Kiel University (CAU) scientists have mimicked the special structure of grasshopper’s feet to create a new multi-surface, artificial frictional system.
A firm grip requires a good contact surface as well as a stable transmission of force at the same time, explains CAU professor Stanislav Gorb:
“In order to stick to different surfaces, we must switch between the behavior of soft and hard materials, which is actually a contradiction in terms.”
For inspiration, Gorb’s team turned to grasshoppers feet; these insects’ feet are characterized by small, cushion-like appendages.
In a previous research, they were 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 fibers which can transmit a great deal of force.
Nevertheless, recreating such a fibrillar structure would be too time consuming and expensive for industrial applications.
The Kiel team has now demonstrated a similar effect for granulate (a fine-grained material) with the help of the ‘jamming transition’ principle – a process that can be seen in vacuum-packed coffee.
In 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.
For their study, Gorb’s team encased the granulate by a flexible membrane cover and tested the friction properties of their Granular Medium Friction Pad (GMFP) 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.
“In our prototype, we used elastic silicone for the cover and actually filled it with dried coffee grounds,” says Gorb.
Due to their size and rough shape, these particles get entangled with each other very easily, and the jamming transition effect works particularly well.
Image and content: Stanislav Gorb/Kiel University