Freezing of Highly Supercooled Water Nanodroplets

Water is the only substance on earth that occurs naturally in all three states in the atmosphere. The rates at which phase transitions involving water occur, the degree to which liquid water can be supercooled, and the kind of solid phases that form are all fundamentally interesting questions. We have followed the nucleation, growth, and subsequent freezing of pure water droplets in a supersonic nozzle apparatus using both Small Angle X-ray Scattering (SAXS) and Fourier Transform Infrared Spectroscopy (FTIR). SAXS showed the droplets had radii r in the range 3 < r < 6 nm. Because of their small size and the rapid cooling rate, the liquid water droplets only began to freeze below ~212 K – a temperature that is well below the homogeneous freezing limit for bulk water. Furthermore, the freezing temperature decreased with decreasing droplet size, or alternatively, with increasing droplet internal pressure. These position resolved measurements can also be used to derive the homogeneous nucleation rates for ice.