High-aspect-ratio polymer pillars have been of great interest in wetting and dry adhesion due to their unique surface topography. Specifically, tunable dry adhesion as manifested in gecko foot hairs has been attributed to millions of hierarchical fibrillar structures on gecko toe pads, allowing for adhesive attachment and rapid detachment. So far, most gecko-like adhesives are fabricated from polymer pillars with cylindrical cross-section with relatively low adhesive strength (10-30 N/cm2), which often is not tunable. Here, we report on the preparation of tapered nanorods from epoxy and temperature-responsive shape memory polymers (SMPs) with various cross-sections and study the shape effect on adhesion strength. In order to realize those various tapered nanostructures, we fabricated anodized aluminum oxide templates (AAO) with tapered pores by a combination of hard- and mild anodization, and pore etching process. The pore depth (a few tens of nanometers to a few micrometer) and diameters (80-300 nm) of AAO templates were precisely controlled, and cone, pencil, and stepwise structures were realized. We then created various tapered nanorods from epoxy and SMPs. The adhesion force was measured by nanoindentation. It showed that depending on the shapes of the nanorods, the adhesion force gradually increased up to 48 N/cm2 with the indentation depth for triangular shaped nanorods, but decreased for stepwise or pencil shaped nanorods to 24 N/cm2 after indentation depth reached 150 nm. To understand the deformation mechanism of the tapered nanorods and the resulting adhesion behaviors, we performed in-situ indentation experiments in SEM. We confirmed that different nanorods deformed differently depending on the load applied in the normal direction. The results from in-situ imaging corroborated well with finite element analysis and nanoindentation measurements, suggesting that the shape of the nanorods played a key role in mechanical response, thus, dry adhesion strength. Furthermore, we prepared SMP tapered nanrods, demonstrating tunable and strong adhesion via an interlocking mechanism.

Enhanced dry adhesion from tapered nanorods and their shape effect

249th National Meeting (2015)

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