Experimental study of the free surface deformation due to thermal convection in liquid bridges

Optical imaging was used to measure the free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5-cSt silicone oil. We obtained the free surface position averaged over time in both the steady and oscillatory regimes. The deviation of the free surface contour from the corresponding equilibrium shape was determined with an uncertainty of about 2 μm. This deviation grew linearly with the applied temperature difference with a proportionality coefficient depending on the liquid bridge volume at equilibrium. Shrinkage at the upper part of the liquid bridge was slightly greater than bulging at the lower with the sum of the maximum deviations at both parts being about 30 μm near the onset of oscillations. This sum, normalized with the radius of the supporting disks, was of the same order of magnitude as the Capillary number. We observed the influence of thermal expansion, surface tension variation over the free surface, and fluid motion separately. The local mean curvature was also calculated and compared with its value at equilibrium, showing that the hydrodynamic effects were important.