February 4, 2010 | 7
The Celsius scale is an elegant, simple system of measurement: water freezes at 0 degree Celsius and boils at 100 degrees C. (The actual definition of the scale is a bit more complex, involving the so-called triple point of water, but that’s the general idea.) But chefs at high altitudes know that the simplicity of Celsius doesn’t always hold—the higher your kitchen, the lower the boiling point of water. Similarly, water’s freezing point is negotiable, with "supercooled" water able to remain liquid down to around –40 degrees, and even colder at extreme pressures. Supercooling can occur if water is so pure and is kept in such uncontaminated conditions that the molecules have nothing to interact with and crystallize around.
It has long been known that electrically charged surfaces can affect supercooled water’s ability to freeze, a fact unsurprising given that water is a polar molecule whose orientation and hence ability to crystallize can be controlled by electric fields. A study in the February 5 issue of Science explores the interaction between electric fields and supercooled water’s freezing point, even producing the counterintuitive phenomenon of water freezing as its temperature increases. The researchers, from the Weizmann Institute of Science in Rehovot, Israel, experimented with supercooled water on pyroelectric surfaces, materials that develop transient electric charges during temperature changes, either positive or negative depending on whether the pyroelectric is heated or cooled.
By using opposite sides of the same pyroelectric lithium tantalate crystal, the Weizmann team produced two surfaces with opposite charges during cooling—one surface developed a positive charge as the temperature decreased, the other a negative charge.
On an uncharged lithium tantalate surface, the water remained a supercooled liquid down to –12.5 degrees C. But when the pyroelectric surface had a positive charge, the team found that water froze at –7 degrees C; the positive charge somehow promoted the ice-forming process. Conversely, when the surface was negatively charged, the water did not freeze until –18 degrees C.
Even after the negative charge had worn off, the water remained in supercooled liquid form. But when the pyroelectric was heated from –11 degrees to –8 degrees C, thereby generating a positive charge on the surface, the water coalesced into a solid—freezing even as its temperature increased.
Ice photo: © iStockphoto/Milous