Etsuro Yokoyama

We demonstrate the oscillatory phenomenon for the twisting growth of a triclinic crystal through in situ observation of the concentration field around the growing tip of a needle by high-resolution phase-shift interferometry.

We measured velocities V-step of lateral displacement of individual elementary steps on an ice basal face, for the first time, by advanced optical microscopy, under various bulk water vapor pressure p(H2O)(infinity). Distances L between adjacent spiral steps exhibited considerable variation under constant p(H2O)(infinity). Hence, we analyzed V-step as functions of L and p(H2O)(infinity). Then we found that (1) under a constant p(H2O)(infinity), V-step decreased with decreasing distances L when L <= 15 mu m and that V-step remained constant when L >= 15 mu m. We named V-step of L >= 15 mu m (isolated steps) V-step(int) and analyzed dependencies of V-step(int) on p(H2O)(infinity). Then we found that (2) the slope of the V-step(int) vs p(H2O)(infinity) plot gradually decreased with increasing p(H2O)(infinity). We proposed a model that took into account both the volume diffusion of water vapor molecules and the surface diffusion of water admolecules on a terrace. Our model could explain result (1) mainly by the competition of adjacent spiral steps for water admolecules diffusing on a terrace but could not explain the result (2) satisfactorily.

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 &gt;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, &lt;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.