To control ice formation on a plane, even when it’s in flight, researchers created a de-icing method that exploits how frost grows on pillar structures to suspend ice as it forms into a layer that’s easier to remove.
De-icing a plane at the airport prior to takeoff is possible but planes also experience plummeting temperatures and rapid ice formation in flight. Once ice forms on the wings, it can greatly inhibit a pilot’s ability to safely operate the aircraft. Equipping planes with the ability to remove ice while flying at altitudes between 35,000 and 42,000 feet would provide a better set of tools to maintain safety.
The team worked from the knowledge that water droplets behave in different ways, depending on the surface. They aimed to leverage a principle known as Cassie’s Law, which shows that air can be trapped under water drops if the drops are suspended atop a structure that is bumpy and water-repellent. With a structure that could trap air underwater in this “Cassie state,” the team sought to make ice form in a layer with lower adhesion to the surface.
Making a surface water-repellent typically requires a chemical coating that must be periodically replenished and the bumpy surface also tends to wear over time. The team opted to make a water-repellent surface that doesn’t require fragile chemical coatings or ultra-fine bumps. Instead, they developed a simple and durable structure in the form of aluminum millimeter-sized pillars.
The team created an array of pillars, each 1 millimeter tall by 0.5 millimeter wide. The tiny pedestals were machined into a pattern with 1 millimeter between. As the temperature dropped, frost preferentially grew on the tops of the pillars, resulting in elevated frost tips. As more water was added, it was absorbed into this porous frost layer. When water drops were subsequently impacted on the surface, they were caught on the frost pedestals.
These freezing drops created tiny “ice bridges” that sealed the gaps of air in the valleys between the frost-tipped pillars. The water drops were caught by the frost tips, building ice bridges to trap air pockets underneath. Over time, a continuous and air-trapping ice canopy formed over the frost-tipped pillars. Whereas other de-icing methods may still allow a sheet of ice to adhere more directly to a large surface area, these trapped air gaps cause the sheet to be suspended, lowering the amount of adhesion ice has to the surface.
Using larger pillars in place of nanostructures and frost tips in place of a hydrophobic coating, the team realized the same benefit of trapping air underneath the forming ice while avoiding durability concerns. With a weaker bond, it’s possible to use the air pockets to then push ice away.