Temperature-induced flocculation of colloidal particles immersed into the isotropic phase of a nematic liquid crystal

The aim of our theoretical study is to investigate the stability of a liquid crystal colloidal dispersion, i.e., a suspension of rigid spherical particles in the isotropic phase of a nematic liquid crystal. We pay special attention to a temperature dependent liquid crystal mediated attraction between particles which originates in a surface-induced nematic order localized to the vicinity of particles. The attraction strongly increases as the temperature of the nematic-isotropic phase transition (TNI) is approached. In real systems this interaction is accompanied by electrostatic and steric repulsion and by the attractive van der Waals interaction. The major part of our work is dedicated to colloidal dispersions that are stabilized by a screened electrostatic repulsion of charged particles in the presence of counter ions in the solvent. We demonstrate that close to TNI a decrease of temperature for about 1 K can induce a sudden transition of the system from a dispersed to a flocculated state. This transition can only take place if the electrostatic repulsion is sufficiently weak. We present a ``phase diagram'' where we show how the temperature that separates a flocculated from a dispersed state depends on the surface charge density and Debye length.