Grain boundary dynamics under mechanical annealing in two-dimensional colloids.

Two-dimensional (2D) colloids embedded in a soap film are employed to study the structures and dynamics of grain boundaries (GBs). While a soap film is drawn vertically from a concentrated bulk suspension, the colloidal particles self-assemble to form polycrystalline structures with a typical grain size of about 100 lattice spaces. Studies of these grain boundaries reveal that the GB lines are faceted at "atomic" scale, leading to a constant energy per unit length along each line. The measured GB energy as a function of misfit angle is in agreement with Reed-Shockley's theoretical predictions. When an external mechanical vibration is applied to the bulk suspension, the grain boundaries are observed to migrate and grains to rotate under the stress field induced by the vibration. The total length of the grain boundaries is found to diminish logarithmically with time, with the rate constant being a function of the excitation strength. This phenomenon, called "mechanical annealing," provides a robust means to grow large 2D single-domain crystals.