Many invertebrates have the ability to adhere and walk on almost any surface. They can even hang upside down for as long as they want!
In some species (incl. most beetles), this amazing ability is attributed to hairy pads on their tarsomeres. Each pad comprises hundreds of microscopic hairs called setae. We study this hair-mediated adhesion experimentally, by combining microscopy and force measurements. Our long-term goal is to use this knowledge to develop a new generation of biomimetic devices for gripping and handling objects at the micrometer scale.
With a combination of microscopy and high-speed imaging, we quantified the kinematics of freely-walking beetles, from the leg scale to the scale of individual setae. Both attachment and detachment often involve some peeling, both at pad scale and setal scale. Surprisingly, attachment is about 10 times slower than detachment.
What is the mechanism that yields such controlled adhesion on almost any surface ? Like in any good inquiry, we looked first for footprints… which are easily found. When each seta detaches, it often leaves a femtoliter droplet of an oily secretion. Therefore, when the setal tip is in contact with the substrate, the liquid must form a capillary bridge.
Thanks to Interference Reflection Microscopy, we quantified the deflection of each setal tip, and we have compared it to the theoretical deflection that it would experience through bending induced by capillary loads. Our results suggest that the capillary forces are sufficient to explain the observed deformation. Each setal tip is therefore in an “elastocapillary” equilibrium of capillary and elastic forces. Moreover, the net adhesive force calculated from this beam model is in good agreement with the adhesion measurements of individual setae, of the order of 1µN. Surface tension is then a likely contributor to this adhesion mechanism. Our recent review paper discusses how such combination of compliant setal tips with a liquid secretion is a winning strategy for robust adhesion at the insect scale.
This work is done in the framework of the scientific network IAP – microMAST.