Wordsworth might have wandered lonely as a cloud, but understanding how water vapor molecules group together to change phase into liquid water is one of the key requirements for comprehending cloud formation. Unfortunately, this process of nucleation — the first step in phase transitions — is difficult to directly observe and quantify systematically. Lippe et al. report the first laboratory measurements with molecular-level details of water vapor nucleation at 47.5 K and 87.0 K and high supersaturation levels.

The new paper describes a change from a steplike increase of the maximum cluster size to a gradual increase with increasing supersaturation, a systematic trend also observed in propane and toluene. The measured nucleation rates are in agreement with a previous model based on ab initio transition state theory. At 47.5 K, their experimental observations are consistent with predictions from barrierless growth, but not at 87.0 K, which suggests a more complex nucleation process at higher temperatures.

The researchers used a Laval nozzle to produce supersonic gas and examined the uniform post-nozzle flow. After performing single-photon vacuum ultraviolet ionization to ionize the clusters, the researchers measured them using mass spectrometry. They measured the distribution of nucleation cluster sizes at a fixed distance from the nozzle as they increased supersaturation, and determined the range for critical cluster size, which is defined as the average size of the clusters when their growth and decay rates are identical. They also measured the cluster size distribution at fixed supersaturation over time, which allowed them to determine the nucleation rate.

Source: “Water nucleation at extreme supersaturation,” by Martina Lippe, Satrajit Chakrabarty, Jorge J. Ferreiro, Kyoko K. Tanaka, and Ruth Signorell, The Journal of Chemical Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5052482.