Credit: © 2010 AAAS

Supercooled water can remain in the liquid state at temperatures as low as −40 °C — providing no seeding nuclei are available — but it has long been known that electric fields near charged surfaces can influence water's freezing temperature. It has so far, however, proved difficult to study the effect of the electric field alone, because nucleation is also induced by conducting surfaces. Now Igor Lubomirsky and colleagues at the Weizmann Institute of Science, Israel, have used pyroelectric materials — polar insulators that create a transitory electric field when heated or cooled — to do just that (Science 327, 672–675; 2010).

Pyroelectric materials create equal but opposite charges at their opposing surfaces, so Lubomirsky and colleagues could use the same compound, LiTaO3, to study the effects of both positive and negative charges on the freezing of supercooled water. The pyroelectric samples were exposed to a humid atmosphere and cooled until water droplets condensed on their surfaces and later froze. They found that supercooled water froze at −12.5 °C on an uncharged surface, −7 °C on a positively charged surface and −18 °C at a negatively charged surface.

Similar results were observed when experiments were repeated using SrTiO3. Water droplets froze at −4 °C on positively charged pyroelectric quasi-amorphous films (pictured, bottom left) and at −12 °C on non-pyroelectric amorphous films (pictured, top right).

The original version of this story first appeared on the Research Highlights section of the Nature Chemistry website.