Three-dimensional centimeter-sized colloidal silica crystals formed by addition of base

Three-dimensional (3D) centimeter-sized colloidal crystals can be spontaneously formed simply by dropping a NaOH solution (10 mM, approximately 10 microL) into an aqueous dispersion of dilute charged colloidal silica (particle diameter 110 nm, particle volume fraction phi = 0.023, 3-4 mL). The charge number of the silica particle increases with pH. Upon adding the NaOH solution, first, sub-millimeter-sized polycrystals are formed in the upper part of the sample due to charge-induced crystallization. The local phi value in the crystal region becomes nonuniform. The crystals with a high phi value accumulate at the bottom of the cell and then grow upward as columnar crystals. The crystal widths increase discontinuously with the growth, and in some cases, 3D centimeter-sized crystals are formed. The centimeter-sized crystals are also obtainable by the controlled diffusion of the base from its dilute reservoir. The present findings may prove valuable in the fabrication of large 3D single-crystalline photonic materials.