Wei Li Lee
Massachusetts Institute of Technology, USA
Title: Mechanically deformable and programmable nanoscale surface textures with tunable wetting and mechanical properties
Biography
Biography: Wei Li Lee
Abstract
Nanoscale surface textures, when optimally designed, present a unique surface engineering approach to improve surface functionalities. Coupling surface texture with shape programmable polymer may generate tunable surface properties. A shape memory polyetherurethane is used to prepare 250 nm-pillar arrays. The mechanical deformation via stretching and recovery of the pillar arrays are investigated as a function of aspect ratios. Scanning electron microscopy and atomic force microscopy analyses show the higher aspect-ratio (2.5:1) pillars exhibiting more deformation in height than low (l:1) aspect-ratio pillars under the same applied macroscopic strain. In the recovery study, the high contribution of surface energy impedes the complete recovery (~70%) of pillars. The nanopillar arrays are shown to perform switchable wetting function and surface mechanical properties (penetration resistance, modulus, buckling/bending) without changing materials or needing continuous external stress or energy inputs. For wetting, the 1:1 pillars exhibited decreasing water contact angles as the applied tensile strain (É›) increased from 0% to 200%, whereas the contact angles on the 2.5:1 pillars increased when changing É› from 0% to 100%, and began to decrease at high strain (É›=200%). In terms of surface mechanical behavours, the 2.5:1 pillars yielded a higher unloading modulus compared to the 1:1 pillars as a result of distinct buckling/bending mechanisms. In addition, the modulus could be further engineered by deforming the pillar structure and its arrangement. This study provides insights into how the surface functionalities can be tuned by manipulating geometrical designs of surface patterns and varying applied levels of stretching during shape programming.