Soiled adhesive pads shear clean by slipping: a robust self-cleaning mechanism in climbing beetles

Abstract

Animals using adhesive pads to climb smooth surfaces face the problem of keeping their pads clean and functional. 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 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, we found that their gait was significantly affected. Specifically, soiled pads slipped 10 times further than clean pads, with more particles deposited for longer slips. Like previous studies, we found that particle size affected cleaning performance. Large (45 μm) beads were removed most effectively, followed by medium (10 μm) and small (1 μm). 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 fibres. By contrast, dock leaves showed an overall larger affinity to the beads and thus reduced the need for cleaning. Self-cleaning through slippage provides a mechanism robust to particle size and may inspire solutions for artificial adhesives.

Statistical properties of defect-dependent detachment strength in bioinspired dry adhesives

“…Other topics of interest include systematic assessment of the role of surface contaminants [50], where introduction of a known size distribution of particles to the substrate may provide insight as to how these give rise to defects and in turn control fibril strength. Intentional introduction of fabrication imperfections [51] could also provide fundamental insight into the mechanics governing fibril adhesive strength.…”

Liquid secretion and setal compliance: the beetle's winning combination for a robust and reversible adhesion

“…Beetle's hairy pads are particularly efficient at self-cleaning, more than the smooth pads of other insects [51], and much more than gecko's dry hairy pads [52]. In a few steps, possibly with sliding [53], the wet hairy pads can be completely depleted from most contaminant particles. Only those of the order of 10 µm (i.e.…”

“…The exact way in which the secretion helps self-cleaning is not yet known, but many particles are found in footprint droplets [51]. The secretion could certainly behave as a lubricant that would favor the sliding of particles to the sides of the bridge [53]. On the other hand, viscous hydrodynamic forces would hardly be sufficient to detach a particle of size R H already in contact with the cuticle.…”

“…53 (Amador et al, 2017): This paper quantifies the self-cleaning ability of wet hairy adhesive pads. It shows that in a free walk, beetles with soiled pads perform sliding movements to efficiently clean them.…”

Functional diversity of attachment and grooming leg structures is retained in all but the smallest insects

“…From the energetic perspective, there is some optimum between passive and active grooming strategies (Amador & Hu, 2015). At the level of an individual adhesive pad of the tarsus, self‐cleaning can be affected via shearing or slipping, and has been demonstrated in Phasmatodea (Clemente et al ., 2010; Clemente & Federle, 2012), Coleoptera (Clemente et al ., 2010; Amador, Endlein & Sitti, 2017) and Formicidae (Hefetz et al ., 2001; Anyon et al ., 2012; Orchard, Kohonen & Humphries, 2012). Besides preventing bacterial or fungal fouling, grooming also enhances olfactory acuity (Böröczky et al ., 2013) and is, for example, considered necessary for platygastroid parasitoids after breaking through the egg chorion (Austin, Johnson & Dowton, 2005).…”