Unstable Thermocapillary Flow at a Gas‐Liquid Phase Boundary

We present results of thermocapillary experiments in the vicinity of a bubble under a heated wall. Thermocapillary convection is a flow along the interface of two fluids, which is caused by local gradients of the surface tensions σ. The aim of the present work is to study this flow phenomenon at higher Marangoni-numbers Mg. At sufficiently high values of Mg the flow exhibits oscillatory fluid motion and eventually becomes turbulent. For the detection of the flow velocity and temperature field, we applied a PIV method and a modified differential interferometer. In order to separate buoyancy effects from thermocapillary convection, we chose an experimental setup, where a bubble is positioned in a liquid matrix under a heated wall. We could observe various regimes of Marangoni flow, from steady flow over regular oscillating up to highly irregular oscillating flows at Mg up to 63,000. However, we also found unexpected stable flow regions and additional evaporation processes. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)