The Importance of Disclinations: Colloidal Dispersions in a Themotropic Nematic Solvent

Hard sphere particles in isotropic organic solvents form a liquid at volume fractions below 49%. We show that if hard spheres are mixed into an anisotropic organic solvent, a thermotropic nematic liquid crystal, complex gel structures form for all volume fractions studied, Ф > 0.5%. For a uniaxial nematic liquid crystal to accommodate spherical colloidal particles, defects in the local nematic order must occur. Both our experiments and simulations reveal that two different gel structures are formed with a transition at volume fraction of 10%. In the dilute limit each particle supports an independent disclination (defect line) wrapped around the particle, but for higher concentrations a network of disclinations is formed which entangles dispersed particles. We show through confocal microscopy and rheological measurements that the behaviour of the disclinations entirely determines the structure and rheology of the colloid-nematic composite.