(494a) Characterizing the Rheology, Slip and Velocity Profiles of Lamellar Gel Networks

We report a thorough rheological characterization of a lamellar system consisting of a mixture of a cationic surfactant-behentrimonium methosulfate, a fatty alcohol mixture of cetyl and stearyl alcohol and water at surfactant/fatty alcohol mole fractions ranging from 0.1-0.6 and water mass fractions of 85% to 97%. Rheological measurements unveiled interesting signatures of these fluids such as its highly pronounced shear hysteresis behavior, memory of the highest shear subjected to it and a solid-like creep response with the existence of an apparent “yield-stress”.

Particle-image velocimetry showed that these systems develop fracture planes and shear bands at low shear rates of 0.001 s^-1 and evolve to a complete plug flow at higher rates starting from 0.01 s^-1. The tendency to slip even after the usage of serrated geometries confirmed that we cannot apply the traditional definition of “viscosity” to these systems. Further experiments with parallel plates at different gaps for fixed velocity and fixed shear rates independently showed the viscosity to be gap-dependent and exhibiting some characteristics of solid-like friction. Thus, this work sheds light on the rheo-tribological signatures of lamellar gels and its relevance on the rheological measurements which governs the formulation and processing of these commercial cosmetic emulsions.