Temperature measurements of supercooled micrometer sized water droplets

The thermodynamic properties of water exhibit a variety of anomalies. Many of them are enhanced in the supercooled region. These properties are essential for various chemical and physical processes such as the development of life or the formation of clouds. One interpretation predicts that water's anomalies are the consequence of a hypothetical liquid-liquid phase transition below the homogeneous nucleation temperature at -41°C. The investigation of water in that temperature region is extremely challenging, as at such low temperatures liquid water crystallizes extremely fast within a few microseconds. Water jets consisting of micrometer sized droplets injected in vacuum offer due to the evaporative cooling with cooling rates up to 106 Ks-1 the unique opportunity to examine the crystallization dynamics of water in this extremely supercooled region. However, the accurate knowledge of the instantaneous temperature of the droplets is a challenge as it is not possible to have a thermometer inside such objects. Previous experiments estimated the droplet temperature with calculations based on the kinetic model of evaporative cooling, using experimental parameters that are not always available with the necessary precision. Therefore, the direct and precise measurement of the droplet temperature is a mandatory step for further investigations of the crystallization dynamics of supercooled water. For this purpose Raman light scattering is ideal. Resonances in the Raman signal allow determining the droplet size as a function of residence time in the vacuum with a precision better than 0.2 %. This droplet size can be translated with a Knudsen evaporative model into a temperature and is compared with results from ordinary Raman thermometry. It was found that water can remain liquid down to 230.6 +/- 0.6 K. Raman spectra of water in the OH stretch region are found to exhibit a gradual blue-shift upon cooling.