Low Kapitza falling liquid films

- Thickness measurements on 2D wave on falling liquid film.
- Light absorption method and Hough transform for wave analysis.
- Dimensionless correlations for wave amplitude, wavelength and celerity at Low Ka.
- Dimensionless correlation for the traveling wave shape.

Low Kapitza liquids, such as mineral or vegetable oils, have a low surface tension and a high viscosity. In these liquids, capillary forces and extensional viscous dissipation play an equal role in damping interface deformation, resulting in a configuration which challenges the long wavelength assumption at the origin of low-dimensional models for falling films. This paper presents time-resolved 2D thickness measurements of a vertical falling film of Dipropilene Glycol (Ka=3.7±2%) using the light absorption technique. The test cases are in the low Reynolds number regime (Re=1-6), with a flow rate pulsing at frequencies in the range fp=12-24Hz. Different liquid film responses to the perturbation are reported, including growing waves, fully developed traveling waves and sinusoidal waves under period doubling instability. For the fully developed waves, simple dimensionless correlations for wave celerity, wavelength and wave profiles are proposed using the Skhadov scaling. The wave characteristics are compared to those of 2D waves in high Kapitza liquid films, as described by Nosoko's correlation (Nosoko et al., 1996).