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 degrees. 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.