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Biomimicry, a promising toolbox for tribology

The tribological performance of a product and its internal components is important to extend its lifespan, limit wear and breakdowns, or quite simply to avoid mechanical losses through friction.

 

Despite past improvements, progress remains to be made, particularly on rubbing surfaces. In automobiles, for example, it has been estimated that 17% of combustion energy is lost in internal mechanical friction. All parts of the driveline are affected: from the engine to the wheels and the transmission.

 

Any improvement would also limit the noise emitted by a product or an internal part, and even reduce thermal losses.

An expert in friction, lubrication, and saving energy, nature has extensive know-how in this area.

To move, species must use efficient techniques to minimise the energy they use. Whether on land or in the sea, life is a specialist when it comes to minimising friction: its drag must be reduced as much as possible.

 

Lubrication and wear reduction techniques are an art mastered by living beings. To guarantee their survival and physical integrity, they must minimise damage due to contact with the external environment.

 

To feed itself, living things have also developped formidable techniques of predation and capture: nature is full of merciless and dead-end traps based, among other things, on exceptional lubricating properties.

A specialist in the field, Bioxegy explains why and how biomimicry makes it possible to design new approaches and technologies in tribology. We provide you with a selection of evocative examples.

Nepenthes Alata: the art of creating non-stick surfaces.

Carnivorous plants feed on insects which serve as nutritional supplements. Each species has developped its own technique, which is extremely effective in attracting, capturing and digesting its prey. Nepenthes Alata is a species with a remarkable capture strategy.

 

At the end of each leaf there is an urn that can reach several tens of centimeters in length, and which constitutes a formidable trap for each insect.

 

The entrance to the trap is covered with nectar which attracts them and leads them directly to the inside of the urn where the walls are so slippery that it is impossible for them to cling to it to escape. The insects then fall into the digestive zone where they will be quickly anesthetised and then dissolved.

 

Where do these non-stick properties come from? The internal walls of the slippery area are super-hydrophobic and microscopically rough: they are covered with wax crystals which prevent insects from attaching to them.

The company Adaptive Surface Technologies was inspired by this microstructure to create a non-stick, ultra-repellent and self-healing surface coating.

 

Named SLIPS ®, this liquid coating is applicable to a wide range of materials such as metals and ceramics, and repels all kinds of liquids and foul particles. This provides unrivaled performance, particularly compared to sprayed superhydrophobic surfaces.

 

In the automobile industry, such a coating opens up new avenues of solutions to reduce friction and improve the lubrication of systems.

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Crédits images : ©Adaptive Surface Technologies ©Bioxegy © I. Scholz, M. Bückins, L. Dolge, T. Erlinghagen, A. Weth, F. Hischen, J. Mayer, S. Hoffmann, M. Riederer, M. Riedel, W. Baumgartner

Snakes: reduce friction without lubrication to ensure good movement speed.

Snakes are crawling animals. To move quickly on all kinds of ground, they must control their friction in two aspects: gripping to better propel themselves and sliding to prevent dragging.

Their skin is made up of evenly distributed individual scales that reduce friction and wear.

 

Another remarkable reptile is the sand fish: it glides over the sand, like a snake through water. For this reason, it has a special exterior skin, whose resistance to erosion is greater than that of steel!

Its skin is made of keratin and sulfur, which gives it an extremely low coefficient of friction.

Researchers from the Karlsruhe Institute of Technology in Germany were interested in the morphology of the skin and scales of these reptiles, which allowed them to create bio-inspired metallic surfaces with unrivaled friction coefficients. On dry surfaces, they found that this design resulted in a 40% decrease in friction compared to an equivalent unmodified flat surface.

 

Studying the potential of biomimicry to create surface textures is a relatively new area of research full of opportunities. In many industries, such as watchmaking, such a surface morphology could greatly reduce wear at the level of dry contact between two metallic parts, for example internal cogs.

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Crédits images : ©Christian Greiner, Michael Schäfer

Fish skin: design
a revolutionary industrial lubricant

Fish have a gland that secretes a specific mucus which attaches to the skin and envelops the body.

Composed of mucoproteins, this mucus plays a crucial role: it allows the fish to protect itself against the penetration of pathogenic microorganisms, but also to move quickly in the water thanks to its very low coefficient of friction.

This mucus is also very effective in making the fish more elusive for predators: it slips.

A team of Chinese researchers sought to reproduce the properties of this powerful mucus. They succeeded in developping a hydrophilic substance, called hydrogel.


Containing particular chemical compounds, it varies according to pH and temperature, which modify the molecular chains: conformational changes occur.

The coefficient of friction is adjustable and can be very low or very high:
At a high pH (7) and ambient temperature (~20°C), the coefficient of friction is very low: 0.05. At a low pH and high temperature (32°C), the friction coefficient becomes very high: 1.2.

 

This bio-inspired lubricant with modular friction is particularly promising and could be used in numerous industrial mechanical components, all sectors combined!

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Expériences menées sur le mucus des poissons-chats | Crédits images : © Yang Wu, Xiaowei Pei, Xiaolong Wang, Yongmin Liang, Weimin Liu & Feng Zhou

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