Many animals, including insects, spiders, geckos and tree frogs, can climb smooth surfaces by using adhesive pads on their feet. Hairy pads exhibit a brush-like array of thin fibres (setae) and are common among spiders, geckos, flies and beetles. As these pads are inherently sticky, they attract dirt particles. Without cleaning, the pads would foul quickly and become unusable. While there may be several mechanisms used for cleaning, like grooming, brushing with legs or flushing with fluids, these mechanisms are time or energy consuming, and so would greatly impede locomotory performance.
Here, a self-cleaning mechanism is proposed whereby soiled feet would slip on the surface due to a lack of adhesion but shed particles in return. Our study offers an in situ quantification of self-cleaning performance in fibrillar adhesives, using the dock beetle as a model organism. After beetles soiled their pads by stepping into patches of spherical beads (Figure a), we found that their gait was significantly affected. Specifically, soiled pads slipped 10 times further than clean pads (Figure e). Like previous studies, we found that particle size affected cleaning performance. Large (45 $\mu$m) beads were removed most effectively, followed by medium (10 $\mu$m) and small (1 $\mu$m) (Figure b-d). Consistent with our results from climbing beetles, force measurements on freshly severed legs revealed larger detachment forces of medium particles from adhesive pads compared to a flat surface, possibly due to interlocking between fibers (Figure f). By contrast, dock leaves showed an overall larger affinity to the beads and thus reduced the need for cleaning. Overall, pad slippage and high adhesion of dock leaves were found to promote effective self-cleaning and reduced fouling. Self cleaning through slippage provides a mechanism robust to particle size and may inspire solutions for artificial adhesives, climbing robots or grippers.