Experimental investigation of the effect of tip shape in gecko-inspired adhesive devices under asymmetric detachment

Background It is usually challenging to achieve surfaces that are highly adhesive, yet can be detached easily. Geckos’ foot hairs, however, can be easily detached from a surface even though they adhere to it strongly enough to hold the animal’s body against gravity. Inspired by Nature, a combination of strong adhesion and easy detachment in adhesion system can be achieved and the possibility of reversible adhesion will be opened up. Geckos’ ability to climb vertical walls and hang upside-down on ceilings have attracted the attention of numerous scientists who, through careful observations, have linked these remarkable properties to microscopic hairs present on the surface of gecko’s foot [1–4]. These findings were corroborated by measurements of the adhesive force of a single gecko foot hair [5]. Insects, such as beetles, also use foot hairs to improve their grip on surfaces [6, 7]. Several devices mimicking these foot hairs were fabricated, which showed improved adhesive properties [8–14]. In particular, a mushroom-like structure exhibited strong adhesive behavior by equalizing the stress distribution at a contact tip [15–19]. In contrast, an asymmetric stress distribution lowers the detachment force. When a tangential force is applied, a moment is generated in the structure and the stress at the contact area is asymmetrically distributed. The stress at the contact edge increases with the applied moment until the stress is reached at which point the detachment occurs [20]. Hence, the detachment force decreases when the moment increases, and the control of the moment is an important Abstract Gecko’s foot hairs exhibit significant frictional anisotropy that enables a strong foot grip in a specific direction and an easy detachment in the opposite direction. In this study, we fabricate adhesive devices with frictional anisotropy mimicking gecko’s foot hair based on oblique micro-beam arrays. The devices adhesion force is strongly anisotropic along the beam tilting direction and depends on the stress distribution at the contact area which, in turn, is affected by the geometry of the beam tips. This dependence is investigated by fabricating and testing micro beam arrays with various tip shapes.