Experimental investigation of liquid films in gravity-driven flows with a simple visualization technique

A visualization technique based on light absorption is used to monitor the thickness profile of a liquid film flowing on an inclined plane with high spatial and temporal resolutions. Surface waves are observed for a certain range of experimental parameters as expected from the classical stability analysis from Benjamin (J Fluid Mech 2:554–574, 1957). The liquid films are found systematically thicker than predicted by Nusselt (Z Ver Dtsch Ing 60:541, 1916) in the case of ideal viscous flows. We interpret this increase in thickness as a consequence of the propagation of waves on the films. The wave dynamics are in qualitative agreement with the asymptotic development from Anshus (Ind Eng Chem Fundam 11:502–508, 1972). Although the wavelength distribution is rather broad, space-time analysis indicates a well-defined phase velocity. Representing the wave velocity of a corrected Reynolds number allows to superimpose the experimental data into a single master curve.