The combined effects of magnetic field and magnetic field gradients on convection in crystal growth

The effect of an external magnetic field on convection in crystal growth is investigated numerically. The geometry considered is a cylindrical vessel filled with crystal solutions. The vessel partially heated from below is placed outside or inside a superconducting magnet. Numerical simulations show that the convection can be suppressed or promoted mainly due to the location of the vessel in the magnet. When the vessel is located above the center of the magnet, the convection is reduced more greatly than that at the center of the magnet. However, when the vessel is located below the center of the magnet, the convection is greater than that at the center of the magnet and that outside the magnet in absence of the field. The above results demonstrate that both magnetic effects on convection in solution will occur due to the magnetic field and its gradient. Generally, the magnetic field will induce the Lorentz force to damp the convection when the solution is electrically conducting. On the other hand, the magnetic field gradient resulted from the inhomogeneity of the field will produce the magnetization force and the magnetic buoyancy to suppress or promote the convection when the vertical magnetization force is opposite to or the same as the gravity. The above results can qualitatively explain recent experimental findings in protein crystal growth and show the potential of using both the magnetic field and field gradient to control convection in nonconducting or low conducting solutions, especially in the process of crystal growth from aqueous solutions. (C) 2004 American Institute of Physics.