Adhesion hysteresis of Janus nanopillars fabricated by nanomolding and oblique metal deposition

Abstract

We present a simple method to fabricate Janus-faced nanopillars showing the direction-sensitive behavior as a dry adhesive. Polymeric nanopillars were first molded from an etched SiO2 substrate containing high aspect ratio nanoholes, followed by the oblique metal deposition on polymer nanopillars such that metal layers in different thicknesses were selectively deposited only on one side of the nanopillars. We found that the Janus nanopillars show the asymmetric adhesion behavior: it exhibits the strong shear attachment when pulled from a polymer contact (∼20 N/cm2) compared with the relatively weak adhesion when pulled from a metal layer in contact with a glass substrate (∼10 N/cm2). The difference in the shear adhesion strength is believed to originate from different interfacial adhesions between different sides of the Janus nanopillars and a target substrate. In addition, the Janus nanopillars could be further allowed to bend by thermal annealing or E-beam irradiation. The bending mechanism for nanopillars bending toward the metal layer by thermal annealing can be explained by the mismatch in thermal expansion coefficients between metal and polymer, followed by the plastic deformation of polymeric pillars. What is strikingly interesting is the fact that these bent Janus nanopillars show even greater adhesion hysteresis: the strong shear attachment when pulled from the bent direction (∼31 N/cm2) in contrast to the easy detachment from the opposite direction (∼4.1 N/cm2) just like gecko-like adhesives. We also demonstrate that this type of relatively simple bending process to produce Janus-faced nanopillars has a distinct advantage for large-area fabrication.