A firm grip on any surface

In their everyday life, insects often have to cope with both rough and smooth as well as sticky surfaces. They achieve a firm grip through special hooks or tiny hairs on their feet. While these different requirements are not a problem for many insects, technical applications are less flexible.

They are usually specifically developed for a particular application – such as summer or winter tyres, for example – and are not able to adapt to different surfaces. Inspired by the special structure of grasshopper's feet, an interdisciplinary research team at Kiel University (CAU) has now developed an artificial frictional system that works on a wide variety of surfaces.

Their combination of a soft silicone membrane, filled with fine-grained granulate, adapts itself to almost any surface and creates a firm grip under slight pressure. The simple production method also enables industrial applications, as reported by the research team in the current issue of the scientific journal Advanced Materials Interfaces.

Switching between soft and hard material properties

A firm grip requires a good contact surface as well as a stable transmission of force at the same time. “In order to stick to different surfaces, we must switch between the behaviour of soft and hard materials, which is actually a contradiction in terms,” explained Stanislav Gorb, Professor of Functional Morphology and Biomechanics at the CAU.

While soft materials allow a large contact area with the wide variety of surface textures, hard materials enable an optimal transmission of force. Therefore, the bionics expert and his team sought a way to switch between the two material properties. In addition, they wanted a solution simple and economical to produce, so that it can also be used for industrial applications.

They were inspired by the special feet of grasshoppers, which are characterised by small, cushion-like appendages. In previous research, Gorb and his team was able to show that these cushions are encased with a rubber-like film, which provides good frictional and adhesive contact with the surface. On the other hand, the inside of the cushions consists of particularly stable fibres which can transmit a great deal of force. Recreating such a fibrillar structure would be too time consuming and too expensive for industrial applications, however.

Now, the Kiel research team has demonstrated a similar effect for granulate, i.e. a fine-grained material. To do so, they used a principle of so-called “jamming transition”. “You know that from vacuum-packed coffee: the coffee powder is compressed and forms a dense mass, as hard as a rock. When the packet is opened for the first time, the powder becomes loose, and thus behaves quite differently, like a fluid,” described Halvor Tramsen, who along with Lars Heepe is one of the physicists in the research team.

High frictional forces on smooth, structured and dirty surfaces

They encased the granulate by a flexible membrane cover and tested the friction properties of their “granular cushion” GMFP (granular medium friction pad) on smooth, structured and dirty surfaces. Thanks to the soft and stretchable membrane, the cushion attached perfectly to the various surfaces.

The scientists then exerted pressure on the cushion, which compacted the granulate inside, and solidified the whole cushion. This stiffness and the large contact area with the surface together generate high frictional forces through which the cushion can no longer be moved. The cushion displayed similarly high friction on all three types of test surfaces.

How the frictional principle of the granulate cushion works on other surfaces is illustrated by a model developed by Professor Alexander Filippov, a theoretical physicist and Georg Forster Research Fellow in the Kiel working group. His numerical model also allowed the interaction of granulate and membrane to be tested for other materials and particle sizes.

“In our prototype, we used elastic silicone for the cover and actually filled it with dried coffee grounds,” explained Gorb. Due to their size and their rough shape, these particles get entangled with each other very easily, and the jamming transition effect, i.e. the switch between the properties of soft and hard materials, works particularly well.

In principle, it is certainly conceivable to also use dried coffee grounds for industrial applications with recycling benefits. After all, this coffee residue is readily available, free of pollutants and cheap, said Gorb. Research on other granular materials and membranous surfaces is already planned.

Original publication:
Maximizing Friction by Passive Jamming. Halvor T. Tramsen, Alexander E. Filippov, Stanislav N. Gorb, Lars Heepe. Adv. Mater. Interfaces 2020, https://doi.org/10.1002/admi.201901930

Photos are available to download:
https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-1.png
Caption: The special structure of grasshoppers' feet inspired the scientists from Kiel University for an artificial adhesion system that works on different surfaces.
Foto/Copyright: Stanislav Gorb

https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-2.png
Caption: The stable fibres inside and the rubber-like outer shell provide a stable hold for the grasshopper.
Foto/Copyright: Stanislav Gorb

https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-3.png
Caption: Inspired by the grasshopper's feet: The elastic membrane of the adhesive system developed at CAU adapts well to uneven surfaces. If light pressure is applied to it in the second step, the granular particles inside move closer together. This increase the stiffness of the entire material and it cannot be moved from the spot.
Foto/Copyright: Halvor Tramsen

https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-4.jpg
Caption: Simple and cheap fabrication: The research team's prototype consists of a silicone pad filled with dried coffee grounds as granules.
Photo/Copyright: Siekmann, CAU

https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-5.jpg
Caption: The flexible cover conforms to even rough surfaces so that the silicone pad cannot be moved, even under heavy pressure.
Foto/Copyright: Siekmann, CAU

https://www.uni-kiel.de/de/pressemitteilungen/2020/074-halt-6.jpg
Caption: Professor Stanislav Gorb and Halvor Tramsen are part of the interdisciplinary Kiel research team at Kiel University.
Foto/Copyright: private

Contact:
Professor Stanislav N. Gorb
Working Group “Functional Morphology and Biomechanics”
Zoological Institute at Kiel University
Tel.: +49 (0)431/880-4513
E-mail: sgorb@zoologie.uni-kiel.de
Website: http://www.uni-kiel.de/zoologie/gorb/topics.html

Details, which are only a millionth of a millimetre in size: this is what the priority research area “Kiel Nano, Surface and Interface Science – KiNSIS” at Kiel University has been working on. In the nano-cosmos, different laws prevail than in the macroscopic world – those of quantum physics. Through intensive, interdisciplinary cooperation between physics, chemistry, engineering and life sciences, the priority research area aims to understand the systems in this dimension and to implement the findings in an application-oriented manner. Molecular machines, innovative sensors, bionic materials, quantum computers, advanced therapies and much more could be the result. More information at www.kinsis.uni-kiel.de

Press, Communication and Marketing, Dr Boris Pawlowski, Text/editing: Julia Siekmann
Postal address: D-24098 Kiel, Germany,
Telephone: +49 431 880-2104, Fax: +49 (0)431 880-1355
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Professor Stanislav N. Gorb
Working Group “Functional Morphology and Biomechanics”
Zoological Institute at Kiel University
Tel.: +49 (0)431/880-4513
E-mail: sgorb@zoologie.uni-kiel.de
Website: http://www.uni-kiel.de/zoologie/gorb/topics.html

Maximizing Friction by Passive Jamming. Halvor T. Tramsen, Alexander E. Filippov, Stanislav N. Gorb, Lars Heepe. Adv. Mater. Interfaces 2020, https://doi.org/10.1002/admi.201901930

https://www.uni-kiel.de/en/details/news/074-halt