Interferometric observation of the effects of gravity on the horizontal growth of ice crystals in a thin growth cell

The effects of gravity on the horizontal growth of crystals were investigated in a thin growth cell, which is normally assumed to be two-dimensional. Three-dimensional interfacial patterns and the solute concentration distribution during directional growth of ice crystals in a water–KCl solution were obtained using a Mach–Zehnder interferometer. It was found that morphological instability of a straight interface occurs not only in the lateral direction but also in the thickness direction of the growth cell (100 μm in thickness). Side view images of a thin layer were first obtained during horizontal growth and upward growth, where the components of gravitational acceleration in the thickness direction were 1g and 0g, respectively. The effects of gravity were clearly observed as follows. In the former case of 1g, the concentrated solution near the growth interface subsided. Simultaneously, the lower side of the interface retreated due to depression of the freezing temperature. In the latter case of 0g, the solution remained stationary and distortion of the interfacial pattern was not observed. In addition, top view images of horizontal growth indicated that the mass balance of solute at the interface, which is usually assumed, was lost as soon as instability occurred in the thickness direction. It should be emphasized that three-dimensional analysis that takes into account the effect of gravity is essential for understanding interfacial patterns even in the case of a thin growth cell.