J. F. A. Soltero

On the shear banding flow of elongated micellar solutions

Abstract Under steady shear flow, elongated micellar solutions show shear stress saturation above a critical shear rate due to the formation of shear bands that result in non-homogeneous flow. Long transients and oscillations accompany this stress plateau. When measurements are done with a controlled stress rheometer, frequently a metastable branch is observed. At higher shear rates, a second upturn is observed above a second critical shear rate, which indicates that homogeneous flow is recovered. Here, a model consisting of the codeformational Maxwell constitutive equation coupled to a kinetic equation to account for the breaking and reformation of the micelles is presented to reproduce the features described above in steady shear flow. The model also predicts a second metastable branch and long transients at higher shear rates and the existence of an inflexion point in stress-shear rate plots above which no shear banding behavior is detected.

Dynamics of worm-like micelles: the Cox-Merz rule

Abstract The viscoelastic behaviour of worm-like micelles in small-amplitude oscillatory, steady simple shear and uniaxial extensional flows are analyzed with a model that couples the Oldroyd-B constitutive equation with a kinetic equation that accounts for the structural changes induced by the flow. In some cases, the constitutive equation predicts a viscoelastic behaviour that is consistent with the Cox–Merz rule. Departures from this rule are also predicted. Experimental data obtained for two worm-like micellar systems indicate that in these solutions, the Cox–Merz rule is not usually followed, in agreement with the predictions of our model. In uniaxial extensional flow, the model predicts a strain hardening in the extensional viscosity at low extensional rates and a strain-thinning at high extensional rates.

Flow regimes of dilute surfactant solutions

Abstract The shear-thickening of dilute surfactant solutions is investigated using water/cethyltrimethylammonium tosylate (CTAT) systems. We show that shear induced structures (SIS) are obtained in Couette and parallel-plate flows when the shear stress is higher than a critical shear stress, independently of the type of shear cell. Three different regimes are observed in Couette flow, confirming the results of Hu and co-workers on TTAA/NaSal/water systems. Flow regimes in parallel-plate flow geometries are studied with the aid of particle image velocimetry (PIV). Particular attention is paid to the calibration of the experimental set-up using pure water in order to distinguish between deviations of the rheometric velocity profile due to the onset of purely hydrodynamic instabilities from those due to the occurrence of SIS. It is noteworthy that spatial and temporal fluctuations in the local velocity fields are observed when SIS exists, even when rheometric measurements show an apparently steady-state situation. When no SIS is present we found that the surfactant solutions behave like Newtonian fluids. Laminar secondary flows are present for both cells under the same conditions as those found for classical hydrodynamics.

Thermal transitions and state of water in binary mixtures of water andn-decanephosphonic acid or its sodium salts

The state of water and several transitions were examined in the systemsn-decanephosphonic acid (DPA)—water and the sodium salts of DPA—water. Temperature — composition phase diagrams are reported. The results show that several liquid crystalline phases plus isotropic liquid, and two solid phases (a waxy solid phase and a crystalline phase) are formed. Several types of water were detected: bulk-like water, interfacial water and hydration water.

Thermoresponsive poly(N-isopropylacrylamide) nanogels/poly(acrylamide) nanostructured hydrogels

ABSTRACT Here we report the preparation and characterization of nanostructured thermo-responsive poly(acrylamide) (PAM)-based hydrogels. The addition of slightly crosslinked poly(N-isopropylacrylamide) (PNIPA) nanogels to AM reactive aqueous solution produces nanostructured hydrogels that exhibit a volume phase transition temperature (TVPT). Their swelling kinetics, TVPTs and mechanical properties at the equilibrium-swollen state (Heq) are investigated as a function of the concentration of PNIPA nanogels in the nanostructured hydrogels. Nanostructured hydrogels with PNIPA nanogels/AM mass ratios of 20/80 and above exhibit higher Heq and longer time to reach the equilibrium swelling than those of the conventional PAM hydrogels. However, the PNIPA nanogels possess thermo-responsive character missing in conventional PAM hydrogels. The TVPT of nanostructured hydrogels depends on PNIPA nanogel content but their elastic and Young moduli are larger than those of conventional hydrogels at similar swelling ratios. Swelling kinetics, TVPT, and mechanical properties are explained in terms of the controlled in-homogeneities introduced by the PNIPA nanogels during the polymerization.

Time-Dependent Rheological Behavior of Liquid Crystalline Dispersions

The rheological response of Aerosol OT (AOT)/water liquid crystalline dispersions is reported here using shear flows. The dispersions exhibit an apparent yield stress and strong non-Newtonian behavior. Steady state and pre-shear dynamical experiments reveal shear-induced structural changes. Under increasing-and-decreasing shear stress experiments, the dispersions exhibit anti-thixotropic hysteresis loops. Once a critical stress is surpassed, an additional thixotropic loop is observed at high shear stress levels. This inverse loop at high shear stresses depends on the previous shear history and on both the rate of change and the maximum attained value of shear stress. The number density of the globular structures in the sheared sample is larger than in one non-sheared sample, but their sizes are smaller than those of the non-sheared sample.