The instability of surface tension driven flow in models for floating zones under normal and reduced gravity

The time dependent thermocapillary flow in floating half-zones has been investigated experimentally under microgravity and under normal gravity by heating from above (opposite direction of thermocapillary and buoyant forces) and by heating from below (parallel driving forces). The critical Marangoni numbers Ma c under the three conditions differ only slightly for the small lengths of floating zones possible under gravity with the tendency that heating from below stabilizes the steady flow state. The direction of gravity plays a stronger role for the type of oscillation spectrum in the range Ma » Ma c where we observed transition to turbulence. The extrapolation of measurements of Ma c on melts with three different Prandtl numbers predicts for silicon a Ma c (Si) of the order of 100. This corresponds to a critical temperature gradient of the order of 5 K cm -1 for silicon.