Sliding-induced non-uniform pre-tension governs robust and reversible adhesion: a revisit of adhesion mechanisms of geckos

Abstract

Several mechanisms have been proposed in the literature to explain the robust attachment and rapid, controllable detachment of geckos' feet on vertical walls or ceilings, yet, it is still debatable, which one is ultimately responsible for geckos' extraordinary capabilities for robust and reversible adhesion. In this paper, we re-examine some of the key movements of geckos' spatula pads and seta hairs during attachment and detachment, and propose a sequence of simple mechanical steps that would lead to the extraordinary properties of geckos observed in experiments. The central subject under study here is a linear distribution of pre-tension along the spatula pad induced by its sliding motion with respect to a surface. The resulting pre-tension, together with a control of setae's pulling force and angle, not only allows for robust and strong attachment, but also enables rapid and controllable detachment. We perform computational modelling and simulations to validate the following key steps of geckos' adhesion: (i) creation of a linear distribution of pre-tension in spatula through sliding, (ii) operation of an instability envelope controlled by setae's pulling force and angle, (iii) triggering of an adhesion instability leading to partial decohesion along the interface, and (iv) complete detachment of spatula through post-instability peeling. The present work not only reveals novel insights into the adhesion mechanism of geckos, but also develops a powerful numerical simulation approach as well as additional guidelines for bioinspired materials and devices.

Gecko toe and lamellar shear adhesion on macroscopic, engineered rough surfaces

“…Russell (Russell, 1981;Russell, 1986;Russell, 2002) proposed that the intermediate structures are used to cushion the foot against the surface, and to allow the flexible lamellae to conform more closely to the surface to which they are adhering. Another theory on the function of the intermediate structures is that the size, shape and angle of the larger hierarchical structures aid in rapid detachment of the foot (Gao et al, 2005;Cheng et al, 2012). Other investigators have also shown analytically that the hierarchical structures uniformly distribute the adhesive force across the attachment pads, resulting in stronger and more robust adhesion on rough surfaces (Kim and Bhushan, 2007;Chen et al, 2008).…”

Recent advances in gecko adhesion and friction mechanisms and development of gecko-inspired dry adhesive surfaces

“…Numerical simulations show that sliding causes the spatulae to become well-aligned or ordered, leading to an increase in the real contact area and to a more stable configuration during sliding [69,70]. Cheng et al [71,72] proposed that a pre-tension can increase the adhesion force of the seta at small peeling angles.…”

Design of gecko-inspired fibrillar surfaces with strong attachment and easy-removal properties: a numerical analysis of peel-zone

“…Peeling tests have been widely applied to investigate bio-inspired conformal adhesion of fibrillar surfaces [27,[33][34][35]. Therefore, it is very important to understand the fundamental principles of various peeling behaviours to develop innovative gecko-inspired surfaces/adhesives [30]. Yet, the geometry of peel-zone, the deformation of fibrillar structures during peeling process and their contributions to peel strength ( peel force per unit width) have not been analysed quantitatively and systematically.…”

“…The peel angle, the length and shape of the peel region of the adhesive tape can significantly influence its mechanical peeling properties [6,17,[30][31][32]. It is practicable to fabricate adhesive surfaces with specially designed micro-fibrillar structures to modulate their reversible peeling and mechanical properties.…”

“…Previous studies have considered the easy removal of strong adhesive surfaces with vertical micro-fibrillar structures similar to that of a real gecko that can attach and detach from surfaces via controlling the peeling angle of its setae by gripping in or releasing its toes, in which the adhesion force can vary by more than two or three orders of magnitude [18,30]. The detachment of *Author for correspondence (tianyu@mail.tsinghua.edu.cn).…”