LAUSR

Experiments provide evidence for two liquid phases in supercooled water droplets When a substance remains liquid below its melting point, it is said to be in a metastable supercooled state. In the region where the substance can be supercooled, the crystal is still the stable state, but crystallization can be avoided if the cooling occurs fast enough. The supercooled phase diagram of water has received particular attention (1). The anomalous thermodynamic properties of water point to the possible existence of two different liquid phases—one with high density and the other with low density—that become identical at a liquid-liquid critical point in the supercooled phase (C′, see the figure). But whereas mild supercooling of water is moderately easy to achieve, the deeply supercooled region has been out of the reach of experiments. On page 1589 of this issue, Kim et al. (2) use an evaporative cooling technique to cool micrometer-sized water droplets to deeply supercooled temperatures and provide evidence for the postulated critical point.