Convection in a floating zone without rotation

Marangoni convection (MC) and natural convection (NC) were studied on the ground in model experiments involving NaNO3 zones. The effects produced by both convection types could be separated by suppressing MC with the aid of a surface impurity. Streamlines and temperature distributions were measured and evaluated. MC dominates in zones with small diameter. Model experiments conducted with zones of larger diameter show that MC dominates near the free surface and NC in the zone interior. The application of these results to realistic situations related to the growing of crystals is discussed. The results of an experiment conducted with the aid of a TEXUS rocket for the study of time-dependent MC are also reported. The occurrence of time-dependent MC is largely independent of direction and intensity of the gravitational force. An occurrence of time-dependent MC must be expected even under microgravitational conditions when the critical Marangoni number is exceeded.