Locking effects in plowing-induced nanorippling of polystyrene surfaces

Abstract

We have investigated the influence of the geometric shape bounding the scan pattern on the orientation of the nanoripples evolved from a polystyrene surface scraped by a silicon tip. Atomic force microscopy measurements at ambient conditions are compared with numeric simulations based on a continuum model describing the time evolution of a generic compliant surface, which is indented viscoplastically and sheared elastically at the location occupied by the scanning tip. The tendency of the ripples to align at a steady angle defined by the scan pattern can be significantly modified by the geometric confinement. The analysis of surface ripples resulting from square, circle, star, pentagon, hexagon, and heart-shaped areas allows us to recognize the distinct influence of straight and curved edges, and of sharp corners, on the nanostructuring process. Straight edges inclined in the direction defined by the locking angle result in a uniform ripple pattern covering the entire scanned region originated from them. These results pave the way to predict plowing-induced surface structures by computer simulations and define the most suitable scan path required to obtain a desired ripple pattern. Potential applications include the controlled fabrication of nanostructures for optical devices, antibacterial surfaces, microfluidic channeling, and flexible electronics.