Adhesion analysis of two-level hierarchical morphology in natural attachment systems for 'smart adhesion'

The attachment ability of insects and lizards is well known. The Tokay gecko, in particular, has the most complex adhesion structures. The pads are covered by a large number of small hairs (setae) that contain many branches per seta with spatulae. Seta branch morphology is hierarchical. Hierarchical morphology of setae is responsible for adaptation of a large number of spatulae to rough surfaces. Van der Waals attraction between the large numbers of spatulae in contact with a rough surface is the primary mechanism for high adhesion. In order to investigate the effect of hierarchical structure, for the first time, the two-level hierarchical model has been developed. We consider one- and two-level hierarchically structured spring models for simulation of setae contacting with random rough surfaces and demonstrate the effect of the two-level hierarchical structure on the adhesion force, the number of contacts and the adhesion energy. Tip of spatula in a single contact was assumed as spherical. Rough surfaces with various roughness parameters which cover a common range of most of natural and artificial rough surfaces at the scale of gecko's pad were generated. It was found that significant adhesion enhancements are created with the two-level structure until a certain value of roughness which appears to be related to the maximum spring deformation. We conclude that the hierarchical morphology of a gecko seta is the necessary part for 'smart adhesion' of gecko, the ability to cling on and detach from different smooth, as well as rough surfaces.