Surface Shape Controls Ice

Biological Strategy

Plants

Sam Stier

Image: S. Hermann & F. Richter / Pixabay / Public Domain - No restrictions

Protect From Ice

Freezing temperatures can be difficult for living systems to manage. They can cause ice crystals to form in blood and tissues, which causes damage to cells and ultimately, death. Living systems can’t afford the energy required to melt ice crystals, so instead, many have strategies to prevent crystals from forming in the first place. Many amphibians, plants, and insects have chemicals that act as antifreeze to prevent icy crystal formation. Another challenge ice presents is that it creates a slick surface, requiring friction to traverse. This is why polar bears’ paws have a rough surface that grips ice.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Macro-scale geometry directs where water condenses and frost forms.

Introduction

Water moves between organisms and their environment in a constant, creative dance, the underlying principles of which many humans have striven to understand. For instance, if you pay close attention on a cold morning, you’ll notice that frost appears in certain specific locations on leaves, but not other spots. Corners, edges, and bumps on leaves and other surfaces tend to gather more ice crystals, while flatter or more concave surfaces remain clear. Why do we see this kind of pattern?

The Strategy

Frost forms when water vapor in humid air makes contact with a surface that is below freezing temperature. Every location on a smooth surface is equally suitable for becoming the site of a water droplet. However, the volume of air (and water vapor) around things that “stick out” is greater than the volume of air around embedded, concave surfaces that “stick in”. So there are simply more chances for a quantity of water vapor to condense at these locations. Moreover, as frost appears preferentially on convex features, individual ice crystals attract more ice formation and grow.

Similar principles are at work with water in liquid form condensing on the relatively large bumps on the back of the Namib Desert Beetle (Onymacris unguicularis). Condensation is more likely to happen on these more exposed, convex features that stick out into the air volume than elsewhere, while evaporation tends to remove what little water has condensed on concave features.

The Potential

The formation of ice can range from being inconvenient to being dangerous. A better understanding of how and where water condenses and ice forms has given scientists and designers a new set of ways to manipulate engineered surfaces to reduce frost formation. Without harmful chemicals or complex microscopic texturing, using large-scale geometry alone (i.e., millimeter-plus scales), frost can be better managed on many kinds of surfaces of interest, including aircraft wings, wind turbine blades, windshields, roads, sidewalks, and more.

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Innovation: Academia

Anti-Frost Surfaces Inspired by Mint Leaves

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Bio-inspired surface from Northwestern University contains nanoscale peaks and valleys that help deter frost formation.

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Biological Strategy

The Beetles That Drink Water From Air

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Darkling beetle wing covers and body postures help condense water from humid air.

Last Updated October 4, 2021

References

“Several desert beetle species harvest water using their bumpy backs, but the topography of these surfaces has been overlooked, primarily because most previous studies attributed preferential condensation to surface chemistry (hydrophilic bumps with hydrophobic surroundings) and discounted convex topography as inferior to concave, on the basis of research into microscale and nanoscale textures. However, the beetle bumps are large (millimetres across), and recent studies have reported that the entire bumpy surfaces are homogeneously covered with hydrophobic wax, thus questioning the role of localized surface chemistry in promoting condensation. We considered instead that even in the absence of localized chemical patterning or microscale/nanoscale textures, the specific geometry of convex millimetre-sized surface structures alone could facilitate condensation, and therefore the topography of synthetic bumpy surfaces could potentially be designed to optimize fast, localized droplet growth by focusing vapour diffusion flux at the apex.” (Park et al. 2016: 78)

Journal article

Condensation on slippery asymmetric bumps

Nature | 2016 | Park, K.C., Kim, P., Grinthal, A., He, N., Fox, D., Weaver, J.C. and Aizenberg, J.

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