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

平衡晶体形状是由伍尔夫结构唯一定义的，它给出了固定体积的总、方向相关的表面自由能的最小化。因此，低能取向主导了形状。另一方面，接近平衡的生长和熔体形状是由依赖于表面取向的动力学决定的。最慢的生长取向最终主导了渐近生长形状，留下了全面分子光滑的取向，而最快的熔体取向主导了稳态熔体形状。我们使用高压砧细胞制造冰晶，并检查晶体形状的生长-融化不对称性。在粗化温度以上，平衡形状是部分面形的圆盘。我们发现圆盘晶体的生长形状和熔体形状不对称；(a)邻近表面在生长过程中逐渐腐烂，但在融化过程中发育；(b)粗糙表面随曲率减小而增大，随曲率增大而熔化，随时间线性增大或熔化。这些结果可能是由于分子附着和分离过程之间的动力学不同。在粗糙化温度以下，我们观察到，虽然冰的二维稳定融化形状以六个平面为界，但与面形生长形状的相似性是表面的；这些平面不是正面，而是从棱镜面旋转了30度。用几何模型定量地描述了熔体生长形状演变的动力学过程。

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.