How Do Ice Crystals Grow inside Quasiliquid Layers

: A microscopic understanding of crystal-melt interfaces, inseparably involved in the dynamics of crystallization, is a long-standing challenge in condensed matter physics. Here, using an advanced optical microscopy, we directly visualize growing interfaces between ice basal faces and quasiliquid layers (QLLs) during ice crystal growth. This system serves as a model for studying the molecular incorporation process of the crystal growth from a supercooled melt (the so-called melt growth), often hidden by inevitable latent heat diffusion and/or the extremely high crystal growth rate. We reveal that the growth of basal faces inside QLLs proceeds layer by layer via two-dimensional nucleation of monomolecular islands. We also find that the lateral growth rate of the islands is well described by the Wilson-Frenkel law, taking into account the slowing down of the dynamics of water molecules interfaced with ice. These results clearly indicate that, after averaging surface molecular fluctuations, the layer by layer stacking still survives even at the topmost layer on basal faces, which supports the kink-step-terrace picture even for the melt growth.