Heterogeneous freezing of super cooled water droplets in micrometre range-freezing on a chip

Abstract. A new setup to analyse the freezing behaviour of ice nucleation particles (INPs) dispersed in aqueous droplets has been developed with the aim to analyse ensembles of droplets with sizes in the micrometre range, in which INPs are immersed. Major disadvantages of conventional drop-freezing experiments like varying drop sizes or interactions between the water-oil mixture and the INP, were solved by introducing a unique freezing-chip consisting of an etched and sputtered 15 × 15 × 1 mm gold-plated silicon or pure gold film. Using this chip, isolated micrometre-sized droplets can be generated with sizes similar to droplets in real world clouds. The experimental set-up for drop-freezing experiments was revised and improved by establishing automated process control and image evaluation. The new set-up is economical, quick in handling the sample, precise in measurement and the results are more next to real conditions than former approaches. We were able to show the efficiency and accuracy of our setup by comparing measured freezing temperatures of different INPs (Snomax^®, K-feldspar, birch pollen (Betula pendula) washing water, juniper pollen suspension (Juniperus communis) and ultrapure water) with already published results. The T₅₀ values of ultrapure water (T₅₀ = −37.2 °C), birch pollen washing water (T₅₀ = −18 °C) and juniper pollen (T₅₀ = −22.7 °C) match the data given in literature. Microcline shows higher freezing temperatures (T₅₀ = −16.4 °C) than literature values from us and others, which can be explained by different preparing/milling parameters. The slightly lower freezing temperature of Snomax^® (T₅₀ = −8.9 °C) received by using the freezing-chip, compared to measurements already published, can be explained by different concentrations and droplet sizes. Our measurements and comparisons with the literature data show the important impact of droplet size, INP concentration and number of active sites on the T₅₀ values. Here, the new set-up exhibits its strength in reproducibility and accuracy which is due to the defined and isolated droplets. Finally, it opens a temperature window down to −37 °C for freezing experiments which was not accessible with many former approaches and allows determination of IN also with weak nucleation activity.