Adhesion and friction of the smooth attachment system of the cockroachGromphadorhina portentosaand the influence of the application of fluid adhesives

Abstract

Two different measurement techniques were applied to study the attachment of the smooth foot pads of the Madagascar hissing cockroach Gromphadorhina portentosa. The attachment of the non-manipulated adhesive organs was compared with that of manipulated ones (depletion or substitution by artificial secretions). From measurements of the friction on a centrifuge, it can be concluded that on nanorough surfaces, the insect appears to benefit from employing emulsions instead of pure oils to avoid excessive friction. Measurements performed with a nanotribometer on single attachment organs showed that, in the non-manipulated euplantulae, friction was clearly increased in the push direction, whereas the arolium of the fore tarsus showed higher friction in the pull direction. The surface of the euplantulae shows an imbricate appearance, whereupon the ledges face distally, which might contribute to the observed frictional anisotropy in the push direction. Upon depletion of the tarsal adhesion-mediating secretion or its replacement by oily fluids, in several cases, the anisotropic effect of the euplantula disappeared due to the decrease of friction forces in push-direction. In the euplantulae, adhesion was one to two orders of magnitude lower than friction. Whereas the tenacity was slightly decreased with depleted secretion, it was considerably increased after artificial application of oily liquids. In terms of adhesion, it is concluded that the semi-solid consistence of the natural adhesion-mediating secretion facilitates the detachment of the tarsus during locomotion. In terms of friction, on smooth to nanorough surfaces, the insects appear to benefit from employing emulsions instead of pure oils to avoid excessive friction forces, whereas on rougher surfaces the tarsal fluid rather functions in improving surface contact by keeping the cuticle compliable and compensating surface asperities of the substratum.

A comparison of tarsal morphology and traction force in the two burying beetles Nicrophorus nepalensis and Nicrophorus vespilloides (Coleoptera, Silphidae)

“…Tarsal insect attachment usually implies directionality, i.e., the friction forces depend on the sliding direction, which is a major feature ensuring controllability, i.e., the ability of the tarsi to rapidly attach to and detach from a variety of surfaces. Whereas such behaviour has mainly been demonstrated for smooth tarsal attachment systems [19,49–50], little such evidence exists for hairy systems [51–53]. As expected, in both the investigated Nicrophorus species, the friction forces of the fore tarsi increase in the pull direction corresponding to the alignment and the apical structure of the tenent hairs.…”

“…For the nanotribometer experiments, we used the same silanized Al 2 O 3 grinding disc surfaces as previously described by Betz and co-workers [19]. The roughness parameters of the micro-rough and the rough Al 2 O 3 grinding discs were as determined by Betz et al (Table 1 in [19]) and amounted to 0.05 µm (“nano-rough”), 3 µm (“micro-rough”) and 11 µm (“rough”) (given as R a values). These surfaces were better wettable with non-polar substances (such as diiodomethane) than with polar substances.…”

“…However, in contrast to both the other surfaces, the surface with 11 µm roughness showed a higher surface polarity and an improved wettability towards polar liquids such as water. As surface free energies of real solids are always influenced by surface texture, the calculated surface free energies of 33.3 mN·m −1 (micro-rough surface) and 42.2 mN·m −1 (rough surface) (Table 2 in [19]), respectively, should not be considered as absolute values, which can only be determined for ideal, perfectly smooth surfaces [20].…”

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

“…Although there is currently no direct evidence of such emulsion for the beetle pads, the corresponding friction forces depend on humidity, which suggests that water could nevertheless be present in the contact zone [27]. The only other formulated hypothesis to explain static friction is the transition from the classical elasto-hydrodynamic lubrication regime (where there is a continuous liquid film separating the cuticle from the substrate) to the boundary / mixed lubrication regime (where there is local formation of dry rubber-like contacts between the cuticle and the substrate) [35,48]. It is unknown in which lubrication regime the setal tips work, and it would certainly depend on both the volume of liquid in the bridge and the substrate roughness.…”

“…Most studies focused on (1) dry fibrillar adhesives usually with micrometer-sized mushroom-shaped surface structures ( [56] and refs. therein), (2) fabrication of functionally graded fibrillar adhesives [57], (3) mimicking the secretion itself [48], (4) mimicking the secretion process [58,59], and (5) dry fibrillar adhesives under flooded conditions [60]. However, a thorough implementation of a controlled secretion supply in combination with compliant thin plate-like fibrillar tips has not yet been realized.…”