Department of Physics, Rutgers University-Newark, Newark, NJ, USA.
Department of Physics, Lehigh University, Bethlehem, PA, USA.
Nature. 2023 Aug;620(7974):557-561. doi: 10.1038/s41586-023-06283-2. Epub 2023 Aug 16.
Supercooled water droplets are widely used to study supercooled water, ice nucleation and droplet freezing. Their freezing in the atmosphere affects the dynamics and climate feedback of clouds and can accelerate cloud freezing through secondary ice production. Droplet freezing occurs at several timescales and length scales and is sufficiently stochastic to make it unlikely that two frozen drops are identical. Here we use optical microscopy and X-ray laser diffraction to investigate the freezing of tens of thousands of water microdrops in vacuum after homogeneous ice nucleation around 234-235 K. On the basis of drop images, we developed a seven-stage model of freezing and used it to time the diffraction data. Diffraction from ice crystals showed that long-range crystalline order formed in less than 1 ms after freezing, whereas diffraction from the remaining liquid became similar to that from quasi-liquid layers on premelted ice. The ice had a strained hexagonal crystal structure just after freezing, which is an early metastable state that probably precedes the formation of ice with stacking defects. The techniques reported here could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.
过冷水滴被广泛用于研究过冷水、冰核形成和液滴冻结。它们在大气中的冻结会影响云的动力学和气候反馈，并通过次生冰核生成加速云的冻结。液滴的冻结发生在几个时间尺度和长度尺度上，而且足够随机，使得两个冻结的液滴不可能完全相同。在这里，我们使用光学显微镜和 X 射线激光衍射研究了在 234-235 K 左右均匀冰核形成后，真空环境中数万滴微小水滴滴冻的情况。基于液滴图像，我们开发了一个七阶段冻结模型，并使用该模型对衍射数据进行计时。冰晶的衍射表明，在冻结后不到 1 ms 内就形成了长程有序的晶体结构，而剩余液体的衍射则变得类似于预融冰上的准液态层。刚冻结的冰具有应变的六方晶体结构，这是一种早期亚稳状态，可能先于具有堆叠缺陷的冰的形成。这里报道的技术可以帮助确定其他条件下的冻结动力学，例如云滴冻结，或者帮助理解其他材料中的快速凝固。
