Frost forms on the convex regions of these mint leaves, but not on the concave veins. Photo: Stephan Herb.Researchers at Northwestern University have discovered a new way to significantly reduce frost formation on any surface. Their finding, reported in a paper in the Proceedings of the National Academy of Sciences, could help decrease the amount of energy needed for de-frosting and potentially result in fewer canceled flights, which can be grounded by even the slightest layer of frost.
By tweaking the texture of any material's surface, the team was able to experimentally reduce frost formation by up to 60%. The millimeter-scale surface texture comprises a jagged series of peaks and valleys, which the researchers first observed in nature. The team also showed that this texture should theoretically reduce frost formation by up to 80%.
"This idea came from looking at leaves," said Northwestern's Kyoo-Chul Park, an assistant professor of mechanical engineering in Northwestern's McCormick School of Engineering, who led the study. "There is more frost formation on the convex regions of a leaf. On the concave regions (the veins), we see much less frost. We found that it's the geometry – not the material – that controls this."
People who live in cold climates are all-too-familiar with frost, which forms when humid air vapor or condensation makes contact with a surface that is below-freezing. Every winter, people scrape frost off their cars or worry about it killing their plants. But frost is more than a nuisance. Frost on airplane wings can create drag, making flight dangerous or even impossible. And when accumulating inside freezers and refrigerators, frost can greatly reduce the energy efficiency of appliances.
But frost doesn't form on everything. Some objects, such as leaves, have a rippling geometry that causes frost to form on the peaks but rarely in the valleys. "People have noticed this for several thousands of years," Park said. "Remarkably, there was no explanation for how these patterns form."
Through experimental work and computational simulations, Park and his collaborators found that condensation is enhanced on the peaks and suppressed in the valleys of wavy surfaces. The small amount of condensed water in the valleys then evaporates, resulting in a frost-free area. Even when Park and his team used a surface material that attracts water, they found that the water still evaporated from the valleys when below the freezing point.
Park used this new information to find the optimal surface texture for preventing frost formation, which turned out to be a surface containing millimeter tall peaks and valleys with small (40–60°) angles in between. Although a thin line of frost still forms on the peaks of this surface topography, it can be defrosted with considerably less energy, bypassing the need for using liquids with lower frosting points or surface coatings, which can be easily scratched.
"The no-frosting region initiates the defrosting process," Park said. "So it would reduce the materials and energy used to solve frosting problems. All we have to do is provide others with the guidelines to design these serrated surfaces."
This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.