Effects of Drop Size and Salt Concentration on the Freezing Temperature of Supercooled Drops of Salt Solutions

Abstract This study investigates the freezing temperature of supercooled drops of salt solution (SDSS), targeting practical applications such as drop-based water purification of millimetric drops (diameter: 7.9 mm - 17 mm). Experiments are conducted to investigate the freezing behaviour of individual SDSS with different salt concentrations (0% (w/w) – 6 % (w/w)). It has been found that the freezing of SDSS is stochastic in nature and occurs in a range of temperature; this feature is more pronounced for smaller drops and higher NaCl concentrations. However, the median freezing temperature of SDSS for all NaCl concentrations increases as the drop size increases, yet with a diminishing increasing rate. A theoretical model based on the water activity theory of salt solution (Koop et al., 2000, Nature 406: 611) is adopted for predicting the heterogeneous freezing temperature of SDSS. As long as the relation between drop size and shift in water activity is known, the freezing temperature of SDSS can be predicted for different drop sizes and salt concentrations. Good agreements are achieved between the predicted freezing temperatures of SDSS and the measured values for large drops and/or low salt concentrations.