Enhanced Compliant Adhesive Design and Fabrication with Dual-Level Hierarchical Structure

Synthetic dry adhesives inspired by the nano- and micro-scale hairs found on the feet of geckos and some spiders have been developed for almost a decade. Elastomeric single level micro-scale mushroom shaped fibres are currently able to function even better than natural dry adhesives on smooth surfaces under normal loading. However, the adhesion of these single level synthetic dry adhesives on rough surfaces is still not optimal because of the reduced contact surface area. In nature, contact area is maximized by hierarchically structuring different scales of fibres capable of conforming surface roughness. In this paper, we adapt the nature's solution and propose a novel dual-level hierarchical adhesive design using Polydimethylsiloxane (PDMS), which is tested under peel loading at different orientations. A negative macro-scale mold is manufactured by using a laser cutter to define holes in a Poly(methyl methacrylate) (PMMA) plate. After casting PDMS macro-scale fibres by using the obtained PMMA mold, a previously prepared micro-fibre adhesive is bonded to the macro-scale fibre substrate. Once the bonding polymer is cured, the micro-fibre adhesive is cut to form macro scale mushroom caps. Each macro-fibre of the resulting hierarchical adhesive is able to conform to loads applied in different directions. The dual-level structure enhances the peel strength on smooth surfaces compared to a single-level dry adhesive, but also weakens the shear strength of the adhesive for a given area in contact. The adhesive appears to be very performance sensitive to the specific size of the fibre tips, and experiments indicate that designing hierarchical structures is not as simple as placing multiple scales of fibres on top of one another, but can require significant design optimization to enhance the contact mechanics and adhesion strength.