Colloidal Mineral Liquid Crystals. Formation & Manipulation

Abstract

The central topic of this thesis is the formation, manipulation and characterization of colloidal mineral liquid crystals. Liquid crystals are liquids containing ordered anisometric particles. A range of liquid crystalline phases exists, from solely orientationally ordered nematic phases to orientationally and 1D or 2D positionally ordered smectic and columnar phases. Liquid crystals have several applications but are mostly known for their application in LCD’s: Liquid Crystal Displays, which contain a nematic liquid crystal. Colloidal liquid crystals generally form spontaneously. For potential applications however, it is important to be able to controllably form and tune the desired liquid crystal phase. The key feature of the research described in this thesis is therefore how the formation of these liquid crystals can be manipulated. This thesis is split in two parts. Part I describes the mineral goethite, which consists of board-like colloids. Various methods to influence the liquid crystal phase behaviour are described. These include the earth gravitational field, a depletant and an external magnetic field. It is described how letting the colloids slowly sediment in the earth gravitational field solves the particle polydispersity problem. The nematic phase for example, was found to absorb particles that do not fit in the smectic phase. Additionally, observations of fractionated crystallization in both the smectic and the columnar phase are discussed. Another approach to influence the phase behaviour is by changing the inter-particle interactions by introducing a spherical depletant into the suspension. This thesis describes how the depletant-induced attraction between the particles promotes the formation of the rare biaxial (in which particles are orientationally ordered in three dimensions) smectic phase. The final method studied is an external magnetic field. We describe how the magnetic field can induce phase transitions between the three different nematic phases (biaxial, prolate and oblate uniaxial), depending on the particle aspect ratios. Additionally it is described how a magnetic field was used to reorient the goethite columnar phase and to study its reorientation pathway with synchrotron small angle X-ray scattering, revealing nanoshear between layers of particles. In Part II of this thesis the mineral gibbsite, consisting of plate-like colloids, is described. Although liquid crystals are generally studied with scattering techniques, in Part II we focus on direct space imaging of the liquid crystals with confocal laser scanning microscopy. This technique enables characterization of the liquid crystals in real-space and time at the particle level. This requires large and fluorescent particles. Different general synthesis methods to prepare such platelets are described. Additionally, results of a preliminary confocal study are discussed, which reveals the observation of single particles as well as their liquid crystalline phase. Since liquid crystals of larger particles are more challenging for X-ray scattering studies, a 3D X-ray scattering method is described which helps make unambiguous identification of the liquid crystal phase.