Growth dynamics and pattern formation of ice and CCl4 crystals under pressure

平衡晶体形状由Wulff结构唯一地定义，该结构使固定体积的总的、与取向有关的表面自由能最小化。因此，低能取向主导了形状。另一方面，平衡附近的生长和熔体形状由表面取向相关的动力学决定。最慢的生长取向最终主导了渐近生长形状，留下了完全多面的分子光滑取向，而最快的熔体取向主导了稳定的熔体形状。利用高压砧室制备了冰晶，考察了冰晶的生长-熔化不对称性。在一定的粗化温度下，平衡形状是一个部分小平面的圆盘。我们发现盘状晶体的生长和熔化形状不对称；(A)邻近表面在生长过程中逐渐衰减，但在熔化过程中发展；(B)粗糙表面随着曲率的减小而生长，但随着曲率的增加而融化，而随着时间的推移呈线性增长或熔化。这些结果可能是由于分子附着和脱离过程的动力学不同所致。在一个粗化温度下，我们观察到，当冰的二维稳定融化形状被六个平面包围时，与小面生长形状的相似性是表面的；这些平面不是真正的小面，而是相对于棱镜小面旋转了30度。用熔体生长形状演化的几何模型定量描述了熔体生长的动力学过程。

Equilibrium crystal shapes are uniquely defined by the Wulff construction that gives a minimization of the total, orientation-dependent, surface free energy for a fixed volume. Thus, the low energy orientations dominate the shape. On the other hand, growth and melt shapes near equilibrium are determined by surface orientation dependent kinetics. The slowest growing orientations ultimately dominate the asymptotic growth shape, leaving a fully faceted molecularly smooth orientations, whereas the fastest melt orientations dominate the steady state melt shapes. Using a high pressure anvil cell we produced ice crystals and examined the growth-melt asymmetry in crystal shape.Above a roughening temperature an equilibrium shape is a partially-faceted disk. We found asymmetry between growth and melt shapes of the disk crystal ; (a) the vicinal surfaces gradually decay during growth but develop during melting ; (b) the rough surfaces grow with decreasing curvature but melt with increasing curvature while grow or melt linearly with time. These results may be due to the different kinetics between the molecular attachment and detachment processes.Below a roughening temperature we observe that while the two dimensional steady melt shapes of ice are bounded by six planes, the similarity with faceted growth shapes is cosmetic ; the planes are not proper facets but rotated 30 degrees from the prism facets. The dynamics is described quantitatively using a geometric model for the melt-growth shape evolution.