Researchers at MIT and GE have dealt a setback to the development of coatings to fight ice buildup, saying that such coatings do not work and may even lead to more ice.
New research shows that the use of a super-hydrophobic (water-repellent) coating presents “serious problems,” MIT said in a news release.
The reason such a coating would not work has to do with frost—ice that forms on a surface directly from a vapor state or by freezing of condensed droplets, MIT said.
The formation of frost could completely defeat the water-repelling properties of a surface that normally would inhibit ice buildup—and, in fact, could actually promote ice formation, according to a study published this month in Applied Physics Letters.
But the team did find a promising alternative: a more complicated, patterned surface.
‘A Big Open Question’
It has been “a big open question” whether the super-hydrophobic behavior of certain surfaces would extend to preventing the buildup of ice on those surfaces, says Kripa Varanasi, the d'Arbeloff assistant professor of mechanical engineering.
In their paper, however, Varanasi and his team report that when super-hydrophobic surfaces are exposed to supersaturated air (such as found in clouds), frost readily forms— thus defeating the purpose of the coating.
“If frost forms, it actually aggravates the problem” by providing a kind of foundation on which ice quickly can build up to form a thick layer, Varanasi says. “We need to be able to control this first phase, when ice nucleation occurs.”
Ice buildup can cause problems on airplane wings, engine turbine blades, electrical lines, oil rigs and other structures and can make even basic operations treacherous for people trying to work on slippery surfaces. Preventing icy buildups usually means using deicing materials (salt or glycol), sprinkled or sprayed on a surface, or activating heating coils embedded in the surface material.
Deicing chemicals can be toxic, and require constant application, and heating coils waste energy, so researchers have been looking for better ways of handling the problem, ideally through a passive method—one based on chemical or physical properties of the surface, and requiring no ongoing input of energy or work. That is why research had turned to coatings.
Super-hydrophobic coatings cause water to bead up into droplets instead of spreading out across a surface. Many researchers had assumed that the coatings would also prevent ice from forming or adhering to the surface.
‘Where Not to Look’
But using an environmental scanning electron microscope to study the process, Varanasi and his team found that there is a limit to how much the coatings could prevent ice from beginning to stick to a smooth surface.
That’s significant, “because it shows clearly that frost formation imposes severe constraints on the development of textured icephobic surfaces,” says Neelesh Patankar, associate professor of mechanical engineering at Northwestern University. “This was not previously known.”
The fact that the team showed that micron-sized textures would not work is “an important result in the field since it possibly tells us where not to look,” Patankar said.
A better option, the team found, might be complex nano-scale texturing of the surface itself. This could drastically improve the hydrophobic qualities, even on a moving surface, by preventing the forming droplets from finding a suitable flat surface to stick to, the researchers said.
“We have to go another way; we have to do textures,” Varanasi says.
Getting the size, configuration and roughness of these textures right will be the subject of future research. The patterning has to be something that can be manufactured on large surfaces at reasonable cost.
“That will be the real breakthrough,” Varanasi says—when someone finds a way to produce “scalable manufacturing techniques, with material that can survive in these kinds of applications.”