Supercooled water drops are often used for studying supercooled water and ice nucleation. They also occur naturally in clouds, where their freezing impacts the formation of precipitation and the radiative balance of Earth. The freezing of supercooled water drops is a complex stochastic process including nucleation, dendritic ice growth, and shape changes. As a result, no two droplets freeze the same. To account for this randomness, we collected snapshot images and X-ray diffraction from tens of thousands of drops freezing in vacuum after homogeneous ice nucleation near -39 °C. Based on the drop images, we developed a seven-stage model of freezing and determined average properties for all freezing stages.
The stochastic nature of nucleation limits the time resolution of experiments that aim to capture the dynamics of subsequent solidification. We mitigated this problem by using the freezing model to “time” our X-ray diffraction profiles. These time-resolved profiles captured the formation of long-range order in the ice crystals, which occurred in less than 1 ms. The ice formed just after freezing had a strained hexagonal structure. This structure is distinct from the stable hexagonal ice and from known metastable forms of ice, and has a strain and microstructure that make it sufficiently metastable to transform to other metastable phases. The experimental and analysis techniques we used could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.
Dr. Claudiu Stan got his undergraduate degree in Physics from the University of Bucharest, and his Ph.D. in Physics from MIT, advised by Wolfgang Ketterle. He was a postdoc in George Whitesides’ group in the Chemistry and Chemical Biology Department at Harvard, and a Research Associate at the SLAC National Accelerator Laboratory. His previous work includes realizing the Bose-Einstein condensation of molecules, discovering the first magnetically-tuned Feshbach resonance between different species of ultracold atoms, and developing microfluidic instruments for the study of supercooled water and of ice nucleation.
Dr. Stan’s current research is focused on the properties and the phase transitions of metastable liquids, and consists in designing and interpreting X-ray laser experiments to investigate the dynamics of crystallization and cavitation in water at length and time scales previously inaccessible. Part of this research involves experimental fluid dynamics at small length scales and extreme conditions, where he used X-ray laser ablation to achieve very large negative pressures in water drops, and extremely intense sound waves in water jets. These phenomena are also relevant to protein crystallography at X-ray lasers, and he was part of the teams who demonstrated serial femtosecond protein crystallography at megahertz data acquisition rates. Dr. Stan is also investigating the freezing of supercooled water droplets using X-ray lasers, which is the topic of the seminar.