New Structure Developed For Smooth And Structured Surfaces

2023 Author : Hannah Pearcy | [email protected] . Last modified: 2023-05-24 11:12

The structure of technical applications is usually developed specifically for a specific benefit and cannot adapt to different substrates. Take tires, for example: summer tires have the best grip on dry, warm roads, while winter tires are made of a soft rubber compound and are therefore not suitable for warm, but for colder temperatures.

Nature, on the other hand, can do it differently: insect feet can cope with both rough and slippery surfaces. Based on these findings, an interdisciplinary research team from the Christian Albrechts University in Kiel (CAU) has now developed an artificial friction system that works on different surfaces.

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The peculiarity of insect feet

When looking for food, insects often have to cope with both rough and smooth and slippery surfaces. For example, hook-like claws or adhesive hairs on their feet give them firm hold. The grasshopper's feet, for example, are characterized by small, cushion-like additions. On the one hand, these pillows are covered with a rubber-like film that provides good contact with the adhesive surface. On the other hand, they consist of particularly stable fibers on the inside, which can transmit a large force. Replicating such a fiber structure would, however, be too time-consuming and expensive for an industrial application.

The research team develops a structure that adapts to almost any surface under slight pressure. Their combination of a soft silicone cover, filled with fine-grained granules, creates a stable hold. The simple manufacture also makes industrial use possible, the research team writes in the current issue of the journal Advanced Materials Interfaces.

Two material properties in one solution

A good contact surface and stable power transmission are required for a firm hold. "In order to adhere to different surfaces, one would have to switch - actually a contradiction - between the behavior of soft and solid materials," explains Stanislav Gorb, Professor of Functional Morphology and Biomechanics at the CAU. While a soft material condition allows a large contact area to the surface, a firm condition allows a large power transmission. Together with his team, the bionic engineer was looking for a way to switch between the two material properties. In addition, the solution should be simple and inexpensive to manufacture so that it can also be used for technical applications.

The Kiel research team has now been able to demonstrate a similar effect to the pillows on the grasshopper feet for granules, i.e. a granular mass. For this they used the principle of the so-called "jamming transition". "You can imagine it like in 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 thus behaves very differently, almost like a liquid,”describes Halvor Tramsen, together with Lars Heepe, one of the two physicists in the research team.

High frictional forces on all test areas

They encased the granulate with a flexible membrane cover and tested the hold of their "granulate cushion" on smooth, structured and dirty surfaces. Thanks to its soft cover, the pillow fits perfectly on the various surfaces. The scientists then exerted pressure on the pillow, which thickened the grains inside and solidified the entire pillow. This strength and the large contact surface with the surface create high frictional forces that prevent the cushion from moving. It achieves a much better grip on all three types of test surfaces than pure silicone rubber or as a cushion filled with liquid.

A model developed by Professor Alexander Filippov, theoretical physicist and Georg Forster research fellow in the Kiel working group shows how the principle of maximizing the friction of the granule cushion works on other substrates. His calculations also make it possible to test the interaction between granulate and membrane for other materials and particle sizes.

CFRP components

When haute couture embroidery meets bionics

"In our prototype, we used stretchable silicone for the case and - actually - filled it with dried coffee grounds," explains Gorb. Due to their particle size and their rough shape, these particles get caught very easily and the effect of the "jamming transition", ie the change between the properties of soft and solid materials, comes into play particularly well. In principle, it is quite conceivable to use dried coffee grounds in the sense of recycling also for industrial applications. After all, the residue is readily available, free of pollutants and inexpensive, says Gorb. Research into other materials and substrates is already planned.