Measurements of liquid surface relief with moon-glade background oriented Schlieren technique

Measurements of liquid surface topography are performed using new modification of Background Oriented Schlieren technique, which is based on the apparent distortion of a background pattern image reflected by the deformed surface. The optical setup comprises a camera and a backlit background, both installed above the liquid surface. The camera images the background pattern reflection. The apparent displacement of the pattern features with respect to the reference image, taken for the unperturbed liquid surface, is proportional to the local slope of the surface. This enables one to obtain instantaneous surface relief map by solving Poisson equation. Compared to the popular free-surface synthetic Schlieren technique, our approach allows measurements of the surface topography over deep liquid layers and for opaque liquids or multiphase flows. It is also more sensitive, which enables reliable measurements of perturbations as small as 1 pm. Experiments are performed for propagation of the circular surface waves, for convection driven by heating of a horizontal wire installed below the liquid surface and for Rayleigh-Benard-Marangoni convection in liquid heated from below. Attenuation coefficient of the gravity-capillary waves is determined for wave frequencies about 10 Hz. Comparison of the results, obtained in glycerol, distilled water and silicone oil, shows the remarkable difference between the surface deformation patterns, observed in liquids, which exhibit Marangoni convection, and liquids, where it is suppressed by the surface film. In distilled water and glycerol elevated surface is always observed in hot regions, whereas in silicone oil the effect of convection on surface relief depends on the competition between the buoyancy-driven and surface-tension-driven flows.