Effect of the width of a bar substrate on the dynamic curvature of a thermocapillary depression and the shape of the thermocapillary response

Abstract

The effect of the width of an absorbing bar substrate on the radial symmetry of a thermocapillary depression is investigated by studying the velocity field of a convective vortex induced by the thermal effect of a He-Ne laser beam in a thin layer of transparent liquid. The beam reflected from the depression forms an elliptical fringe pattern on a screen placed at the beam cross section. This pattern is caused by the fact that the radius of the curvature of the depression across the substrate is smaller than that along the substrate. This circumstance is explained by the higher losses to viscous friction for the flow directed along the bar. In the case of a flat and infinitely extended substrate, the substrate begins to affect the shape of the fringe pattern when the bar width becomes comparable with the diameter of the thermocapillary depression. As the bar width increases, the eccentricity of the pattern increases. An increase in the liquid viscosity leads to a decrease in the depression diameter, and the fringe pattern tends to a circular shape.