Shear-Thickening Dilute Surfactant Solutions: Equilibrium Structure As Studied by Small-Angle Neutron Scattering

Abstract

We have combined rheology and small-angle neutron scattering experiments to investigate dilute aqueous solution of cetyltrimethylammonium tosylate (CTAT). Rheology experiments have been performed under controlled strain conditions as function of concentration (φ ) 0.05 -1%) and temperature (T ) 23-45 °C). At low surfactant concentration and under steady shear conditions, the CTAT exhibits a continuous increase of the apparent viscosity with increasing shear rates, also termed a shear-thickening transition. The critical shear rate for shear thickening varies strongly with the temperature according to an Arrhenius behavior, but very weakly with concentration. On the same solutions that were studied by rheology, neutron scattering was performed at rest to search for a correlation between the rheological features of the shear-thickening transition and the equilibrium micellar structure. We have found that (i) for all concentrations that display the thickening transition, the local morphology is that of cylindrical micelles, with a radius R ) 21.0 ( 0.5 Å; (ii) all spectra exhibit a scattering correlation peak due to strong electrostatic repulsion between charged micelles; (iii) the shear-thickening transition in CTAT displays the same rheological features for solutions prepared either in the dilute (φ < φ*) or in the semidilute (φ* < φ < 0.8%) regimes.

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

“…These concentrations were chosen since according to the recent literature [6,7,20,21], they are above the second critical micellar concentration (∼ 0.04%) at which cylindrical micelles start to form, and below the overlap concentration φ * which is the onset of semidilut regime. For CTAT at ambient temperature, the overlap concentration was estimated around φ * ∼ 0.7% using transient electric birefringence [20] and fluorescence recovery [21], whereas viscosity measurements suggest a slightly lower value at φ * ∼ 0.5%.…”

“…From electric birefringence, as well as from small-angle neutron scattering measurements, CTAT aqueous solutions at φ = 0.2% are suspensions of short anisotropic aggregates of average length 200-300Å(aspect ratio 4-6). The intermicellar distance was evaluated from the position of the first maximum of the structure factor, and was found to be around 600Å [7]. Figure 1 shows the characteristic flow curve for CTAT at φ = 0.24% and T = 25 • C. The stationary shear viscosity (filled circles) exhibits a clear Newtonian behavior forγ < 30 s −1 at a level that compares well to the viscosity of the solvent, η 0 = 1 ± 0.1 mPas.…”

“…It is now established that a prerequisite for the shear thickening is that the amphiphile molecules self-assemble in the quiescent state into locally cylindrical micelles. The shear thickening has been observed in solutions of rodlike aggregates in the dilute regime (case i) [1][2][3][4][5][6][7] but also in solutions of wormlike micelles in the entangled regime (case ii) [4,5,[8][9][10][11][12][13]. Interestingly, some characteristic features of the transition are similar for rodlike and wormlike assemblies.…”

“…Under steady shear however, above a critical strain rate notedγ c the shear viscosity increases, as a new and more viscous phase develops in the cell. The shear-induced phase (SIP) has been thoroughly investigated by rheological measurements [1][2][3][4][5][6][7][8]11], flow-birefringence [3,5,14] and small-angle neutron scattering [2][3][4]6]. The two latter techniques revealed a strong degree of alignment of this new phase with respect to the flow velocity.…”

Rheology of Wormlike Micelles: Equilibrium Properties and Shear Banding Transitions

“…The continuous lines through the data are calculated using b = 380 Å and b = 360 Å, respectively. Persistence lengths of the order of 400 Å are typical for neutral wormlike micelles, and have been found repeatedly in the recent literature [27,53,98,100,102,107]. For polyelectrolyte micelles, the total persistence length can be larger than the above characteristic values [108,109].…”