At what temperature do liquids freeze

The electrical charge of the surface determines the different freezing points of supercooled water

Rehovot (Israel) - Usually water freezes to ice at 0 degrees. But without interference, it can even be cooled to minus 40 degrees without solidifying. Just shaking it is enough to suddenly solidify the liquid. Electric fields can also influence the freezing behavior of supercooled water. Israeli scientists now report in the journal "Science" that even the polarity of the electrical charge on the surface on which a drop of water lies plays an important role.

"The freezing point of water is a critical factor in many areas. It plays a role in the survival of cold blooded animals, in the cryopreservation of cells and tissues, in the freezing of cereals and snow production," say David Ehre and his colleagues from the Rehovot Institute Examples. Therefore, they investigated the solidification of supercooled water more precisely as a function of electrical fields. They discovered that this electrofreezing depends on the type of charge on a surface. To do this, they deposited drops of water on thin layers of lithium tantalate and strontium titanate. These crystalline substances are pyroelectric and can become electrically charged when the temperature changes. In their experiments, the water, which was subcooled to minus eleven degrees, remained liquid on the negatively charged lithium tantalate surface. However, when it was heated to minus eight degrees, the polarity changed and the surface was positively charged. The surprising result: Despite the higher temperature, the water droplet solidified immediately.

Similar experiments with strontium titanate, in which the scientists followed the freezing under the microscope and with X-ray diffraction, allowed even more detailed insights. A drop on a positively charged surface first freezes at the interface between substrate and water. On the other hand, on a negatively charged surface, solidification began at the boundary between water and air. A precise explanation of these effects cannot yet be given by Ehren and colleagues. But they suspect that the polarity of the electric field has an influence on the alignment of the water molecules and thus on the formation of the ice nuclei.

As early as 1861, the French chemist L. Dafour had discovered that supercooled water could be made to freeze using electrical fields. This effect is explained by the formation of tiny ice crystals, which act as germs for the shock-like freezing of the liquid. Further experiments could help explain the variable freezing of water at different freezing points.