Source of Electrofreezing of Supercooled Water by Polar Crystals

Polar crystals, which display pyroelectricity, have a propensity to elevate, in a heterogeneous nucleation, without epitaxy, the freezing temperature of supercooled water (SCW). Upon cooling, such crystals accumulate an electric charge at their surfaces, which creates weak electric fields, <kV·cm^-1, that are thousands of times lower than necessary for inducing homogeneous ice nucleation. By performing comparative freezing experiments of SCW on the same surfaces of three different polar crystals of amino acids, we demonstrate that preventing the formation of charge at these surfaces, by linking the two hemihedral faces of the polar crystals with a conducting paint, reduces the temperature of freezing by 2-5 °C. The temperature of ice nucleation was found to be correlated with the amount of the surface charge, thus implying that the surface-charge-induced interactions affect the interfacial water molecules that trigger freezing at a higher temperature. This finding is in contrast to previous hypotheses, which attribute the enhanced SCW freezing to the effect of the electric field or capture of external ions or particles. Possible implications of this mechanism of freezing are presented.