Rheology of defect networks in cholesteric liquid crystals

The rheological properties of cholesteric liquid crystals containing networks of defects are investigated. A network of linear defects of the "oily-streak" type is stabilized when colloidal particles are dispersed into the cholesteric liquid crystals. This network converts the rheological response of a presheared cholesteric liquid crystal from fluidlike to solidlike and leads to the formation of a "defect-mediated" solid. The frequency-dependent complex shear modulus G*(omega) is measured, for samples with and without inclusions, in both the linear and nonlinear viscoelastic regimes. The linear elastic response mediated by the defect network is discussed in terms of a model analogous to the theories of rubber elasticity. All our data for G*(omega) are fitted to a simplified theoretical form, and the values and variations of the fitting parameters, in the various regimes investigated, are discussed in terms of the properties of defect structure present in the samples. Similar rheological properties are expected to arise from particle-stabilized oily-streak defect networks in layered systems such as smectic-A and lyotropic L(alpha) phases.