From rough to smooth to slippery – some animals manage to hold onto very different surfaces. German bionics engineers have once again taken the structures responsible for this as a model: Inspired by grasshopper feet, they have developed a granulate cushion that can provide support on very different surfaces. The concept uses the effect of the so-called “jamming transition” and seems ingeniously simple: The prototype is a soft silicone cover that is filled with coffee grounds.
Gecko foot, spider leg or snail slime – nature has created sophisticated systems that give animals amazing-looking abilities to move around on different surfaces. For a long time, researchers have been trying to technically replicate the biological systems and structures so that they can be used by humans. In some cases, the implementation was at least partially successful. But one aspect of the animal role models is often difficult to implement: Technical systems are usually not very flexible. They are usually developed specifically for specific substrates – they are hardly all-round solutions. Some concepts are also quite complicated and expensive.
Uncomplicated all-round solutions are required
In order to develop better solutions, the researchers led by Stanislav Gorb from the Christian Albrechts University in Kiel continue to explore the patents of nature.
Her focus is on systems that guarantee a firm hold through the combination of an optimal contact surface and stable power transmission. In addition, the concepts should enable simple and inexpensive replication.
“In order to adhere to different surfaces, you would have to switch – actually a contradiction – between the behavior of soft and solid materials,” says Gorb. While a soft material condition allows a large contact area to the surface, a solid condition allows a large power transmission, explains the scientist. He and his colleagues found the appropriate model for the grasshoppers: in addition to claws, these insects have pillow-like elements on their feet to hold them on surfaces. Through detailed investigations, the scientists were able to show that these cushions are characterized by two features: they are covered by a rubber-like layer that ensures good contact with the surface to be adhered to, and inside there are stable fibers that ensure a strong grip when pressed.
Coffee grounds give stable grip
Technically reproducing a corresponding fiber structure would be too expensive for an industrial application, the researchers say. But, as her study shows, granules – a granular mass – can also convey the effect of the fibers. As you explain, the concept has the effect of the so-called “jamming transition”. “You can imagine it like a pack of coffee: the coffee powder is pressed together tightly by pressure and forms a dense mass, solid like a stone. When the package is opened, the powder falls loose and behaves very differently – almost like a liquid, ”is how co-author Halvor Tramsen describes the effect.
In fact, the researchers also used dried coffee grounds in the development of their granule pillow. They encased this granulate with a flexible membrane covering made of silicone, which corresponds to the rubber-like structure of the grasshopper foot cushions. The researchers then tested their granulate pillow on smooth, structured or soiled surfaces. They were able to document the physical processes and effects precisely: thanks to its soft cover, the pillow fits perfectly to the various surface structures. If you then put pressure on the pillow, the grains inside compress and the whole structure solidifies.
Potential is emerging
As the researchers report, the combination of large contact area and strength created high frictional forces, which means that the cushion can no longer be moved when moving parallel to the surface. As a result, it developed a much better grip on the three different test surfaces than pure silicone rubber or as a pillow filled with liquid. The researchers also developed a model to maximize the friction of the concept on various substrates. The scientists say that it allows the interaction of granules and membrane to be simulated for other materials and particle sizes.
However, as you emphasize, coffee grounds could be an optimal filling material for such applications: due to the particle size and their rough shape, the small particles get caught very easily and the change between soft and solid state is particularly good. This could even result in a new way of recycling coffee residues, the researchers say. They are convinced that the simple production of appropriate granule pillows could make the concept interesting for industrial use. In further investigations, they now want to further explore the corresponding potential and performance of their system.
Source: Christian Albrechts University of Kiel, technical article: Adv. Mater. Interfaces, doi: 10.1002 / admi.201901930