等温条件下雪的有序动力学
Ordering dynamics of snow under isothermal conditions
摘要：
利用X射线层析成像技术研究了实验室新雪在不同温度$T=-3,-9,-19 ^{\circ}C$等温条件下的形态演变。双连续冰-蒸汽系统的集体动力学由两点（密度）关联函数$C(\mathbf{r},t)$和类似于孔径分布的特定厚度分布的演变来监测。我们观察到的动态缩放的情况下，并揭示从根本上不同的类的长度尺度：第一类包括平均冰厚度和（逆）比表面积（每冰体积测量）单调增加，并遵循幂律。动力学指数与局部守恒序参量的粗化一致。第二类长度尺度是从不同坐标方向上原点处的$C(\mathbf{r},t)$的斜率导出的。所有这些尺度都表现出较慢的增长与反常的幂律标度。两种不同的幂律的存在是定量一致的分形聚集体的粗化，并揭示了持久性的幂律相关性在初始条件下，雪包含树枝状结构。在低温下，这些结构甚至持续一整年。第三类长度尺度可以由$C(\mathbf{r},t)$的第一个过零点来定义。过零点在重力方向和水平方向之间表现出强各向异性的非单调演化。我们把这种行为归因于冰网络的大规模结构弛豫，这显然使小规模的界面弛豫不受影响。然而，反之亦然，它仍然是一个问题，结构的流动性是如何引起的界面粗化。
英文摘要：
We have investigated the morphological evolution of laboratory new snow under isothermal conditions at different temperatures $T=-3,-9,-19 ^{\circ}C$ by means of X-ray tomography. The collective dynamics of the bicontinuous ice-vapor system is monitored by the evolution of the two-point (density) correlation function $C(\mathbf{r},t)$ and a particular thickness distribution which is similar to a pore size distribution. We observe the absence of dynamic scaling and reveal fundamentally different classes of length scales: The first class comprises the mean ice thickness and the (inverse) specific surface area (measured per ice volume) which increase monotonically and follow a power law. The dynamic exponent is in accordance with coarsening of a locally conserved order parameter. A second class of length scales is derived from the slopes of $C(\mathbf{r},t)$ at the origin in different coordinate directions. All these scales show a slower growth with anomalous power law scaling. The existence of two different power laws is quantitatively consistent with coarsening of fractal aggregates and reveal the persistence of power law correlations in the initial condition where the snow contains dendritic structures. At low temperatures these structures persist even over an entire year. A third class of length scales can be defined by the first zero crossings of $C(\mathbf{r},t)$. The zero crossings display a non-monotonic evolution with a strong anisotropy between the direction of gravity and horizontal directions. We attribute this behavior to larger scale structural relaxations of the ice network which apparently leave the small scale interfacial relaxations unaffected. However, vice versa it remains the question how structural mobility is induced by interfacial coarsening.
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