Liquid crystal drops in suspensions of colloidal plates

Abstract

This thesis deals with liquid crystalline phases and in particular liquid crystal drops in suspensions of colloidal gibbsite platelets. In Part I we deal with some general aspects of liquid crystalline phases in suspensions of gibbsite platelets, to provide a basic framework for the research described in this thesis. First of all, we report on the birefringence of liquid crystalline phases in suspensions of gibbsite platelets and show that for solvents with a refractive index close to that of gibbsite intrinsic birefringence dominates, whereas for solvents with a considerably different refractive index the form birefringence due to the anisometric particle shape prevails over the intrinsic birefringence. Secondly, we show that a bimodal distribution of the platelet aspect ratio leads to remarkable phase behaviour, including an isotropic-nematic density inversion and a three phase coexistence. We demonstrate experimentally and theoretically that both phenomena can be attributed to fractionation with respect to the platelet aspect ratio. Part II deals with static properties of nematic droplets, tactoids. We study the shape and director field of tactoids as a function of size and solvent and show that small tactoids have an oblate shape and a uniform director field, whereas larger tactoids are spherical with a radial director field. We also study the deformation of tactoids in an external magnetic field. A detailed analysis of the tactoid shapes and director fields as a function of size and magnetic field strength provides us with values for the splay elastic constant, the interfacial tension and the anchoring strength, the relevant material parameters in this field. In Part III we study coalescence and breakup of tactoids and nematic threads. We show that coalescence of small tactoids (with a uniform director field) with the isotropic-nematic interface proceeds along similar lines as the coalescence of isotropic droplets with the interface in isotropic fluids. Coalescence of large tactoids (with a radial director field) with the isotropic-nematic interface is remarkably similar to the transport of an immiscible drop through a liquid-liquid interface. The buoyancy driven coalescence of two tactoids with an asymmetric radial director field appears to proceed as in the case of isotropic fluids. We show that droplet snap-off and breakup of a nematic thread are strongly influenced by the liquid crystalline order present in the thread. In the case of weak anchoring the thinning neck has a uniform director field and the snap-off is determined by the viscous properties of the nematic phase and by thermal fluctuations. On the other hand, in the case of strong anchoring the thinning neck has an escaped radial director field and the snap-off is hampered for topological reasons. To conclude, in part IV we study the role of columnar droplets, both in the formation of the columnar phase and the devitrification of the glassy state. In both cases a key-role is played by stacks of platelets.