Magnetically controlled convection in a paramagnetic fluid

Abstract

CONVECTION in a liquid is important for problems involving heat transfer and crystal growth from a melt. The driving force for convection is usually the density difference between hot and cold regions of the fluid. If the fluid has a magnetic susceptibility that varies with temperature, magnetic forces, rather than buoyancy, can be made to drive convective motion. Studies on ferrofluids (suspensions of ferromagnetic particles¹) have shown that magnetic convection can be initiated in a homogeneous magnetic field^2,3 and enhanced in a field gradient⁴. We show here that the strong magnetic fields available from superconducting magnets can be used to induce magnetic convection in normal paramagnetic fluids, such as solutions of paramagnetic salts or melts of paramagnetic solids. We have used a magnetic field both to enhance and to suppress buoyancy-driven convection in a solution of gadolinium nitrate, the sign of the effect depending on the relative orientation of magnetic-field and temperature gradients. The effect might be exploited in heat-transfer devices or to control microstructures in crystal growth.