Journal of Rheology, Vol.54, No.4, 881-913, 2010
Wormlike micellar solutions: II. Comparison between experimental data and scission model predictions
Although many constitutive models for wormlike micellar solutions have been proposed, few quantitative comparisons have been made with detailed rheological measurements. The majority of comparative studies focus on the linear viscoelastic properties of micellar solutions, which are well described by monoexponential Maxwell-like behavior. In the present work we compare the predictions of a prototypical two-species reptation-reaction model [developed in Part 1, Vasquez , "A network scission model for wormlike micellar solutions: I. Model formulation and viscometric flow predictions," J. Non-Newtonian Fluid Mech. 144(2-3), 122-139 (2007)] with rheological measurements performed using a concentrated cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) solution in a range of steady and transient shear flows. The model captures the continuous rupture and reformation of the long entangled chains that form a physically entangled viscoelastic network and the enhanced breakage rates that occur during imposed shearing deformations. In homogeneous shearing flows, the model describes numerous qualitative features of the linear and nonlinear rheologies, including a strongly strain-dependent damping function during large strains, agreement with the Lodge-Meissner rule at moderately large strains, large rate-dependent first normal stress coefficients in steady shear flow, and pronounced stress overshoots during start-up of steady shear. The present model cannot predict the second normal stress difference observed experimentally or the persistent agreement with the Lodge-Meissner rule observed experimentally at very large strains. Homogeneous flow calculations with this simplified two-species model cannot capture quantitatively the full range of transient dynamics observed experimentally. More complex time-dependent test protocols, including step-jumps (up and down) in deformation rate and applied stress, are used to reveal the slow temporal dynamics associated with evolution of the shear-banding plateau. Such experiments help to provide insight into additional features (such as diffusion coefficients for stress-microstructure coupling) that are required for fully quantitative rheological equations of state describing these concentrated wormlike micellar solutions. (C) 2010 The Society of Rheology. [DOI: 10.1122/1.3439729]
Keywords:damping;non-Newtonian flow;shear deformation;shear flow;stress effects;stress relaxation;surfactants