Abstract
Insect climbing footpads are able to adhere to rough surfaces, but the details of this capability are still unclear. To overcome experimental limitations of randomly rough, opaque surfaces, we fabricated transparent test substrates containing square arrays of 1.4 µm diameter pillars, with variable height (0.5 and 1.4 µm) and spacing (from 3 to 22 µm). Smooth pads of cockroaches (Nauphoeta cinerea) made partial contact (limited to the tops of the structures) for the two densest arrays of tall pillars, but full contact (touching the substrate in between pillars) for larger spacings. The transition from partial to full contact was accompanied by a sharp increase in shear forces. Tests on hairy pads of dock beetles (Gastrophysa viridula) showed that setae adhered between pillars for larger spacings, but pads were equally unable to make full contact on the densest arrays. The beetles' shear forces similarly decreased for denser arrays, but also for short pillars and with a more gradual transition. These observations can be explained by simple contact models derived for soft uniform materials (smooth pads) or thin flat plates (hairy-pad spatulae). Our results show that microstructured substrates are powerful tools to reveal adaptations of natural adhesives for rough surfaces.
Highlights
Many insects are capable of producing adhesion on natural substrates, which usually possess some degree of surface roughness [1]
We find a transition from full contact to partial contact and a consistent reduction in shear forces for dense arrays of tall pillars in insects with smooth and hairy pads
While a loss of adhesion on rough substrates has been reported in previous studies on smooth and hairy footpads of insects [2,19,20,24,27,29,30], our contact images reveal in detail how contact area is affected by surface roughness
Summary
Many insects are capable of producing adhesion on natural substrates, which usually possess some degree of surface roughness [1]. The smooth pads as found in ants, cockroaches and stick insects [5,11 –14] possess a soft, specialized adhesive cuticle which can deform and adjust to surface roughness. The internal fibrous structure of smooth pads may be important for their deformability [14,15], for the shearinduced lateral increase in contact area [16] or for the efficient transfer of tensile forces, but its detailed function is still unclear. For both smooth and hairy pads, small length scales of surface roughness may be further compensated by the
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