A new electrostatic de-icing technique that exploits the natural charge separations in growing frost crystals has been developed by Jonathan Boreyko and colleagues at Virginia Tech in the US. The team used high-speed cameras to show how ice particles are broken off and propelled away from chilled surfaces when liquid water is suspended above them. Their discoveries could significantly improve our ability to remove stubborn frost layers from surfaces including aircraft and car windscreens.
Spontaneous charge separations in growing ice crystals have been studied for decades. For atmospheric scientists, the effect is key to understanding how clouds become charged during thunderstorms. However, one related effect, characteristic of frost formation, has remained largely unexplored until now.
When surfaces including glass and metal are chilled in humid air, ice crystals with branching, tree-like structures called dendrites can form. As these crystals grow, their upper branches will gradually become warmer, while their bases will remain cold. This generates higher concentrations of thermally activated negative ions, including hydronium and hydroxide in the branches, creating an excess of negative charge in those regions.
Jumping the gap
Borekyo’s team explored the idea that this charging effect could be exploited to develop better techniques for de-icing frosty surfaces. In their experiment, they prepared layers of dendrites on both glass and metal surfaces and suspended thin films of liquid water a few millimetres above them. Since water molecules are strongly polar, they became aligned in the presence of the negatively charged dendrite branches. This generates an electrostatic attraction between the branches and the liquid water; causing branches to dramatically break off and jump across the gap to stick to the water (see video).
Since no airflow or applied voltage is involved in the process, Borekyo and colleagues could non-invasively capture these jumps using a high-speed camera and compare their observations with numerical simulations. Their images showed strong agreement with the simulations; enabling them to precisely measure the electrostatic forces involved, and to determine their dependence on the temperature gradient across the dendrites.
Moth-eye nanostructures make good anti-icing coatings
The results could now provide fresh insights for atmospheric scientists studying how growing ice crystals drive electrification in thunderclouds. In addition, the research could lead to practical new electrostatic de-icing techniques; suitable for removing built-up frost from surfaces including aircraft, air conditioning units, and car windscreens on cold winter mornings.
Borekyo’s team now plan to scale up their technique in future research. By replacing water films with high voltage, actively charged electrodes, they could cause larger masses of ice, including entire dendrites, to be propelled away from surfaces.
The research is described in ACS Nano.