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Rheo-optic and rheo-NMR studies of the flow behaviour of wormlike micelles solutions

Allan Raudsepp School of Chemical and Physical Sciences, Victoria University, Wellington, New Zealand

In the appropriate conditions surfactant molecules in solution may self-assemble into long semi-flexible polymer-like rods called wormlike micelles. Wormlike micelles solutions can show pronounced viscoelasticity and are sometime observed to flow in a peculiar way that suggests that the viscosity of the fluid varies discontinuously. This discontinuous banding has generated considerable interest because of its general implications for the flow behaviour of complex fluids. Despite years of study the underlying mechanism for this flow instability is still not well understood.
Here I report on our recent investigations into the flow behaviour of semi-dilute wormlike micelles solutions composed of the surfactant cetylpyridinium chloride (CPCl) and counterion sodium salicylate (NaSal) in brine ([CPCl]/[NaSal]=2, variable weight fraction, in [NaCl]=0.5 M brine) using rheo-optical technique based on homodyne photo-correlation spectroscopy (PCS), diffusing wave spectroscopy (DWS), ellipsometry and direct observation along with rheo-nuclear magnetic resonance (NMR) and conventional mechanical rheometry. Partitioning into stable high viscosity/low shear rate/high birefringence/low turbidity and low viscosity/high shear rate/low birefringence/high turbidity bands was observed in strongly shear-thinning samples in a cylindrical-Couette flow cell geometry using a combination of PCS and ellipsometry. Stable shear banding was also demonstrated in strongly shear thinning samples in the cylindrical-Couette geometry using DWS – DWS measurements suggested that unstable shear flow may be present in the parallel-plate flow geometry. Fluctuating turbid rings were observed directly in strongly shear thinning samples in the parallel-plate geometry – subsequent rheo-NMR measurements indicated that these rings were correlated with formation of a fluctuating high strain band (Fig. 1).

Fig 1. Sequence of transverse rheo-NMR velocity and shear rate images showing shear banding of a wormlike micelles solution in the parallel-plate geometry (r=10 mm, d=0.75 mm).