Overhang Structures Repel Various Liquids

Biological Strategy

Springtails

Eleanor Banwell

Image: Andy Murray / CC BY SA - Creative Commons Attribution + ShareAlike

Protect From Fungi

Fungi–which include molds and mushrooms, amongst others–play important roles in every ecosystem, but can also create diseases in plants and animals. Fungi can rapidly reproduce, so living systems must fend them off early to avoid their spread and potential, physical harm. Living systems use both physical and chemical strategies to protect from fungi, and must ensure that these strategies don’t harm the threatened living system itself. For example, a pumpkin’s skin has antifungal proteins to prevent fungal growth without harming the pumpkin itself.

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Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Protect From Dirt/solids

When dirt and other small solids adhere to living systems, they can slow them down, create blockages, reduce their ability to carry out vital functions, or cause surface wear and tear. Due to electrostatic forces, it’s easy for dirt and other solids to adhere to surfaces, so living systems must overcome those forces. An earthworm, for example, uses a small electric current to keep soil particles from adhering to its body as it moves through the soil. This enables it to move more efficiently by reducing drag.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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Skin of springtails repels liquids with undercut structures that stabilize air bubbles.

Design application: water and oil repellent option for fabric surfaces.

Springtails are tiny arthropods less than 6mm long. They get their name from their furcula, a tail-like appendage that they can use to launch themselves large distances and out of danger. Springtails live in moist leaf litter and other decaying organic matter where they primarily eat dead plant material and fungi. Springtail habitats are wet and, as these animals respire through their skin, repelling moisture is critically important for their survival. They are evolutionarily ancient and are very well adapted to this challenging environment that is full of oily substances released from decaying matter, as well as potentially harmful microorganisms.

Surface roughness is important for repellency as textured surfaces trap an air bubble (or plastron) at the surface, preventing water and other substances from contacting it directly. Water’s surface tension makes it relatively easy to repel, and many organisms have rough surfaces that function in this way. Springtail skin is remarkable in that it resists a far wider range of substances, including oils and other low surface tension liquids, even when subjected to high pressure.

Springtail skin is rough across multiple length scales. It has relatively large bumps that are themselves coated in smaller “primary granules”. Primary granules are unique and convoluted structures approximately 250 nm across. Made from even folds in the skin, they are arranged in a regular lattice, forming nano-scale cavities that trap air. The granules have undercut overhangs that orient surface-tension forces in a way that makes the structures exceptionally effective at stabilizing the plastron. This gives springtail skin its incredible repellency.

Image: Ellie Banwell / Public Domain - No restrictions

The tiny structures that protrude from the skin of springtails preserve an air bubble that prevents substances wetting the surface.

Image: Andy Murray / CC BY SA - Creative Commons Attribution + ShareAlike

Leaf Structure Retains Air Layer Underwater

Giant salvinia

The leaf structure of the Salvinia water fern retains a layer of air when submerged in water due to water-resistant hairs that possess water-attracting tips.