Microstructure and crystal order during freezing of supercooled water drops

Supercooled water droplets are widely used to study supercooled water^1,2, ice nucleation^3-5 and droplet freezing^6-11. Their freezing in the atmosphere affects the dynamics and climate feedback of clouds^12,13 and can accelerate cloud freezing through secondary ice production^14-17. Droplet freezing occurs at several timescales and length scales^14,18 and is sufficiently stochastic to make it unlikely that two frozen drops are identical. Here we use optical microscopy and X-ray laser diffraction to investigate the freezing of tens of thousands of water microdrops in vacuum after homogeneous ice nucleation around 234-235 K. On the basis of drop images, we developed a seven-stage model of freezing and used it to time the diffraction data. Diffraction from ice crystals showed that long-range crystalline order formed in less than 1 ms after freezing, whereas diffraction from the remaining liquid became similar to that from quasi-liquid layers on premelted ice^19,20. The ice had a strained hexagonal crystal structure just after freezing, which is an early metastable state that probably precedes the formation of ice with stacking defects^8,9,18. The techniques reported here could help determine the dynamics of freezing in other conditions, such as drop freezing in clouds, or help understand rapid solidification in other materials.