Adhesive hairy foot pads on insects self-clean more efficiently than smooth pads because hair tips reduce the area for particle attachment.

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An insect’s adhesive feet are critical to its ability to move around, climbing on vertical surfaces and hanging upside down while finding food or evading predators. Unlike most man-made adhesives, however, insect feet can attach and detach from a surface many times and still remain functional, even while encountering contaminants like dirt, pollen, and flaky wax from leaf surfaces. How do their sticky feet stay clean?

There are two main kinds of foot pads among insects: smooth foot pads, like those found on grasshoppers, ants, and stick insects; and hairy foot pads, like those found on beetles and flies. Smooth foot pads have even and soft surfaces, while hairy foot pads are covered with bristles that range from microns to millimeters long. Both kinds of foot pads secrete fluids that aid in adhesion, which depends on the amount of contact between the foot surface and the substrate: the lower the contact area between the two—for instance because contaminating particles are covering the foot—the lower the adhesion. Both kinds of foot pads can also self-clean by walking. Stepping on a substrate removes contaminating particles from the feet when those particles are more attracted to the substrate than the feet. When it comes to self-cleaning, however, hairy foot pads appear to have a leg up on smooth ones.

Smooth foot pads seem to require a shear force (parallel to the surface, as a brushing movement would be) to remove particles from the feet, but hairy foot pads can shed contaminating particles even when the foot just lifts up from the substrate (perpendicular to it). Based on models of hairy adhesive systems, it appears that hairs are easier to clean because their tips have inherently small surface areas; a contaminating particle attached to a narrow tip needs to move only slightly to the side for it to encounter empty space and disengage. While some particles could get stuck on the sides or between setae, on average, more particles are likely to be shed from hairy foot pads than from smooth foot pads. Furthermore, the hairs’ small surface area raises the likelihood that contaminating particles will be more attracted to the external substrate the insect is walking on, rather than its foot. Smooth and soft foot pads, on the other hand, can deform around a contaminating particle, increasing the contact between the two. As a result, hairy foot pads seem to recover from contamination much faster than smooth foot pads.

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References

“The smooth pads of stick insects exhibited self-cleaning only in the presence of a shear movement. By contrast, forces for the hairy pads of beetles were able to recover even when steps consisted only of pull-offs. This suggests that beetles might self-clean by a mechanism similar to the one proposed for dry gecko setae by Hansen and Autumn (Hansen and Autumn, 2005). Hansen and Autumn suggested that self-cleaning results from the greater attraction of particles to the surface than to the tips of the setae, resulting from the contact geometry, stiffness and low surface energy of fibrillar adhesive systems. Self-cleaning in the beetles’ adhesive pads may be further enhanced by microscale shear movements of the setae. In a hairy adhesive system, contaminating particles need to be moved by only a very small distance to reach the edge of a seta’s adhesive contact zone. When pressed onto the substrate, the tip of a seta may simply push particles aside, thereby moving them to a place where they adhere more strongly to the substrate and less to the seta.” (Clemente et al. 2010:641)

Journal article
Evidence for self-cleaning in fluid-based smooth and hairy adhesive systems of insectsJournal of Experimental Biology, 213: 635-642February 15, 2010
Clemente CJ; Bullock JMR; Beale A; Federle W

“The dense setae of the hairy pad type may simply allow easier shedding of contaminating particles than for smooth pads through normal forces (acting perpendicular to the surface) as suggested by previous studies (Clemente et al., 2010; Hui et al., 2006; Hansen and Autumn, 2005), or through a rolling action (Hui et al., 2006) where small lateral movements of the setae may facilitate cleaning via rolling the particle off the pad.” (Orchard et al. 2012:284-285)

Journal article
The influence of surface energy on the self-cleaning of insect adhesive devicesJournal of Experimental Biology, 215: 279-286January 15, 2012
Orchard MJ; Kohonen M; Humphries S

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