Colloidal Crystals of Spheres and Cubes in Real and Reciprocal Space

Abstract

Colloidal suspensions, consisting of nano to micro-meter sized particles dispersed in a liquid, are ubiquitous in daily life. Examples are milk, blood and paint. One of the most remarkable phenomena exhibited by concentrated suspensions of colloidal particles is the spontaneous self-organization in structures with long-range spatial and/or orientational order, the so-called colloidal crystals. A well-known example of a colloidal crystal is the natural gemstone opal that consists of regular arrays of small spherical silica colloids. The periodic arrays of spheres diffract visible light, giving rise to the opal’s fascinating play of colors. The self-organization of colloidal particles is strongly influenced by their shape. A relatively small change from a sphere to a rounded cube already gives rise to new structures. This thesis is an in-depth study of the effect of colloidal particle shape, namely spheres and cubes, on the self-organization and the final crystal symmetries that can be achieved. The thesis research employs state-of-the-art X-ray diffraction and microscopy techniques for the detailed characterization of colloidal crystal structures prepared using various self-assembly techniques. Furthermore, the thesis shows that by making use of thermo-responsive particle systems, defect formation and diffusion can be studied in situ. The thesis work also reveals the subtle structural variations that arise by changing the particle shape from a sphere to that of a rounded cube. In particular, the roundness of the cube corners combined with the self-organization pathway, convective assembly or sedimentation, is shown to markedly affect final crystal symmetries. In addition, the influence of a magnetic core and the accompanying magnetic attractions between the cubes on the sedimentation behavior and crystal structures of the cubes is investigated, along with their directed assembly in an external magnetic field.