2006

Abstract:

In bulk crystalline solids, plastic deformation due to shear usually leads to the formation and multiplication of defects which decreases the degree of long range order. By contrast, shear has recently been found to remove defects and induce order in several highly confined soft materials. An example that sparked great interest are experiments with sphere-forming block copolymer (PS-PEP), which self-assembles into nanometer-sized spheres or micelles. In a thin film, the spheres form a polycrystalline material composed of two-dimensional grains with local hexagonal symmetry but random orientation. Remarkably, the application of simple or oscillatory shear produced a nearly-defect free triangular lattice when the film thickness was exactly two or three layers. In this talk, we will use Brownian dynamics simulations to identify the microscopic mechanism responsible for this ordering phenomenon. We show that the global alignment process can be explained in terms of a local grain boundary migration mechanism. Based on the simulations, we construct both microscopic and continuum models that predict the boundary migration velocity as a function of local structure and shear stress. In the continuum picure, shear induced potential energy gradients act as the driving force for migration, and the grain boundary mobility is sensitive to the degree of mismatch between the grains.