Investigation of early stages of crystallization in supercooled atomic liquids by time-resolved x-ray scattering from microscopic laminar jets in conjunction with machine learning-based data analysis.

液体冻结是一种无处不在的状态变化，影响着我们日常生活的许多方面，但人们对这种如此熟悉的相变的许多方面仍然知之甚少。在微观层面上，结晶被经典地视为一个两步过程。过冷液体中的热波动--也就是低于其熔点--最初会触发新有序相小的局域核的自发形成，随后它会增长到宏观尺寸。然而，结晶过程远比这种经典描述所代表的要复杂得多，而且这个过程的许多细节仍然缺乏足够的微观理解。在这里，我们的目标是将欧洲X射线自由电子激光的X射线散射与原子元素Ar和氪的微观液体喷流结合起来，作为研究过冷液体结晶早期阶段的有效方法。这种方法独特地解决了晶体成核过程的时间尺度很短和固有的随机性质的问题，因为通常没有关于自发固体形成的空间和时间坐标的先验知识。结合计算机模拟和最先进的基于ML的数据分析技术，我们希望能够对结晶过程的许多基本方面获得更深入的见解，从而为当前的晶体形核和生长理论提供新的基准。

Freezing of a liquid is a ubiquitous change of state that affects many aspects of our daily life, but many aspects of this so familiar phase transition are still poorly understood. At the microscopic level, crystallization is classically viewed as a two-steps process. Thermal fluctuations in the supercooled liquid — that is, below its melting point — initially trigger the spontaneous formation of a small, localized nucleus of the new ordered phase, which subsequently grows to macroscopic dimensions. However, crystallization is by far more complex than represented by this classical description and many details of the process still lack an adequate microscopic understanding. Here we aim to combine x-ray scattering at the European X-ray Free-Electron Laser with microscopic liquid jets of the atomic elements argon and krypton as a powerful approach to the study the early stages of crystallization in supercooled liquids. This approach uniquely addresses the problems of very short time scale and the inherently stochastic nature of the crystal nucleation process as no a priori knowledge of the spatial and temporal coordinates of the spontaneous solid formation is in general available. In combination with computer simulations and state-of-the-art ML-based techniques for data analysis, we expect to be able to gain deeper insights into many fundamental aspects of the crystallization process, thereby providing new benchmarks for current theories of crystal nucleation and growth.