Effect of Screening of Intermicellar Interactions on the Linear and Nonlinear Rheology of a Viscoelastic Gel

Abstract

We report our studies of the linear and nonlinear rheology of aqueous solutions of the surfactant cetyl trimethylammonium tosylate (CTAT) with varying amounts of sodium chloride (NaCl). The CTAT concentration is fixed at 42mM and the salt concentration is varied between 0mM to 120mM. On increasing the salt (NaCl) concentration, we see three distinct regimes in the zero-shear viscosity and the high frequency plateau modulus data. In regime I, the zero-shear viscosity shows a weak increase with salt concentration due to enhanced micellar growth. The decrease in the zero-shear viscosities with salt concentration in regimes II and III can be explained in terms of inter-micellar branching. The most intriguing feature of our data however, is the anomalous behavior of the high frequency plateau modulus in regime II (0.12≤ 1.42). In this regime, the plateau modulus increases with an increase in NaCl concentration. This is highly counter-intuitive, since the correlation length of concentration fluctuations and hence the plateau modulus G • are not expected to change appreciably in the semi-dilute regime. We propose to explain the changes in regime II in terms of the unbinding of the organic counterions (tosylate) from the CTA + surfaces on the addition of NaCl. In the nonlinear flow curves of the samples with high salt content, significant deviations from the predictions of the Giesekus model for entangled micelles are observed.

Effect of silica colloids on the rheology of viscoelastic gels formed by the surfactant cetyl trimethylammonium tosylate

“…The value of α in the Cole-Davidson model [16] gives an estimate of the width of the distribution of relaxation times. Multiexponential relaxation processes are seen in dilute micellar solutions where α < 1, with α increasing to 1 with increasing surfactant and/or salt concentrations [23,24]. On adding SiO 2 to semidilute aqueous solutions of CTAT, we find a similar increase in the values of α suggesting an approach towards single exponential stress relaxation.…”

A Rheological Approach to Viscoelastic Worm‐Like Micelles of Tunable Properties

“…External stimuli such as light and temperature are found to tune the size and shape of selfassembling molecules and thereby control their assembly in solution [5,6]. The flow behavior of surfactant solutions have been studied extensively, both theoretically and experimentally [7,8]. In aqueous cetyl trimethyl ammonium bromide (CTAB) thread-like micelles appear above 11 wt.% and form an isotropic phase at 27°C.…”

Order and chaos in soft condensed matter

“…Their d-JS-φ model (spatially non-local Johnson-Segalman model with concentration coupling) calculations predict a positive value for the slope of the plateau in the banded region of the flow curve, and the slope increases with the strength of the coupling between flow and concentration fluctuations ( figure 4). In a recent experimental work, Bandyopadhyay and Sood [27] have shown that the slope α of the stress plateau, which we find is a power law σ ∼ γ α , for worm-like micellar solutions of surfactant cetyltrimethylammonium tosylate (CTAT) can be tuned by adding salt (NaCl) ( figure 5).…”