Simulation of synthetic gecko arrays shearing on rough surfaces

Abstract

To better understand the role of surface roughness and tip geometry in the adhesion of gecko synthetic adhesives, a model is developed that attempts to uncover the relationship between surface feature size and the adhesive terminal feature shape. This model is the first to predict the adhesive behaviour of a plurality of hairs acting in shear on simulated rough surfaces using analytically derived contact models. The models showed that the nanoscale geometry of the tip shape alters the macroscale adhesion of the array of fibres by nearly an order of magnitude, and that on sinusoidal surfaces with amplitudes much larger than the nanoscale features, spatula-shaped features can increase adhesive forces by 2.5 times on smooth surfaces and 10 times on rough surfaces. Interestingly, the summation of the fibres acting in concert shows behaviour much more complex that what could be predicted with the pulloff model of a single fibre. Both the Johnson-Kendall-Roberts and Kendall peel models can explain the experimentally observed frictional adhesion effect previously described in the literature. Similar to experimental results recently reported on the macroscale features of the gecko adhesive system, adhesion drops dramatically when surface roughness exceeds the size and spacing of the adhesive fibrillar features.

Stick–slip friction of gecko-mimetic flaps on smooth and rough surfaces

“…The trailing edge of a contact junction is associated with detachment of the individual flaps from the silica surface in a JKR peeling fashion [4,5]. The flaps that are about to detach from the silica surface are shown in red in figure 7b,d.…”

“…When Schallamach waves are responsible for the peeling (JKR mechanism) [4,5] and sticking of an adhesive interface, the work of adhesion can be estimated by…”

“…The mechanisms for attachment and high adhesive forces of gecko spatula and setae have been measured and modelled by Johnson-Kendall-Roberts (JKR)-type theories [4,5], whereas the ease of detachment from surfaces requires a peel-off theory [6][7][8]. The effects of end-shape and size of microfibres on adhesion have been investigated experimentally [9,10] and theoretically [11].…”

“…According to this model, the adhesion of a gecko footpad [12] or its mimic to a substrate depends on the applied shear force [4,6] and explains the very low detachment forces observed in climbing geckos. Anisotropic fibrillar synthetic adhesives mimicking the gecko footpad functionality have been previously fabricated [5,[13][14][15][16][17][18][19][20][21][22][23][24] and were used to study adhesion and frictional properties on silica surfaces. The mechanism of operation of these structures involved application of a small pre-load (several millinewtons) followed by shearing the structures against the surface of interest for several micrometres to allow the real surface area of contact to be maximized and hence attain a good grip.…”

An anisotropic large displacement cohesive zone model for fibrillar and crazing interfaces

“…As recently investigated in Zhou et al (2014), the angle b can have a deterministic value only if the texturing is regular as in the form of cylinders, see Fig. 3(a), or in case of sinusoidal waviness as considered in Gillies and Fearing (2014). In the more general case of random roughness, a statistical distribution of angles b is expected to arise from the actual distribution of the local slopes of the profile height field.…”