Dynamic Adhesion and Self-cleaning Mechanisms of Gecko Setae and Spatulae

Geckos can freely climb on walls and ceilings against their body weight at speed of over 1ms-1. Switching between attachment and detachment seem simple and easy for geckos, without considering the surface to be dry or wet, smooth or rough, dirty or clean. In addition, gecko can shed dirt particles during use, keeping the adhesive pads clean. Mimicking this biological system can lead to a new class of dry adhesives for various applications. However, gecko’s unique dry self-cleaning mechanism remains unknown, which impedes the development of self-cleaning dry adhesives. In this dissertation we provide new evidence and self-cleaning mechanism to explain how gecko shed particles and keep its sticky feet clean. First we studied the dynamic enhancement observed between micro-sized particles and substrate under dry and wet conditions. The adhesion force of soft (polystyrene) and hard (SiO2 and Al2O3) micro-particles on soft (polystyrene) and hard (fused silica and sapphire) substrates was measured using an atomic force microscope (AFM) with retraction (z-piezo) speed ranging over 4 orders of magnitude. The adhesion is strongly enhanced by the dynamic effect. When the retraction speeds varies from 0.02 µm/s to 156 µm/s, the adhesion force increases by 10% ~ 50% in dry nitrogen while it increases by 15%~70% in humid air. A dynamic model was developed to explain this dynamic effect, which agrees well with the experimental results. Similar dynamic enhancement was also observed in aqueous solution. The influence of dynamic factors related to the adhesion enhancement, such as particle inertia, viscoelastic deformations and crack propagation, was discussed to understand the dynamic enhancement mechanisms. Although particles show dynamic enhancement, Gecko fabrillar hair shows a totally different trend. The pull off forces of a single gecko seta and spatula was tested by AFM under different pull-off velocities. The result shows that both the spatula and the seta have a rate independent adhesion response in normal retraction, which is quite different from micro-particles. Further research indicated the shape of the contact area was a key factor to the dynamic effect. In order to verify this hypothesis, artificial gecko spatula made of glass fibers was nanofabricated by a focus ion beam (FIB) and tested by AFM. These manmade spatulae also show a rate independent adhesion response. The dynamic adhesion of a single gecko seta and spatula were simulated with finite element analysis and the results also confirm the rate independent phenomena.. In conclusion, self-cleaning is induced by dynamic effect during gecko locomotion. The relative dynamic adhesion change between particles and seta makes it possible for gecko to shed the dirt particles while walking.Finally, the fatigue property of gecko seta was examined with the atomic force microscope under cyclic attachment/detachment process, mimicking gecko running. The adhesion force versus cycles has been tested and evaluated. Fatigue mechanism of gecko seta was also analyzed based on the experimental findings.