Properties of the solid-liquid interface layer of growing ice crystals: A dynamic light scattering study

The dynamics of solidification are investigated at the ice-water interface by means of Rayleigh-Brillouin spectroscopy. If a critical growth velocity is exceeded, Rayleigh scattering occurs in a layer which has a thickness between 1.4 and 6 μm and a density between water and ice, close to that of water. At constant intensity the width $\Gamma$ of the central line is proportional to the square of the scattering vector $\overrightarrow{\mathrm{k}}$, $\Gamma =D_i{k}^2$, independent of the orientation of $\overrightarrow{\mathrm{k}}$ relative to the interface. The measured diffusion constants $D_i$ are in the range $1.4\times {10}^{-8\mathrm{}}<D_i<5.7\mathrm{}\times {10}^{-8\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$ and isotropic in space. The intensity of the scattered light depends on the heat flow from the liquid into the solid and on the growth velocity. The linewidth of the scattered light decreases with increasing scattering intensity, and vice versa. We interpret $D_i$ as an effective thermal diffusion constant that describes the transport of heat involving fluctuations of order and disorder in the layer (Frenkel's "structure diffusion"). The increase of the total scattering intensity corresponds to an increase of the isothermal compressibility by a factor of 700 as compared to water.