Importance of Loading and Unloading Procedures for Gecko-Inspired Controllable Adhesives

Abstract

The importance of loading and unloading procedures has been shown in a variety of different methods for biological dry adhesives, such as the fibers on the feet of the Tokay gecko, but biomimetic dry adhesives have yet to be explored in a similar manner. To date, little work has systematically varied multiple parameters to discern the influence of the testing procedure, and the effect of the approach angle remains uncertain. In this study, a synthetic adhesive is moved in 13 individual approach and retraction angles relative to a flat substrate as well as 9 different shear lengths to discern how loading and unloading procedures influence the preload, adhesion, and shear/friction forces supported. The synthetic adhesive, composed of vertical 10 μm diameter semicircular poly(dimethylsiloxane) fibers, is tested against a 4 mm diameter flat glass puck on a home-built microtribometer using both vertical approach and retraction tests and angled approach and retraction tests. The results show that near maximum adhesion and friction can be obtained for most approach and retraction angles, provided that a sufficient shear length is performed. The results also show that the reaction forces during adhesive placement can be significantly reduced by using specific approach angles, resulting for the vertical fibers in a 38-fold increase in the ratio of adhesion force to preload force, μ', when compared to that when using a vertical approach. These results can be of use to those currently researching gecko-inspired adhesives when designing their testing procedures and control algorithms for climbing and perching robots.

Bioinspired fibrillar adhesives: a review of analytical models and experimental evidence for adhesion enhancement by surface patterns

“…This arose from the difference in contact area between the top, flat edge and curved edge faces of the pillar. In a separate study the authors demonstrated significant preload dependence on adhesion after shearing towards the curved face [71]. Similarly, Parness et al have shown that wedge-shaped pillars, with a wide base and narrow tip, exhibit negligible normal adhesion unless the pillars are pre-sheared to increase the contact area [55].…”

Influence of Humidity on Grip and Release Adhesion Mechanisms for Gecko-Inspired Microfibrillar Surfaces

“…39 Our work aims to address these topics while also exploring the grip-release mechanisms of our gecko-inspired adhesives in humid and wet environments. Geometrically anisotropic halfcylinder microfibers, 44,45 as well as tilted half-cylinder microfibers, 46 yield high adhesion and low adhesion states by controlling the loading path (approach−shear−retraction). Shear in one direction results in a large contact area and high friction and adhesion, referred to as the "grip state".…”

Coupled effects of applied load and surface structure on the viscous forces during peeling

“…20 Detachment via peeling is also desirable because the adhesion force can be modulated by varying the peel angle. [21][22][23][24] Geckos 25 and tree frogs 3 have both been shown to splay their limbs to control their adhesion to surfaces where the peel angle is kept low to maintain contact and increased to pull out. In biomimetic systems, the effect of structured surfaces on peeling has been well-studied in dry [26][27][28] and wet 28,29 environments, but less so in completely ooded conditions.…”