Effect of particulate solids on the rheology of a lyotropic gel medium

Abstract

Experimental studies of the rheology of concentrated suspensions of particulate solids (marble) in a lyotropic gel phase (potassium palmitate with 40% water) were carried out. Such gels are viscoplastic and are characterized by strong colloidal interactions. Suspensions with lyotropic gels as a continuous phase serve as models for systems such as household detergent products, and processed foods. For comparison, experiments were also carried out on suspensions of the same solids in a viscous Newtonian medium (silicone oil). The steady shear experimental data for both the systems were fitted by the Bingham model. The entrance pressure drop for capillary flow of the gel-marble suspensions was interpreted in terms of an entrance yield stress. The latter when extrapolated to zero extrusion speeds was found to be nearly independent of die geometry, and was found to increase exponentially with solids volume fraction. The Bingham yield stress showed a similar behavior with volume fraction though its magnitude was found to be one to two orders smaller. For fixed volume fractions and shear rate, the apparent relative viscosity was found to be significantly larger for suspensions with the gel medium as compared to the Newtonian medium. The increase in the storage modulus of the gel suspensions with solids concentration showed a fundamentally different trend as compared to the Newtonian medium, increasing by two orders of magnitude on the addition of just 5% (v/v) of the particulate solids. Such behavior is typical of systems described by isostrain constitutive models, and might be a result of the formation of a sample-spanning structure of particles bridged by the gel.