Thermocapillary Liquid Bridges and Marangoni Convection under Microgravity—Results and Lessons Learned

Abstract

We describe the first measurements of the critical Marangoni number of thermocapillary flow in liquid bridges under microgravity. The experiments were conducted during the ballistic flight of rockets with small liquid bridges established by melting the equivalent cylindrical rod of solid sodium nitrate under microgravity. Fine thermocouples in the melt indicated the onset of flow oscillations by the increase of temperature oscillations. The critical Marangoni number M a c was measured to be approximately 1·104 for Prandtl number P r = 9. This type of experiment was especially simple, effective and suitable for execution during the short microgravity–time available during sounding rockets flights. The lessons learned from these experiments have been collected. In a second part of this paper we review five more sophisticated microgravity experiments in sounding rockets in which the surface tension driven flow was visualized by tracers and suitable illumination. Two experiments are devoted to the very first demonstration of the classical cellular Benard-Marangoni instability of thin liquid layers with free surface, heated from below. We could demonstrate in another experiment chaotic thermocapillary flow in a cubic cuvette filled with oil up to a flat free surface. Two experiments dealt with liquid bridges (LBs); in one the LB was nearly as long as possible at all. Here we could observe the helical nature of the hydrothermal wave in the geometry of the LB and measured a much smaller critical Marangoni number than in the short liquid bridges on ground. In the other experiment we investigated particle accumulation structures (PAS) and could show that PAS develops independently from gravity forces though it was indicated that gravity has an influence on PAS–formation by changing the flow field.