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.

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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.