D25 Contact tribometry

Topic

The wear and friction experienced by the insect's adhesive setae as they slide across various substrates necessitate a precise characterization of this behavior using tribometry—a technique that records in detail the friction arising from the contact between the seta tip and the supporting surface.

This contact friction does not follow the classical law of dry friction—whereby frictional force is simply proportional to the applied normal load—but rather an extended version proposed by Derjaguin, which incorporates an additional term accounting for adhesion. Consequently, friction exists even when the normal load is zero or slightly negative, as the adhesion itself maintains contact between the seta and the surface.

This behavior, which differs significantly from conventional dry contact, is characteristic of bioadhesive systems in general; it enables the insect's seta to withstand substantial tangential shear without requiring a normal load to press it against the surface, relying instead solely on the force of adhesion to maintain contact.

As the insect's leg moves slowly across the surface, this adhesive contact friction can manifest in two distinct ways—with the Prandtl-Tomlinson model allowing for the prediction of which will occur: a regime of smooth, continuous sliding, or a "stick-slip" regime, in which the seta momentarily adheres to the surface before jumping forward in discrete steps, only to stick again. When this tribological contact between the mushroom-shaped structure and the surface occurs at high sliding speeds, friction itself generates additional localized heating right at the tip—known as "flash temperature." This effect is particularly significant for insects walking on hot surfaces, as the additional heat can alter the properties of the adhesive wax coating the tip.

All this friction at the contact point—whether during continuous sliding or stick-slip motion—dissipates a certain amount of energy during each movement cycle, represented by the area enclosed within the friction loop. This energy cannot be recovered; consequently, the insect's metabolism must continuously replenish it to allow the leg to keep moving across the surface cycle after cycle.

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