Analysis and measurement of stress distributions in gecko toes and synthetic adhesives [electronic resource]
Keywords: Applied Physics.

The adhesive pads on gecko toes are complex systems containing structures at different size scales. Each toe is covered in flaps of skin called lamellae, which are in turn covered in arrays of microscopic hair-like structures known as setae. The tip of each seta splits into hundreds of even smaller nanoscale structures (spatulae) which produce adhesion through intermolecular van der Waals forces. Using this adhesive system, geckos can stick to a wide range of surfaces. One of the most interesting properties of gecko adhesive is controllable adhesion. An adhesive is called controllable if the stickiness can be switched on or off so it can be easily and repeatedly attached and detached. In gecko adhesive, the adhesion is controlled by the shear force: geckos can control their adhesive simply by applying a downwards shear force to their toes. In previous work, a controllable synthetic adhesive was developed that used shear force to control the adhesion similarly to gecko adhesive. The synthetic adhesive consisted of wedge-shaped microstructures made of polydimethylsiloxane (PDMS) silicone rubber, known as microwedges. This thesis presents a new micromachining manufacturing process for microwedge adhesives, which produces stronger adhesives with more varied geometries, enabling practical applications such as grasping and climbing devices for robots and humans. In addition, this thesis investigates the distribution of adhesive stress in natural gecko adhesive and synthetic microwedge adhesive through a combination of experimental measurements and theoretical modeling. In order for an adhesive system to produce the maximum possible adhesive force, the force must be uniformly distributed over the adhesive area. However, until now it was unknown how forces are distributed in gecko adhesive. To address this question and gain understanding of the gecko's adhesive system, the stress distribution over the toes of a live tokay gecko (Gekko gecko) was measured using a custom optical tactile sensor with 100 micrometer spatial resolution based on frustrated total internal reflection (FTIR). Additionally, the stress distribution in the synthetic microwedge adhesive is investigated with a theoretical model that describes the elastic deformation and adhesive interactions of adhesive microstructures. Adhesion is modeled using a cohesive zone model, where the normal and tangential forces generated along the side of the microwedge depend on the separation distance between the microwedge and the surface. Deformation is modeled using a geometrically exact beam model, where the microwedge is treated as a tapered beam undergoing bending, axial, and shear deformation. This modeling approach accurately reproduces the limit curve in force space of microwedge adhesive, describing the relationship between normal and shear force that gives rise to controllable adhesion. In both the tokay gecko toe and the synthetic adhesive, the stress distributions were found to be nonuniform. In the gecko, the normal stress varied significantly at the lamella scale, with compressive stresses observed in some areas even though the net stress over the toe was tensile. Likewise, the model predicts that the normal stress on an adhesive microwedge varies from tensile to compressive along the adhesive interface, with a net stress that is several times smaller than the maximum stress. If the stresses were distributed uniformly, both systems would be capable of supporting much larger loads (around 20 times larger for tokay gecko toes and 5 times larger for microwedges). The proposed model may be useful in evaluating new microwedge structures with modified geometry in order to design a structure that distributes stress more uniformly. Along with the capabilities of the new micromachining process, this could lead to the development of stronger controllable adhesives.