Strong Viscosity Increase in Aqueous Solutions of Cationic C22-Tailed Surfactant Wormlike Micelles

The viscoelastic properties and structure parameters have been investigated for aqueous solutions of wormlike micelles of cationic surfactant erucyl bis(hydroxyethyl) methylammonium chloride with long C22 tail in the presence inorganic salt KCl. The salt content has been varied to estimate linear to branched transition conditions due to screening of the electrostatic interaction in the networks. The local cylindrical structure and low electrostatic repulsion was obtained by SANS data. The drastic power law dependencies of rheological properties on surfactant concentrations were obtained at intermediate salt content. Two power law regions of viscosity dependence were detected in semi-dilute solutions related to “unbreakable” and “living” micellar chains. The fast contour length growth with surfactant concentration demonstrated that is in good agreement with theoretical predictions.

Modification of turbulence caused by cationic surfactant wormlike micellar structures in two-dimensional turbulent flow

An experimental study is performed to investigate the effects of the extensional rheological properties of drag-reducing wormlike micellar solutions on the vortex deformation and turbulence statistics in two-dimensional (2-D) turbulent flow. A self-standing 2-D turbulent flow was used as the experimental set-up, and the flow was observed through interference pattern monitoring and particle image velocimetry. Vortex shedding and turbulence statistics in the flow were affected by the formation of wormlike micelles and were enhanced by increasing the molar ratio of the counter-ion supplier to the surfactant, ξ, or by applying extensional stresses to the solution. In the 2-D turbulent flow, extensional and shear rates were applied to the fluids around a comb of equally spaced cylinders. This induced the formation of a structure made of wormlike micelles just behind the cylinder. The flow-induced structure influenced the velocity fields around the comb and the turbulence statistics. A characteristic increase in turbulent energy was observed, which decreased slowly downstream. The results implied that the characteristic modification of the 2-D turbulent flow of the drag-reducing surfactant solution was affected by the formation and slow relaxation of the flow-induced structure. The relaxation process of the flow-induced structure made of wormlike micelles was very different from that of the polymers.

Strong Viscosity Increase in Aqueous Solutions of Cationic C22-Tailed Surfactant Wormlike Micelles

The viscoelastic properties and structure parameters have been investigated for aqueous solutions of wormlike micelles of cationic surfactant erucyl bis(hydroxyethyl) methylammonium chloride with long C22 tail in the presence inorganic salt KCl. The salt content has been varied to estimate linear to branched transition conditions due to screening of the electrostatic interaction in the networks. The local cylindrical structure and low electrostatic repulsion was obtained by SANS data. The drastic power law dependencies of rheological properties on surfactant concentrations were obtained at intermediate salt content. Two power law regions were detected in semi-dilute solutions related to “unbreakable” and “living” micellar chains. The fast contour length growth with surfactant concentration was demonstrated that is in good agreement with theoretical predictions.

Dual Transient Networks of Polymer and Micellar Chains: Structure and Viscoelastic Synergy

Dual transient networks were prepared by mixing highly charged long wormlike micelles of surfactants with polysaccharide chains of hydroxypropyl guar above the entanglement concentration for each of the components. The wormlike micelles were composed of two oppositely charged surfactants potassium oleate and n-octyltrimethylammonium bromide with a large excess of anionic surfactant. The system is macroscopically homogeneous over a wide range of polymer and surfactant concentrations, which is attributed to a stabilizing effect of surfactants counterions that try to occupy as much volume as possible in order to gain in translational entropy. At the same time, by small-angle neutron scattering (SANS) combined with ultrasmall-angle neutron scattering (USANS), a microphase separation with the formation of polymer-rich and surfactant-rich domains was detected. Rheological studies in the linear viscoelastic regime revealed a synergistic 180-fold enhancement of viscosity and 65-fold increase of the longest relaxation time in comparison with the individual components. This effect was attributed to the local increase in concentration of both components trying to avoid contact with each other, which makes the micelles longer and increases the number of intermicellar and interpolymer entanglements. The enhanced rheological properties of this novel system based on industrially important polymer hold great potential for applications in personal care products, oil recovery and many other fields.

