Daniel R. Moberg, University of California San Diego, La Jolla, CA, USA, and colleagues have determined that the smallest nanodroplets of water in which ice can form consist of 90 water molecules. At this size, the clusters are around 2 nm in diameter. This is a tenth of the size of the smallest virus or one million times smaller than a typical snowflake.
On Earth ice exists mainly in the highly ordered hexagonal crystal structure known as “ice I.” To form the nanodroplets, the researchers expanded a mixture of water and argon through a nozzle with a diameter of 60 µm. The resulting beam was funneled through three distinct zones. Here the temperature was dropped to 150 K (-123 °C; -189 °F). Infrared spectroscopic signatures were used to monitor the transition to ice I in the clusters. Computer models were used to simulate the properties of the nanodroplets.
Experimental and theoretical analysis suggest that the melting transition in the droplets is very different from macroscopic water. When cooled slightly below their freezing temperature, water clusters with less than ∼150 molecules can oscillate in time between the liquid and crystalline state. The liquid–ice transition loses its sharp character.
According to the researchers, their results can be important to understand the state of water confined in proteins and other materials such as interstellar objects, planetary atmospheres, and the mesosphere, an atmospheric layer above the stratosphere.