(523f) Colloidal Crystal Annealing By Rotating Electric Fields

Defective structures such as grain boundaries, dislocations, and vacancies are common in colloidal crystals assembled with the assistance of external fields. High quality colloidal crystals are desirable because of improved mechanical strength and optical reflectivity; using annealing processes can improve crystal quality and therefore contribute to these properties. However, typical annealing strategies based on the application of uniform fields do not complete resolve defect structure. Here, we explore an annealing strategy that creates a defect-free crystal by application of a coplanar rotating alternating current electric field with six-fold symmetry to the self-assembled colloidal crystal. The AC fields was applied and rotationally cycled at constant period through the angles 0°, 60°, and 120°. We analyzed confocal microscopy images of the evolving crystal as a function of the cycle frequency. We measured the hexatic order parameter, crystallite cluster number, and Voronoi diagram to determine the evolution of the microstructure as a function of time. The cyclic fields generated fluid flow and colloidal crystal deformation. The structural deformation led to local particle rearrangements that progressively generated global annealing. By means of this analysis, we report the optimal cycling period to produce the best crystal quality. This annealing approach significantly improves the defect removing efficiency and ultimately generates a defect-free crystal without closed loop control system. We use molecular dynamics simulation to model the assembly and annealing behaviors. This research demonstrates the feasibility of synthesizing near-perfect colloidal crystals by deploying rotating electric fields.