Complex flows of viscoelastic wormlike micelle solutions

Abstract

Micelles are formed by the self-assembly of surfactants in solutions. Wormlike micelles or living polymers are a particularly interesting fluids because their long flexible cylindrical geometry can lead to entanglement even at relatively low concentrations. The rheological response of the wormlike micelles is similar in many ways to polymer solutions. Entangled wormlike micelles provide a model fluid system in many respects. In semi-dilute solutions, wormlike micelles show remarkably simple rheological behaviour; their linear rheology is best described by a single-element Maxwell model with a single relaxation time. The non-linear rheological and flow response of wormlike micelles has proven to be incredibly rich and complex. Wormlike micelles are easy to prepare and not susceptible to aging or shear mechanical degradation. Beyond semi-dilute concentrations, nematic and/or hexagonal phases can be observed, allowing an additional level of complexity to be dialled in by increasing concentration. In strong, steady-shear flows, some wormlike micelles have been shown to shear band. In uniaxial extensional flows, wormlike micelle solutions demonstrate enormous strain hardening of their extensional viscosity and under large extensional stresses, can break apart. This failure and the resulting morphological changes of the micelles in the flow have been linked to a number of interesting elastic instabilities. These rheological properties have led to the broad use of wormlike micelle solutions in consumer products as rheological modifiers. In this review, we will discuss a wide range of complex flows of wormlike micelle solutions with both shear and extensional flow components. We will discuss how flow geometry, flow strength, micelle concentration and micelle structure affect these complex flows with a particular emphasis on flow stability.