A new class of coatings developed at the University of Michigan sheds ice effortlessly from even large surfaces. This could let companies and individuals “ice-proof” cargo ships, airplanes, power lines, and other large structures. The spray-on coatings let ice fall away from structures with just the force of a light breeze, or often the weight of the ice itself. In a test on a mock power line, the low interfacial toughness (LIT) coating shed ice immediately.
Previous ice-repellent coatings worked well on small areas, but in field testing they didn’t shed ice on large surfaces as effectively as researchers had hoped. That’s an issue, since ice tends to cause the biggest problems on the biggest surfaces—sapping efficiency, jeopardizing safety, and necessitating costly removal.
“For decades, coating research has focused on lowering adhesion strength, the force per unit area required to tear a sheet of ice from a surface,” says Anish Tuteja, a researcher at the University of Michigan. “The problem with this strategy is that the larger the sheet of ice, the more force is required. We found that we were bumping up against the limits of low adhesion strength, and our coatings became ineffective once the surface area got large enough.”
The new coatings solve this problem by introducing a second strategy: low interfacial toughness. Surfaces with low interfacial toughness encourage cracks to form between ice and the surface. And unlike breaking an ice sheet’s surface adhesion, which requires tearing the entire sheet free, a crack only breaks the surface free along its leading edge. Once that crack starts, it can quickly spread across the entire iced surface, regardless of its size.
“Imagine pulling a rug across a floor,” says Michael Thouless, mechanical engineering professor at the University of Michigan. “The larger the rug, the harder it is to move. You are resisted by the strength of the entire interface between the rug and floor. The frictional force is analogous to the interfacial strength.
“But now imagine there’s a wrinkle in that rug. It’s easy to keep pushing that wrinkle across the rug, regardless of how big the rug is. The resistance to propagating the wrinkle is analogous to the interfacial toughness that resists the propagation of a crack.”
Thouless said the concept of interfacial toughness is well known in the field of fracture mechanics, where it underpins products like laminated surfaces and adhesive-based aircraft joints. But until now, it hadn’t been applied in ice mitigation. “Traditionally, fracture mechanics researchers only care about interfacial toughness, and ice mitigation researchers often only care about interfacial strength,” Thouless says. “But both parameters are important for understanding adhesion.
“I pointed out to Anish that if he were to test increasing lengths of ice, he would find the failure load would rise while interfacial strength was important, but then plateau once toughness became important,” he continues. “Anish and his students tried the experiments and ended up with a beautiful demonstration of the mechanics, and a new concept for ice adhesion.”
To test the idea, Tuteja’s team used a technique he honed during previous coating research. By mapping out the properties of a vast library of substances and adding interfacial toughness as well as adhesion strength to the equation, they could mathematically predict the coating’s properties without physically testing each one. This let them concoct a wide variety of combinations, each with a specifically tailored balance between interfacial toughness and adhesion strength.
They tested a variety of coatings on large surfaces: a rigid aluminum sheet approximately 3 ft. square, and a flexible aluminum piece approximately 1 in. wide and 3 ft. long that replicated a power line. On every surface, ice fell off immediately due to its own weight. It stuck fast, however, to the control surfaces, which were identical in size; one was uncoated and another was coated with an earlier icephobic coating.
The team’s next step is to improve its LIT coating durability.