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 the 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 polyethylene glycol, and surfactant tetradecyltrimethylammonium bromide 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 this work also highlights the complexity of water freezing as it is influenced by trace concentrations of impurities.