| Most geckos can rapidly attach and detach from most surfaces using the weak but universal van der Waals forces. This ability is attributed to the hierarchical structure of their feet (involving macroscale toe pads, arrays of 'seta' microfibers, and nanoscale spatula tips), and how these anisotropic structures are moved (articulated) to generate strong adhesion and friction forces on gripping that rapidly relax on releasing. Inspired by the gecko adhesive system, various structured surfaces have been fabricated suitable for robotic applications.;First, vast arrays of both vertical and tilted rectangular PDMS micro-flaps were fabricated using micro-electromechanical systems (MEMS) fabrication techniques. Friction and adhesion force properties were investigated using both a Surface Forces Apparatus (SFA) and a custom-built tester with larger contact area. Demonstrating the importance of both asymmetric tilted structures (such as the fibers in geckos) and an optimal articulation mechanism, it was found that the anisotropic structure of the tilted micro-flaps resulted in highly anisotropic adhesion and friction forces when articulated along different directions: high friction and adhesion when sheared along the tilt direction, and low friction and adhesion when sheared against tilt.;Then, the friction and adhesion properties of the tilted micro-flaps were characterized against five different rough glass substrates with non-uniformly spaced asperities. It was found that while adhesion and friction anisotropy was maintained on all substrates, the magnitudes showed an unexpected increase at intermediate height RMS roughness due to a geometric match between the dimensions of the flaps and the average spacing between asperities, leading to interlocking. This both demonstrates the importance of substrate surface structure as well as indicates how fibrillar adhesives could be tailored to achieve higher forces on specific surfaces.;Finally, the importance of both tilt angle and fiber shape was demonstrated with tilted half-cylinder PDMS microfiber arrays. With suitable articulation, the adhesion force was switched from a maximum of 9.4 kPa for strong attachment to a minimum of zero for easy detachment. Practical applications require adhesives to be highly durable in addition to being anisotropic - this material retained up to 77% of the initial adhesion after 10,000 test cycles. |
