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Drop freezing: fine detection of contaminants by measuring the tip angle

In this Letter, we show that the shape of a freezing drop of water can be sensitive to the presence of impurities. We measure the tip angle of water drops frozen on a cold plate. The fine changes in tip angle are robustly captured by our image analysis method, which shows a deviation from that of pure water in solutions with salt (NaCl), polymer (PEG) and surfactant (TTAB) starting at concentrations of 10−6, 10−4 and 10−6 M respectively. The method could be adapted into a portable water purity tester, but the work also highlights the complexity of water freezing as it is influenced by trace concentrations of impurities.

This paper appeared in Applied Physics Letter and is available on arxiv:
https://doi.org/10.1063/1.5144071
https://arxiv.org/abs/2002.08590