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 15x15x1 mm gold-plated silicon or pure gold film
(Pummer et al., 2012; Zolles et al., 2015). 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. 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 (Atkinson et al., 2013; Augustin et al., 2013; Pruppacher and
Klett, 1997; Pummer et al., 2012; Wex et al., 2015; Zolles et al., 2015). Comparison
of our measurements with 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 former traditional
approaches
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