Liquid crystal phase behaviour of colloidal platelets in external fields

Abstract

In this thesis, the liquid crystal phase behaviour of colloidal platelets in external fields is studied. We have specifically investigated the influence of morphological, gravitational, magnetic and centrifugal fields. Part I of this thesis involves sterically stabilised colloidal gibbsite platelets. In Chapter 2, we make use of a morphological field (the sample walls) to create large oriented domains of the columnar liquid crystalline phase. These domains were studied using small-angle X-ray scattering (SAXS), providing unambiguous evidence for the hexagonal nature of the columnar phase, which had been lacking hitherto. Chapter 3 focuses on the effect of the earth’s gravitational field. An initially isotropic-nematic sample develops a third, columnar phase at the bottom on prolonged standing. This is quite well described using a simple osmotic compression model that we developed. We performed Monte Carlo simulations of cut spheres and took data from the literature to supply the equations of state required for the model. In Part II, we subject the sterically stabilised colloidal gibbsite platelets to external magnetic fields. An important observation is the significant magnetic-field-induced orientational order in the isotropic phase, which is studied in Chapters 4 and 5. In Chapter 4, we use SAXS to measure the field-induced order and derive a simple model that relates the scattered intensity of the isotropic phase to the orientational distribution function. Despite its approximations, the model describes the observed SAXS patterns reasonably well and yields a lower bound of the diamagnetic susceptibility anisotropy. In Chapter 5, we measure this value using birefringence measurements of the field-induced order and find reasonable agreement. Chapter 6 focuses on the effect of the magnetic field on the isotropic-nematic phase transition. We find a shift of the transition concentrations to lower values, which is analysed using a Clausius-Clapeyron type approach to the problem. In Chapter 7, we study the Frederiks transition in our nematic phase, i.e., the competition between the wall- and magnetic-field-induced order. From the uniform Frederiks transition, we determine the bend elastic constant of the nematic phase. We use a rotating magnetic field to induce a non-uniform bend-splay transition, involving transient periodic patterns. These patterns are analysed and we find qualitative agreement with theoretical predictions. The last part of this thesis, Part III, is devoted to aqueous suspensions of gibbsite platelets. In Chapter 8, we describe the synthesis of such dispersions and study their phase behaviour. For the first time since Langmuir in 1938, we find isotropic-nematic phase separation in a dispersion of charged plate-like particles. The application of a gravitational field on the suspension results in a three-phase (isotropic, nematic and columnar) equilibrium, well described by the same osmotic compression model used in Chapter 3. In Chapter 9, a centrifugal field of 900 G is applied to the aqueous gibbsite suspension. Surprisingly, the columnar phase is still observed under such extreme conditions. The order of the colour of the Bragg reflections of the columnar phase suggests size fractionation in the sediment, which is confirmed by SAXS measurements and electron microscopy.