Shrinking of Rapidly Evaporating Water Microdroplets Reveals their Extreme Supercooling

TL;DR

This study uses resonant Raman scattering to precisely measure the temperature of rapidly evaporating water microdroplets, revealing their ability to remain liquid at temperatures as low as 230.6 K, thus advancing understanding of supercooled water.

Contribution

It introduces a novel optical method to accurately determine the temperature of evaporating microdroplets, enabling direct observation of supercooled water at unprecedentedly low temperatures.

Findings

Water droplets remained liquid down to 230.6 K.

Resonance-based size measurement achieved better than 0.2% accuracy.

Results challenge previous temperature estimates of supercooled water.

Abstract

The fast evaporative cooling of micrometer-sized water droplets in vacuum offers the appealing possibility to investigate supercooled water - below the melting point but still a liquid - at temperatures far beyond the state-of-the-art. However, it is challenging to obtain a reliable value of the droplet temperature under such extreme experimental conditions. Here, the observation of morphology-dependent resonances in the Raman scattering from a stream of perfectly uniform water droplets has allowed us to measure with an absolute precision of better than 0.2% the variation in droplet size resulting from evaporative mass losses. This finding proved crucial to an unambiguous determination of the droplet temperature. In particular, a fraction of water droplets with initial diameter of 6379$\pm$12 nm were found to remain liquid down to 230.6$\pm$0.6 K. Our results question temperature estimates reported recently for larger supercooled water droplets, and provide valuable information on the hydrogen-bond network in liquid water in the hard-to-access deeply supercooled regime.