Adhesion and friction of an isolated gecko setal array: The effects of substrates and relative humidity

Geckos can climb steadily and quickly on different surfaces under various environmental conditions. Extensive experiments have been conducted using gecko toe, setal array, single seta and single spatula to explore the physical principles of gecko adhesion and friction. This work focuses on the effect of environment (i.e. substrate materials and relative humidity [RH]) and experimental conditions (i.e. preload, sliding velocity, and sliding direction) on gecko adhesion and friction through an isolated gecko setal array. Experimental results show that when sliding a setal array in gripping direction, adhesion and friction can be enhanced by more than 200% by increasing surface energy, 60% by increasing relative humidity, and only slightly by increasing sliding velocity; an appropriate preload increase can also cause an enhancement. When RH >40%, adhesion and friction become saturated, and higher sliding velocity and preload can realize the saturation at a lower RH. When a preload is oversized in gripping direction or when sliding a setal array against substrates in releasing direction, the setal array deforms severely and always exhibits a preload-dominated repulsion. A strong anisotropic property is revealed when sliding a setal array in gripping and releasing direction. These results help understand the remarkable abilities of gecko swiftly controlling strong attachment and easy detachment, which inspire the design and fabrication of gecko-inspired reversible dry adhesives