A Rheo-Optic Study of Wormlike Micelles Solutions

<p>Shear banding, where a fluid spatially partitions into strain rate or shear bands in steadystate simple shear flow conditions, was first observed in wormlike micelles solutions and has since been observed in many other complex fluids. These solutions have been used extensively to explore the relationship between shear (or stress) banding and microstructure in complex fluids. This relationship is difficult to study because of its dynamic nature and there is still no clear consensus as to how banding relates to microstructural changes in wormlike micelles solutions. In this thesis, the rheology of a number of wormlike micelles solutions is examined using both conventional and novel techniques with the view to developing a better understanding of this relationship. The rheology of three wormlike micelles solutions composed of a surfactant cetylpyridinium chloride (CPCl) and counterion sodium salicylate in water with or without the salt sodium chloride were examined using mechanical rheometry and the rheo-optical techniques: homodyne photo-correlation spectroscopy (PCS), diffusing wave spectroscopy (DWS) and ellipsometry. Rheo-mechanical measurements were largely consistent with the predictions of the reptation-reaction model. While signi cant stress fluctuations were noted in one particular flow geometry, they were generally not observed in most rheomechanical measurements presented here, indicating that these fluctuations are not universal and that they are geometry dependent. Shear induced turbidity was directly observed in the cone-plate and parallel-plate geometries with turbid rings forming in samples that showed a stress plateau. The Poisson-renewal model, which extends the reptationreaction model to include the influence of high frequency modes on the linear rheology, was tested experimentally using mechanical rheometry, DWS microrheology and literature data. In most cases the data fitted the model behaviour quite well, giving a physically reasonable estimate of the average length of the micelles. DWS's spatial sensitivity to shear induced relative motion was then used to probe the flow behaviour of selected wormlike micelles solutions in the cylindrical-Couette, cone-plate and parallel-plate geometries. In the cylindrical-Couette, the 'flow-DWS' measurements were largely consistent with rheo-mechanical measurements and indicated that some wormlike micelles solutions were partitioning into apparently stable high and low strain rate bands in the vicinity of the stress plateau. While measurements in the cone-plate and parallel-plate geometries also suggested shear banding in samples that showed a stress plateau, the interpretation was less clear-cut. Homodyne PCS was combined with ellipsometry to examine the spatial relationship between strain rate and birefringence banding in selected wormlike micelles solutions in a cylindrical-Couette geometry. In contrast to the observations of previous workers, it was found here that the birefringence and strain rate bands did coincide. Furthermore, the high strain rate band was observed to be more turbid than the lower strain rate band suggesting a connection between strain rate, optical anisotropy and turbidity.</p>

A Rheo-Optic Study of Wormlike Micelles Solutions

<p>Shear banding, where a fluid spatially partitions into strain rate or shear bands in steadystate simple shear flow conditions, was first observed in wormlike micelles solutions and has since been observed in many other complex fluids. These solutions have been used extensively to explore the relationship between shear (or stress) banding and microstructure in complex fluids. This relationship is difficult to study because of its dynamic nature and there is still no clear consensus as to how banding relates to microstructural changes in wormlike micelles solutions. In this thesis, the rheology of a number of wormlike micelles solutions is examined using both conventional and novel techniques with the view to developing a better understanding of this relationship. The rheology of three wormlike micelles solutions composed of a surfactant cetylpyridinium chloride (CPCl) and counterion sodium salicylate in water with or without the salt sodium chloride were examined using mechanical rheometry and the rheo-optical techniques: homodyne photo-correlation spectroscopy (PCS), diffusing wave spectroscopy (DWS) and ellipsometry. Rheo-mechanical measurements were largely consistent with the predictions of the reptation-reaction model. While signi cant stress fluctuations were noted in one particular flow geometry, they were generally not observed in most rheomechanical measurements presented here, indicating that these fluctuations are not universal and that they are geometry dependent. Shear induced turbidity was directly observed in the cone-plate and parallel-plate geometries with turbid rings forming in samples that showed a stress plateau. The Poisson-renewal model, which extends the reptationreaction model to include the influence of high frequency modes on the linear rheology, was tested experimentally using mechanical rheometry, DWS microrheology and literature data. In most cases the data fitted the model behaviour quite well, giving a physically reasonable estimate of the average length of the micelles. DWS's spatial sensitivity to shear induced relative motion was then used to probe the flow behaviour of selected wormlike micelles solutions in the cylindrical-Couette, cone-plate and parallel-plate geometries. In the cylindrical-Couette, the 'flow-DWS' measurements were largely consistent with rheo-mechanical measurements and indicated that some wormlike micelles solutions were partitioning into apparently stable high and low strain rate bands in the vicinity of the stress plateau. While measurements in the cone-plate and parallel-plate geometries also suggested shear banding in samples that showed a stress plateau, the interpretation was less clear-cut. Homodyne PCS was combined with ellipsometry to examine the spatial relationship between strain rate and birefringence banding in selected wormlike micelles solutions in a cylindrical-Couette geometry. In contrast to the observations of previous workers, it was found here that the birefringence and strain rate bands did coincide. Furthermore, the high strain rate band was observed to be more turbid than the lower strain rate band suggesting a connection between strain rate, optical anisotropy and turbidity.</p>