Shear-thickening transition in surfactant solutions: New experimental features from rheology and flow birefringence

We report on the shear-thickening transition observed in dilute aqueous solutions of cetyltrimethylammonium tosylate (CTAT) at concentrations $\phi\sim 0.2\%$. We have re-examined the kinetics of the shear-thickening transition using start-up experiments at rates above the critical shear rate $\dot{\gamma}_\mathrm{c}$. Using simple well-defined protocols, we have found that the transient mechanical response depends dramatically on the thermal and on the shear histories. Using the same protocols, flow birefringence experiments were carried out. The gap of a Couette cell containing the sheared solution has been visualized between crossed polarizers in steady shear conditions, as well as in start-up experiments. We show that the birefringent shear-induced phase starts from the inner cylinder and grows along the velocity gradient direction, as in a shear banding situation. However, around $\dot{\gamma}_\mathrm{c}$ we have not observed a regime of phase coexistence (isotropic and birefringent)

Shear-induced micellar growth in dilute surfactant solutions

The shear-thickening transition observed in aqueous solutions of cetyltrimethylammonium tosylate (CTAT) is investigated using rheology and small-angle neutron scattering under shear. Above a critical shear rate \dot{\gamma}_\ab{C}, the increase of the apparent shear viscosity is due to the formation of a shear-induced phase. Using a Porod representation to analyze the aggregate local morphology, we demonstrate that the shear-induced viscous state consists of cylindrical micelles strongly aligned in the flow. We also investigate the shear rate and surfactant concentration dependencies of the structure factor peak revealed at low wave vectors. Above \dot{\gamma}_\ab{C}, the systematic shift of the structure factor peak to lower wave vectors is interpreted in terms of the shear-induced micellar growth of the aggregates

Flow-structure relationship of shear-thickening surfactant solutions

The shear-thickening transition in dilute surfactant solutions is investigated using rheology and small-angle neutron scattering. Steady shear experiments on a new thickening system, the tosylate of cethyltrimethylammonium, revealed a continuous increase of the apparent viscosity above a critical strain rate $\dot\gamma_{\rm c}$. Concentration and temperature variations of $\dot\gamma_{\rm c}$ are derived, and the former is found to be in contradiction with a theoretically predicted gelation. From the scattering under shear, we establish a clear correlation between flow and structure. In the thickening region, the patterns are due to the superposition of two coexisting states, one viscoelastic entangled sheared network and one still purely viscous made of short aggregates. At higher $\dot\gamma$, the former state dominates and its increasing orientation results in shear-thinning

Microrheology study of semidiluted deoxyribonucleic acid solutions

Linear Theological behavior and Microrheology measurements of sodium salt calf-thymus DNA aqueous solutions as a function of concentration are reported here. The microrheological behavior was obtained by a combination of experimental techniques: mechanical Rheometry and Dynamic light scattering (DLS). The viscoelastic properties of DNA in water as a function of concentration were performed at 20°C and Theological and microrhelogical curves were performed. The result indicated that for concentrations lower than the entanglement concentration (Ce) the system exhibits a predominantly viscous behavior, whereas for higher concentrations exhibits a predominantly elastic behavior. The plateau modulus (G0) and the zero complex viscosity ( ) follow a power law concentration dependence of the form: G0 ∝ CDNA2.3 and η0 ∝ CDNA3.6, respectively The microrheology results overlap perfectly in a single line with the mechanical rheology results, extending the time resolution to faster breathing modes. © Materials Research Society

Microrheology study of semidiluted deoxyribonucleic acid solutions

Linear Theological behavior and Microrheology measurements of sodium salt calf-thymus DNA aqueous solutions as a function of concentration are reported here. The microrheological behavior was obtained by a combination of experimental techniques: mechanical Rheometry and Dynamic light scattering (DLS). The viscoelastic properties of DNA in water as a function of concentration were performed at 20°C and Theological and microrhelogical curves were performed. The result indicated that for concentrations lower than the entanglement concentration (Ce) the system exhibits a predominantly viscous behavior, whereas for higher concentrations exhibits a predominantly elastic behavior. The plateau modulus (G0) and the zero complex viscosity ( ) follow a power law concentration dependence of the form: G0 ? CDNA2.3 and ?0 ? CDNA3.6, respectively The microrheology results overlap perfectly in a single line with the mechanical rheology results, extending the time resolution to faster breathing modes. Zapotitlán Materials Research Society

Phase behavior of the Pluronic P103/water system in the dilute and semi-dilute regimes

The detailed temperature-composition phase diagram of the P103/water system in the dilute and semidilute regions is reported here using density and ultrasound velocity measurements, differential scanning calorimetry (DSC), rheometry and dynamic (DLS) and static light scattering (SLS). These techniques allow a precise determination of the critical micellar temperature (CMT), the sphere-to-rod micellar transition temperature (GMT) and the cloud point temperature (CPT) as a function of concentration. DLS and SLS measurements were employed to gain information on unimers and aggregate sizes and on the transition from spherical-to-rod micelles. � 2009 Elsevier Inc. All rights reserved

Rheology of the Pluronic P103/water system in a semidilute regime: Evidence of nonequilibrium critical behavior

The linear and nonlinear rheological behaviors of semidilute aqueous solutions of the amphiphile triblock polymer Pluronics P103 in water are reported here. For Csurf ? 20 wt%, micelles are spherical at temperatures lower than ca. 27 �C and grow with increasing temperature to form long polymer-like micelles. These polymer-like micelles exhibit strong viscoelasticity and a shear-banding region that shrinks as the cloud point is approached. Master time-temperature-concentration curves were obtained for the dynamic moduli using traditional shifting factors. In the nonlinear regime, P103 polymer-like micellar solutions follow the master dynamic phase diagram proposed by Berret and colleagues, in which the flow curves overlap in the low-shear-rate homogeneous flow region. Within the nonhomogeneous flow region (confirmed by flow birefringence and small-angle light-scattering measurements), oscillations and overshoots are detected at the inception of shear flow, and two main relaxation mechanisms are apparent after cessation of steady shear flow. Evidence for nonequilibrium critical behavior is presented, in which the order parameter is the difference of critical shear rates that limit the span of the plateau stress. Most of the steady-state and transient features of the nonlinear rheology of the P103 polymer-like micelles are reproduced with the Bautista-Manero-Puig (BMP) model, including the predictions of nonequilibrium critical behavior under flow. � 2009 Elsevier Inc. All rights reserved