Friction in Only One Direction

Nature has amazed us since the beginning of time. It provides the foundation for all of our best ideas. The animal world provides the inspiration behind one of the new inventions in the world of tribology – namely microstructured material designed with spatial variation to create one directional friction.

This product is based upon studies of the feet of the gecko. The toes of the gecko are covered with millions of tiny, spatula-shaped hairs. These hairs allow the gecko to easily run up walls and across ceilings. However, it retains the ability to effortlessly release its grip at the same time.

Tiny folds of skin cover the sole of the gecko’s foot. Covering these folds are tiny hairs that branch out repeatedly until the tips (called seta) are only a few nanometers across. This creates millions of tiny locations where weak molecular forces can be activated, working together to create a force strong enough to allow the gecko to effortlessly hang from the ceiling.

Amazingly, these setae do not prevent the gecko from moving quickly. In fact, they have the ability to attain speeds of up to 20 body lengths per seconds, indicating that the sole of the gecko’s foot not only sticks when needed, but also releases from surfaces easily. This ability comes about due to the number of seta as well as their flexibility, curvature, and the fact that they are all placed at a 60-degree angle. When a gecko wants to move its foot, it simply lifts the foot straight up and effortlessly pulls it away from the surface.

Researchers working at replicating this amazing skill created a material from a silicone elastomer designed with microscopic wedges on its surface. Pulling the material downward towards the surface increases the frictional force. Pulling the material in the opposite direction allows it to easily slide. The randomly placed wedges are of different lengths so that the longer wedges curl over the shorter wedges when the material is pulled in the opposite direction, allowing the material to easily slide.

The original research by Srinivasan A. Suresh published in the Journal of the Royal Society Interface (https://royalsocietypublishing.org/doi/10.1098/rsif.2018.0705) compares material with uniform features creating friction in all directions and non-uniform features creating one directional friction. The first material’s surface consists of wedges all the same height. As a shear force is applied in one direction, the wedges self engage, increasing the contact area and friction and adhesion forces. Applying force in the opposite direction results in the same process occurring. The second material’s surface consists of wedges increasing in length. As a shear force is applied in one direction, the larger flap conforms to the shape of the surface creating a large contact area. However, the friction and adhesion are lower than those created by uniform features. When the force is applied in the opposite direction, the larger flap does not allow other wedges to touch the surface, therefore decreasing the contact area and lowering frictional forces.

Effectively used throughout nature, this spatial variation is rarely seen in manmade materials due to the increased production costs in dealing with the randomness required.

Further information can be found in the reference article:  Srinivasan A. Suresh et al. Spatially variant microstructured adhesive with one-way friction, Journal of The Royal Society Interface (2019). DOI: 10.1098/rsif.2018.0705