Rheology of giant micelles

Abstract

Giant micelles are elongated, polymer-like objects created by the self-assembly of amphiphilic molecules (such as detergents) in solution. Giant micelles are typically flexible, and can become highly entangled even at modest concentrations. The resulting viscoelastic solutions show fascinating flow behaviour (rheology) which we address theoretically in this article at two levels. First, we summarise advances in understanding linear viscoelastic spectra and steady-state nonlinear flows, based on microscopic constitutive models that combine the physics of polymer entanglement with the reversible kinetics of self-assembly. Such models were first introduced two decades ago, and since then have been shown to explain robustly several distinctive features of the rheology in the strongly entangled regime, including extreme shear-thinning. We then turn to more complex rheological phenomena, particularly involving spatial heterogeneity, spontaneous oscillation, instability, and chaos. Recent understanding of these complex flows is based largely on grossly simplified models which capture in outline just a few pertinent microscopic features, such as coupling between stresses and other order parameters such as concentration. The role of 'structural memory' (the dependence of structural parameters such as the micellar length distribution on the flow history) in explaining these highly nonlinear phenomena is addressed. Structural memory also plays an intriguing role in the little-understood shear-thickening regime, which occurs in a concentration regime close to but below the onset of strong entanglement, and which is marked by a shear-induced transformation from an inviscid to a gelatinous state.

Rheo-PIV of a shear-banding wormlike micellar solution under large amplitude oscillatory shear

“…Wormlike micelles are a particularly interesting class of shear banding systems because they are widely used in consumer products, and they have become a canonical model system for probing shear banding. The bulk rheology of these particular systems has been studied extensively by other workers (Rehage and Hoffmann, 1991;Berret et al, 1994;Schmitt et al, 1994;Cates and Fielding, 2006;Lerouge and Berret, 2010). With recent developments in experimental techniques that can be used to directly measure deformation/velocity fields in these fluids (see the review by Manneville (2008)), there has been a growth of interest in connecting measurements of the bulk rheology and the local flow behavior in order to better understand the phenomenon of shear banding and the coupling between kinematic changes in the flow and the resulting stress-shear rate profile.…”

Interplay between elastic instabilities and shear-banding: three categories of Taylor–Couette flows and beyond

“…This is why they are sometimes called "living polymers". Stress relaxation is modified by this additional mechanism, in a way modelled efficiently by Cates' reptation-reaction model [5]. Strikingly, this added complexity at the mesoscopic scale leads to simpler macroscopic properties.…”

Novel experimentally observed phenomena in soft matter

“…When a constant shear rate is imposed in the metastable region of the flow curve, the sample splits up into bands that form layers along the direction of the flow gradient. This is called gradient banding and has been studied in detail in solutions of GWMs [87,90,96,97]. If a constant shear stress is applied, the sample breaks up into bands with layer normals in the vorticity direction.…”

“…The sample splits up into coexisting bands supporting different shear rates (or viscosities) and different microstructures [90][91][92]. This phenomenon is called 'shear banding'.…”

“…[89]. When aqueous solutions of GWMs are subjected to high shear rates, the wormlike chains disentangle, break, and eventually stretch in the direction of flow [90]. A spontaneous retraction accompanies this stretching and gives rise to a stress peak in the flow (stress-vs.-strain) curves of these samples.…”