Enhanced Adhesion and Friction by Electrostatic Interactions of Double-Level Teflon Nanopillars

As dry adhesives resembling the gecko lizard's dry adhesion, double-level nanopillars of extremely high aspect-ratio (27.5-225) are fabricated from a rigid low-surface energy fluoropolymer (Teflon AF) by a replica-molding method. Upon infiltration of Teflon AF melt into anodic aluminium oxide nanopores as the mold, the polymer melt fingers over the pore walls. The fingerlike structure formed during infiltration, subsequently collapses after removal of the mold, developing a unique sheet-like nanostructure on top of the base nanopillars. Concurrent fabrication of the terminating nanostructure helps the fabrication of extremely high aspect-ratio nanopillars which, up to an aspect-ratio of 185, neither collapse at the tip nor bundle. In order to fabricate nanopillars of different topographical properties, the height of the nanopillars as well as the size and density of the terminating nanostructure are carefully controlled by adjusting the processing temperature. However, although gecko and other gecko-inspired adhesives mainly rely on van der Waals interactions for their performance, the adhesion of the fabricated dry adhesives for the most part arises from electrostatic interactions of the applied polymer. In other words, Teflon AF has an exceptional potential to develop electric charges at its surface upon contact with other materials via the so-called contact electrification phenomenon, leading to elevated electrostatic interactions at its surface upon contact. The unique electrostatically driven adhesion of Teflon AF nanopillars helps them to generate extremely large adhesion forces, even up to similar to 100 times larger than those attainable by van der Waals forces. The nanopillars of specific geometrical properties have achieved remarkable adhesion and friction strengths of similar to 1.6 N cm(-2) and 12 N cm(-2), respectively (up to similar to 1.6 and 1.2 times larger than those of a gecko toe pad).