Suppression/reversal of natural convection by exploiting the temperature/composition dependence of magnetic susceptibility

Natural convection, driven by temperature or concentration gradients or bot h, is an inherent phenomenon during solidification of materials on Earth. T his convection has practical consequences (e.g., effecting macrosegregation ) but also renders difficult the scientific examination of diffusive/conduc tive phenomena during solidification. It is possible to halt, or even rever se, natural convection by exploiting the variation (with temperature, for e xample) of the susceptibility of a material. If the material is placed in a vertical magnetic field gradient, a buoyancy force of magnetic origin aris es and, at a critical field gradient, can balance the normal buoyancy force s to halt convection. At higher field gradients the convection can be rever sed. The effect has been demonstrated in experiments at Marshall Space Flig ht Center where flow was measured by particle image velocimetry in MnCl2 so lution in a superconducting magnet. In auxiliary experiments the field in t he magnet and the properties of the solution were measured. Computations of the natural convection, its halting and reversal, using the commercial sof tware FLUENT(R) were in good agreement with the measurements. (C) 2000 Amer ican Institute of Physics. [S0021-8979(00)07319-9].