Layering transitions in colloidal crystals as observed by diffraction and direct-lattice imaging

Abstract

Layering transitions of colloidal crystals confined between two smooth glass surfaces have been studied by transmission diffraction of light as well as direct-lattice imaging with an optical microscope. The polystyrene spheres we studied have diameter Φ=0.305 μm and a surface charge of ∼${10}^4$ electronic charges, and form three-dimensional (3D) colloidal crystals in completely deionized water at a volume concentration above ≊0.3%. A 3D suspension of these spheres in completely deionized water forms both body-centered-cubic (bcc) and face-centered-cubic (fcc) crystalline structures as a function of colloid density n≡(1/$a_s$ ${)}^3$, with lattice constants ranging between 0.7 and 1.5 μm. When the colloid is confined between glass plates separated by distances D∼0.5–1 μm, the spheres form a single-layer 2D fluid. For D near 1 μm, a transition occurs to a single-layer 2D hexagonal crystal. As D increases, the evolution from two- to three-dimensional crystals is observed as a series of structural transitions distinguished by the number of crystal planes between the plates and by the preferred crystal symmetry parallel to the glass boundaries. We present here a study of colloidal crystals confined in a wedge cell that allows diffraction and imaging from the same crystallite. We present diffraction and imaging measurements of structural phases of one- through seven-layer colloidal crystals confined between two smooth glass surfaces, for a range of densities such that 2$a_s$/Φ$a_s$/Φ