correlation between fluorescence intensity and droplet shape can be derived. Furthermore, advancing and receding contact angles can be determined in this manner. A new and particularly important feature of the improved technique is the estimation of the contact line velocity at various locations, all around the moving droplet. The in-plane velocity components show a clear dependency on the Bond number and on the position around the droplet circumference.