Coalescence of water droplets at 125 K

In spite of four decades of research on vitrified glassy water droplets, their flow properties in the deeply supercooled regime below 160 K have remained elusive. However, they are key to astrophysical processes where molecules are transported between water interfaces in the deeply supercooled or glassy state. Here we observe coalescence of a deposit consisting of thousands of micrometer-sized glassy water droplets between ~123 and ~140 K. Both electron microscopy and small-angle X-ray scattering indicate the disappearance of droplet interfaces at the time scale of minutes. Scattering profiles suggest a mechanism involving stretching of droplets and confluence. Remarkably, these phenomena coincide with water’s first glass transition, which thus defines the verge between glassy and liquid water. This marks the observation of coalescence in the ultraviscous domain, whereas previously coalescence has only been recognized for temperatures above 260 K, at which viscosity is thirteen orders of magnitude lower. Our findings indicate a recoupling of viscosity to translational diffusion below 160 K, while decoupling and breakdown of the Stokes-Einstein relation take place above 250 K. Coalescence under cryoconditions might take place in grainy amorphous ice covering interstellar dust particles in molecular clouds, enabling the formation of complex molecules in space.