Title: Healing of defects in granular crystals

Author (Talk): Francisco Lopez Jimenez, MIT

Abstract:

Crystalline materials rarely consist of a pristine single crystal; grains, or defects, often form with similar crystallographic properties but different orientations. Grain boundaries, the interface between two such grains, can become sites for the nucleation of fatigue and failure, as well as decrease the electrical and thermal conductivity of the material. Diffusion can lead to the ‘self-healing’ of these defects, whereby the grain can shrink and eventually disappears. We present the results from an experimental investigation of a macroscopic model system for a two-dimensional granular crystal. An ensemble of millimeter-size steel ball-bearings is first packed into a hexagonal lattice on a horizontal tray. A single circular grain defect is then created by rotating a central portion of the crystal, at a set angle with respect to the outer lattice, and the whole system is vibrated uniaxially by an electromagnetic shaker. Frictional interactions between the the ball-bearings and the surface of the tray set the particles into random motion, acting as the analogue of a thermal source. Digital imaging is used to quantify the structure and dynamics of the defect as a function of the principal parameters of the system, namely: the size of the grain, its orientation and the frequency and amplitude of the vibration. Focus is given to classifying and understanding the healing rate of the defects.