Using Gradient Magnetic Fields to Suppress Convection during Crystal Growth

Using in situ schlieren microscopy, we have studied the growth process of nickel sulfate hexahydrate crystals in a gradient magnetic field under conditions where convection is suppressed, to investigate whether the features that occur here are the same as those that occur during microgravity crystal growth. An expansion of the depletion zone with decreasing effective gravity was observed. If convection is suppressed, the depletion zone keeps expanding and the crystal growth rate decreases significantly, reaching a constant value. Although there remains some residual flow, it is very slow and does not affect the depletion zone. We find that homogeneity of the effective gravity in the milligravity range is sufficient to suppress convection. The results show that crystal growth in gradient magnetic fields is a good alternative for microgravity crystal growth in space.