There is a great need for understanding the relationship between the structure and chemistry of surfactants forming wormlike micelles, and their macroscopic flow properties. Available macroscopic Rheological Equations of State (REoS) are often inadequate to predict flow behaviour in complex geometries or even to describe the full set of rheological measurements. In this paper, we show how the link between surfactant structure and rheology can be explored through the use of mesoscopic particulate simulations. We describe the development of a realistic Brownian Dynamics model, where the persistence length of the micelle is the smallest length scale. Most of the parameters for the BD model are obtained from atomistic Molecular Dynamics simulations to establish the link to the chemistry of the surfactant. As a preliminary result, we calculate the shear viscosity of a solution of entangled wormlike micelles as a function of increasing shear rate. The results can be approximately mapped on an experimental flow curve for this particular system, using a mean field scaling relationship. Finally, as our motivation in this work is to understand the flow of wormlike micellar fluids in porous media, we give preliminary experimental results for the flow in an expansion-contraction capillary using micro Particle Image Velocimetry (μ-PIV). This demonstrates the complex response of these fluids in non-viscometric flows and the challenges they pose to any predictive simulation model.@en

We carry out a stability analysis of the Bautista-Manero (B-M) constitutive equations for extensional flow of wormlike micelles. We show that all solutions for the steady-state extensional viscosity ηE are unstable when the elongational rates ε exceed some critical value. In some cases the only real solution for the extensional viscosity is negative at intermediate values of the elongational rate. This critical elongational rate is not unfeasibly large, e.g., 250 s-1 for a typical EHAC solution. We note that the extension rates at which enhanced pressure drop is observed experimentally in porous media flow is generally well below the onset of the instability in the B-M model. However, the extension rates presented here are only average values for the porous media in question and at the pore scale a wide range of values will be encountered. For this reason, we require an improved rheological equation of state. We first separate the contributions to the viscosity of the solution and the wormlike micelles. Then, we remove the term containing the retardation time λJ. We now find that the extensional flow behaviour is well defined for both the transient and steady-state cases.@en