Crystals and grain boundaries

Materials composed of a single crystal are uncommon in nature: many materials are polycrystalline, consisting of individual crystallites packed together. A grain boundary is a planar defect that occurs where two such crystallites meet: the same crystal structure and chemical composition exists on each side but the orientation differs. Grain boundaries are central to our understanding of deformation and fracture mechanisms, melting kinetics, and transport properties in a wide range of materials. Some examples include: ductility, brittleness and electrical conductivity. The misorientation affects the properties of the boundary such as its surface tension, mobility and stiffness and is therefore of great importance in the study of grain boundaries.

In our group, we study various aspect of grain boundaries, such as their migration dynamics in terms of particle motion and dislocation dynamics. We employ optical tweezers to generate well-defined grain boundary loops in 2D colloidal crystals and to actively deform grain boundaries in optical tweezers by pulling on embedded impurity probe particles. In addition, we study the structure and dynamics of grain boundaries in 3D colloidal crystals.

Pulling a probe particle through a grain boundary in a colloidal crystal using an optical tweezer.
A grain boundary loop generated using an optical vortex. The inner crystal (green) has a different orientation to the outer crystal (red), and will slowly shrink over time.