Dynamics of coalescence in hyperquenched glassy water probed by x-rays

Dynamics, morphology, and structure of glassy water are highly relevant for cryochemical techniques, in particular for cryo-electron microscopy. Here, we study the structural dynamics of a deposit consisting of thousands of micrometer-sized glassy water droplets during and after droplet coalescence using x-ray photon correlation spectroscopy at the micro- and meso-scale. We cover the temperature range from 94 to 161 K, encompassing droplet coalescence, the glass transition, and crystallization to ice I. Our experimental protocol involves heating beyond the coalescence regime, followed by recooling and reheating beyond crystallization, which allows us to disentangle the dynamics of coalescence from those associated with the glass transition. During coalescence, we observe an irreversible ballistic process in the temperature range between 130 and 145 K, with characteristic velocities of ∼0.1-0.2 Å s-1. In addition, samples that are not annealed below 125 K exhibit a q-independent mode (q0) at 130-145 K, which only appears while coalescence is progressing. We regard this to be a collective relaxation connected to a mobile surface layer at the droplet interfaces. After coalescence is complete, we observe significant diffusive dynamics. In particular, we find a sharp increase in diffusivity to ∼2 Å2 s-1 at around 148 K, indicating the onset of pronounced diffusive motion. From these results, we conclude droplet coalescence is primarily governed by ballistic, non-diffusive dynamics below ∼136 K, whereas strongly heterogeneous diffusive dynamics emerge at higher temperatures. We associate the abrupt increase in diffusivity after coalescence with the bulk glass-to-liquid transition.