Frosty leaves are a beautiful sight in the winter. And, it turns out, they hold the secret to creating frost-free surfaces. Inspired by the irregular pattern of frost on leaves, researchers have made textured surfaces that do not form frost on almost 60% of their area. The approach could lead to an alternative approach to anti-frost surfaces that would make refrigerators more energy-efficient, make airplanes and trains safer, and reduce repairs on pipelines.

Many research groups are designing surfaces that reduce or delay ice formation. Microscopic structures on a surface that make it water-repellant can delay ice, but not for too long. More robust strategies involve treating surfaces with special chemicals that can lower the freezing point, or using surface coatings of oils, slippery lubricants, and moisture-absorbing polymer coatings that keep ice from sticking.

But Kyoo-Chul Park, a professor of mechanical engineering at Northwestern University, wanted to design a frost-free surface that does not rely on chemicals or coatings. He says researchers in the field have overlooked macroscopic structures. On a winter walk through campus, he says, “I found inspiration from the beautiful patterns of frost on leaves.” These patterns are discontinuous, with frost forming only on the convex regions of leaves, not on concave areas like the veins.

To understand why, he and his colleagues created rows of 5-mm-tall ridges on an aluminum sheet. They made four samples, each with ridges having a different slope, so that the valleys had angles of 40°, 60°, 90°, and 100°. They found that as moisture in the air hits the cold serrated aluminum, it first condenses on the peaks, so that the droplets grow faster and get bigger at the peaks, gradually getting smaller down the slopes into the valley. Frost also forms first and is thicker at the peaks, gradually thinning down the slopes, while at the very bottom of the valleys the water droplets evaporate before they can freeze.

Aluminum is naturally water-attracting. To see if the material chemistry mattered, the researchers boiled a sample in water for half an hour, which creates a superhydrophilic aluminum oxide hydroxide (or boehmite) layer on the surface, and also made samples that were water-repelling by cleaning and coating them with a fluoroaliphatic phosphate ester. In both cases, they saw the same pattern of enhanced frost at the peaks and none in the valleys. “So this could be applied to any material as long as it has that millimeter-scale texture,” Park says.

The team also numerically simulated the diffusion and the concentration of water vapor near the serrated features. The simulations showed that diffusion of the water vapor played a key role in forming the frost-free zone. This prediction corroborated the experimental reults.

The surface with the smaller valley angles of 40° remained the most frost-free, retaining only thin lines of frost on the peaks, which made up a little over 40% of the surface area. “In theory we could go down to 20% of the area,” Park says. The rest of the ice on peaks could be removed using minimal energy.

Shuhuai Yao, a professor of mechanical and aerospace engineering at the Hong Kong University of Science and Technology, calls this approach clever. “[It] requires just macrotextured patterns with no surface treatment, which is more manufacturable for many kinds of materials,” Yao says. One downside of the design is that melted water may accumulate in the valleys which could “lead to malfunction of the anti-frosting in repeated cycles.” Plus, the textured patterns might degrade the material’s mechanical strength. However, more carefully designed structures could overcome these limitations.

Read the abstract in Proceedings of the National Academy of Sciences.