Measurement of two-dimensional distribution of surface supersaturation over a sodium chlorate crystal surface using multidirectional interferometry

The two-dimensional (2D) distributions of surface supersaturation of sodium chlorate crystals with and without solutal convection have been measured by means of a multidirectional interferometry (MDI) technique coupled with the principles of three-dimensional (3D) computer tomography. When solutal convection was present over a top face, the supersaturation at the center of the face was depleted by a factor of >0.9 with reference to the value at the edges of the crystal. When the convection was suppressed using an upside-down geometry, the depletion of supersaturation at the center of the face was much smaller, <0.4. Therefore, the supersaturation difference between the edges and the face center, which is responsible for the morphological stability due to volume diffusion for the solute, becomes less important compared to the effect of convection due to hydrodynamic reasons. This result should give us a key to solve why the crystal quality is sometimes better in convection-free microgravity conditions because of improved stability of a crystal face caused by more homogeneous distribution of supersaturation over the crystal surface. We measure the 2D distribution of surface supersaturation of sodium chlorate crystals and show that when convection was present over a top face, the supersaturation at the center of the face was depleted whereas when convection is suppressed the depletion was much smaller. This result may show that in convection-free micro-gravity the more homogeneous distribution of supersaturation over the crystal surface leads to the improved stability of a crystal face.