Self-assembly of colloidal spheres and rods in external fields

Abstract

Colloidal particles are applied throughout industry, for example in paints, food, personal care products, ceramics and pharmaceutics. The characterization of the structure and dynamics of colloidal suspensions is therefore important for many industrial applications. Besides their industrial significance, colloids can also be used as a valuable model system to study fundamental questions in condensed matter physics. Because of their size, colloids are much slower than atoms or simple molecules, yet they can display the same equilibrium phase behavior. Furthermore, colloids are easily manipulated with external fields and they are large enough to be observed in real-space with an optical microscope. Phenomena such as crystallization, the glass transition and flow induced behavior of spherical colloids have been extensively studied in 3D real-space over the last decades. However, despite the recent increase in synthesis methods that produce (shape) anisotropic colloids, quantitative 3D real-space studies of anisotropic colloids strongly lag behind. In this thesis, we investigated the self-assembly of not only colloidal spheres but also of suspensions of (shape anisotropic) colloidal rods. The colloidal spheres consisted of poly(methyl methacrylate) (PMMA) and the rod-like particles consisted of silica and both particles were fluorescently labeled. In analogy with the spherical particles, we developed a new method that allowed us to also study the rod-like particles in 3D real-space on the single particle level. We manipulated and directed the self-assembly process by application of three external fields: gravity, shear, and electric fields. By applying shear-flow, we found regimes where we can induce long-rang crystals in the case for spheres or long-range order in the case of the rods. Furthermore, we measured for the first time the equilibrium sedimentation profile of a rod-like particle system in 3D real-space and found good agreement with theory and computer simulations on the liquid-crystalline phase behaviour of hard spherocylinders. By changing the interaction potential of the rod-like particles to long-range repulsive, we demonstrated one of the first instances of a 3D colloidal plastic crystal. When we increased the density of the system of long-ranged repulsive rods, particles failed to crystallize and a 'plastic glass' was found, which could be reversibly switched with an electric field. This work thus highlights the unprecedented possibilities to study the effect of rotations on the structure and dynamics of liquid-crystal phases, plastic-crystal phases and the glass transition on the single particle level.