A design methodology for tunable adhesion and friction using curved and hierarchical structures
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Nature-inspired synthetic gecko adhesive systems show tremendous potential in developing robust, repeatable, residue free adhesive systems as an alternative in a wide range of applications from industrial and medical robotics to aerospace and household applications as these animals demonstrate highly efficient and repeatable adhesive that can stick to surfaces independent of their chemical composition and roughness with adhesive strengths as high as 200 kPa. These adhesive systems are usually comprised of densely packed fibers of high elastic modulus (1-4 GPa), are usually graded in either material as in the ladybird beetle or employ hierarchical structuring evolved from smooth pad as in the gecko, and are curved and tilted to enable strong attachment and easy detachment. While there has been tremendous progress in the design, manufacture, and performance of nature inspired adhesive over the last decade, these synthetic adhesives lag their natural counterparts in two major performance metrics: adhesion to rough surfaces and easy removal. These deficiencies are mainly due to mainly the monolithic construction of fibers as well as the inability to replicate and study the curved/bent nature of the fibers forming the adhesive arrays. In this dissertation, the effect of curvature and tilt, as well as adding material gradients on adhesion and friction of nature inspired synthetic fibers to rough and smooth surfaces is investigated. A cost-effective fabrication method to generate curved fibers with high degree of geometry control is demonstrated. Tilt and curvature together are found to affect friction as well as the coupling between adhesion and friction. These tilted and curved structures demonstrate dynamic friction as high as 164 kPa while only showing 25 kPa in adhesion. A method to create fibers that are graded in material composition is proposed. Using fibers that are softer at the tips, adhesion to rough surfaces with strengths up to 80 kPa is demonstrated. These fibers that are graded in elastic modulus show promise towards universal adhesive systems that can adhere to rough and smooth surfaces.