Forcing the issue: testing gecko-inspired adhesives

Abstract

Materials are traditionally tested either by imposing controlled displacements and measuring the corresponding forces, or by imposing controlled forces. The first of these approaches is more common because it is straightforward to control the displacements of a stiff apparatus and, if the material suddenly fails, little energy is released. However, when testing gecko-inspired adhesives, an applied force paradigm is closer to how the adhesives are loaded in practice. Moreover, we demonstrate that the controlled displacement paradigm can lead to artefacts in the assumed behaviour unless the imposed loading trajectory precisely matches the deflections that would occur in applications. We present the design of a controlled-force system and protocol for testing directional gecko-inspired adhesives and show that results obtained with it are in some cases substantially different from those with controlled-displacement testing. An advantage of the controlled-force testing approach is that it allows accurate generation of adhesive limit curves without prior knowledge of the expected behaviour of the material or the loading details associated with practical applications.

An Electrostatically Actuated Gecko Adhesive Clutch

“…For a clutch application, the conventional load‐pull (LP) test approach used to test adhesives for climbing or lifting applications is less appropriate than a load‐drag‐pull protocol (LDP). [ 37 ] Briefly, in the LP case, the adhesive is brought into contact with a surface at some approach angle and then pulled away. The maximum values of the normal and shear force are recorded for various pull‐away angles, resulting in an empirical limit‐curve that defines the limiting normal and shear adhesive values as a function of loading angle.…”

“…The original wedge data are taken from previous results for Load-Drag-Pull (LDP) testing. [37] Figure 8. Shear stress for the non-stick prisms as a function of normal pressure.…”

“…The shaded region shows variability in normal and shear stresses for the new wedges. The original wedge data are taken from previous results for Load-Drag-Pull (LDP) testing [37].…”

From Adhesion to Detachment: Strategies to Design Tissue‐Adhesive Hydrogels

“…[137] Inspired by this, many studies have successfully fabricated nano-or micropillar dry-adhesive patches, which show good adhesion to human skin for various applications. [138] However, most of these patches are made of hydrophobic elastomers such as polydimethylsiloxane (PDMS) [139,140] and considered as dry adhesives only, as van der Waals forces are easily disrupted in the presence of water, making them unsuitable for wet tissue adhesion. To tackle this problem, Yi et al fabricated the hydrogel micropillar structure using UV cross-linkable poly(ethylene glycol) dimethacrylate (PEGDMA) [141] (Figure 9A).…”

A Polynomial Framework for Design of Drag Reducing Periodic Two-dimensional Textured Surfaces

“…Textures are well documented for their ability to alter and regulate physical response of surfaces, both in the natural and engineering settings; Drag reducing effect of ribs on the denticles of fast swimming shark species [1,2,3] as well as bioinspired riblet-covered surfaces [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18], super-hydrophobicity, super-repellency, and self cleaning of Colocasia esculenta, Euphorbia myrsinites, Nelumbo nucifera (Lotus), and Salvinia leaves, packed with convex papillose cells which are covered by three-dimensional wax crystal [19,20,21,22,23,24], slippery and insect trapping mechanism of the concave textures on the rim (peristome) of Nepenthes pitcher plants [25,26,27,28], which can also act as superior fog harvesters [29], as well as excellent adhesion of textures on the feet of gecko and some artopods [30,31,24] and the textured gecko-inspired engineered adhesives [32,33,34,35,36] are but a few examples.…”