Surfaces engineered to repel water and ice can lose their ice resistance if environmental conditions shift.

'Superhydrophobic' surfaces may prevent ice build-up and have applications in aerospace and power transmission, among others. Dimos Poulikakos at the Swiss Federal Institute of Technology in Zurich and his colleagues ran water droplets down four different hydrophobic surfaces at −15 °C, in an experimental chamber in which humidity and nitrogen gas flow rate could be controlled. Although the droplets did not freeze at the water–surface boundary, at certain levels of humidity and gas flow they did freeze at the water–gas boundary. This occurred because of evaporation at this interface that further cooled the water, and resulted in much higher adhesion of ice to the surface under particular conditions.

Nature Commun. http://dx.doi.org/10.1038/ncomms1630 (2012)