Direct Current Electric Field Assembly of Colloidal Crystals Displaying Reversible Structural Color

We report the application of low-voltage direct current (dc) electric fields to self assemble close-packed colloidal crystals in nonaqueous solvents from colloidal spheres that vary in size from as large as 1.2 mu m to as small as 0.1 mu m. The assemblies are created rapidly (similar to 2 min) from an initially low volume fraction colloidal particle suspension using a simple capacitor-like electric field device that applies a steady dc electric voltage. Confocal microscopy is used to observe the ordering that is produced by the assembly method. This spatial evidence for ordering is consistent with the 6-fold diffraction patterns identified by light scattering. Red, green, and blue structural color is observed for the ordered assemblies of colloids with diameters of 0.50, 0.40, and 0.29 mu m, respectively, consistent with spectroscopic measurements of reflectance. The diffraction and spectrophotometry results were found to be consistent with the theoretical Bragg's scattering expected for closed packed crystals. By switching the dc electric field from on to off, we demonstrate reversibility of the structural color response on times scales similar to 60 s. The dc electric field assembly method therefore represents a simple method to produce reversible structural color in colloidal soft matter.