A new experimental technique for measuring the dynamical free surface deformation in liquid bridges due to thermal convection

The dynamical free surface deformation due to thermal (Marangoni-buoyant) convection in liquid bridges of 5 cSt silicone oil was studied experimentally using optical imaging. The experiments were conducted under ground-based conditions for a wide range of temperature differences between the solid supports, and for several liquid bridge volumes. Oscillatory motion set in at a critical value of the temperature difference and caused oscillation of the free surface. The amplitude and fundamental frequency characterizing the magnitude and shape of that oscillation were obtained by optical imaging. The procedure allows one to describe the oscillation up to sub-micron order. The temperature oscillations at five points inside the liquid bridge were measured by five thermocouples. Their amplitude and fundamental frequency were compared with the corresponding values of the free surface oscillations. The optical imaging revealed the same dynamics as was obtained from the (more intrusive) temperature measurements. The spatial structure of the free surface oscillations along the liquid bridge axis was also analysed.