Structure and Dynamics of Colloidal Grain Boundaries

Abstract

Grain boundaries, or intercrystalline boundaries, are defects that affect the properties of most crystalline solids. The grain boundary energy, of which the orientation-dependence is captured by the interface stiffness, drives grain boundary motion, a key ingredient of the evolution of the microstructure of materials. It is difficult to study the dynamical structure of grain boundaries in situ since atoms are fast and small. The capillary fluctuation model has been used to calculate the interface stiffness of a small number of highly symmetrical grain boundaries. Since hard-sphere colloidal particles mimic atomic behavior, they give us the opportunity to visualize grain boundary dynamics in real time and space using confocal microscopy. In this dissertation, we analyze the structure and dynamics of three random, high-angle boundaries between three-dimensional colloidal crystals of different orientations. The colloidal crystals have a face-centered cubic structure with a few stacking faults and twin boundaries. The grain boundaries are identified as the particles at the end of each crystal, and a sharp transition is observed. We find that these boundaries undergo thermal fluctuations consistent with the capillary fluctuation model; no evidence for coupled parallel motion was found. The stiffness depends strongly on the in-plane direction, and we measure this anisotropy by determining, for the first time in a colloidal system, the stiffness tensor. The principal directions of these tensors are consistent with the crystallographic symmetries of the boundary planes. By determining an upper limit of the grain boundary energy from the excess volume at the interface, we demonstrate that the orientation dependence of the energy significantly contributes to the magnitude and variation of the stiffness.