Studying freezing, cavitation and ablation in water drops with x-ray lasers, optical imaging, and numerical modeling

Under metastable conditions, liquid water undergoes rapid phase transitions such as freezing below 0 °C and cavitation at large negative pressures. In micrometer-scale systems these processes occur on microsecond-nanosecond timescales. To probe such systems, high temporal resolution X-ray laser techniques have been developed, but their results have been limited in accuracy and consistency. This dissertation builds on these techniques and introduces new integrated experimental and data analysis methods that reveal new features of the phase transitions.Freezing of supercooled water drops produces short-lived metastable ice, which is difficult to probe due to the stochasticity of ice nucleation and due to a complex freezing dynamics with multiple stages. We examined tens of thousands of supercooled drops using synchronized X-ray laser diffraction and optical imaging. An image-based stochastic model of freezing, combined with X-ray laser diffraction, yielded time-resolved ice structure in the initial stages of freezing, improving previous temporal resolution by one order of magnitude. The measurements revealed a strained hexagonal ice structure. In parallel, we modeled an experimentally observed phenomenon: nucleation-induced self-propulsion of freezing drops.Large negative pressures in water are difficult to achieve; most experiments produce tensions far less negative than theoretical predictions. Our method includes ablating micrometer-scale drops with an X-ray laser, launching a shock wave that reflects at the surface as a rapidly growing negative pressure wave, which causes spallation when sufficiently strong. Initial studies indicated the generation of large negative pressures, but could not quantify them accurately. We therefore used computational fluid dynamics (CFD) to reproduce the pressure dynamics and quantify negative pressures, and we independently validated the CFD using optical ray tracing simulations. Using additional experiments and modeling, we achieved a high fidelity match between ray traced images and experimental ones. The simulated images are sensitive to small variations of the pressure waves, paving the way to improved accuracy in negative pressure measurements.