Bionics in Tribology: Adhesive and Frictional Dress of Elastomeric Surfaces

Abstract

To secure their locomotion on a variety of natural substrates, terrestrial animals have evolved attachment systems based on hairy and smooth pads. Hairy pads can be either wet or dry and, in most cases, they bear spatula-shaped tips that appeared independently in insects, arachnids and reptiles. Smooth pads are always wet and they usually possess different surface textures, of which the hexagonal one found in bush crickets, mushroom-tongued salamanders and tree frogs is the most striking. Inspired by the questions of what mechanisms are hidden behind the spectacular tribological performance of biological attachment systems and whether they can be used in technical applications, we mimic both spatula-bearing and hexagonally textured surfaces using elastomeric materials. Tested for adhesion and friction, biomimetic surfaces demonstrate a range of properties related to arresting and facilitation of relative motion. In spatula-inspired surface microstructures, these are directional adhesion and friction to load ratio of over 100, which can be used in clean, cheap and safe gripping mechanisms. In hexagonally patterned surfaces, we observe elimination of stick-slip instabilities, suppressing of hydroplaning and ability to tune friction from 50% to nearly 100% of that measured on a smooth reference, which may find application in systems ranging from syringes and hand prostheses on one hand to belt drives and submarine propeller shaft bearings on the other hand.