Inhomogeneous shear flows of wormlike micelles:mA master dynamic phase diagram

Abstract

We report on the nonlinear mechanical response of surfactant wormlike micelles subjected to steady shear flow. The system placed under scrutiny is made of cetylpyridinium chloride, sodium salicylate, and salted water at 0.5M NaCl ͑abbreviated as CPCl-Sal͒. We have investigated solutions in the intermediate concentration ranges, at ϭ6%, 8% and ϭ12% for temperatures ranging between 20°C and 50°C. In these T and ranges, we first confirm that the present system is a true Maxwell fluid. As a consequence, the nonlinear rheology can be formulated in terms of the normalized quantities, *ϭ/G 0 and ␥ *ϭ␥ R , where G 0 is the elastic plateau modulus and R the terminal relaxation time of the Maxwell fluid. Second, we demonstrate that the flow curves of the CPCl-Sal wormlike micelles * ͑␥ *͒ are invariant with respect to relative changes in temperature and concentration. This enables us to define superimposition procedures between flow curves obtained at different temperatures and concentrations and to derive the so-called ''master dynamic phase diagram'' of the CPCl-Sal wormlike micelles. One crucial feature of this master phase diagram is the existence of a critical behavior at TϭT c . Here, the stress * levels off progressively up to a plateau that is reduced to a single flat point of coordinates ͑*ϭ0.9, ␥ *ϭ3͒. The low-temperature regime (TϽT c ) is again identified by a shear stress plateau beginning by a true discontinuity of slope in the * ͑␥ *͒ behavior. At high T ͑ϾT c ͒, * increases monotonously at all shear rates. In order to examine the state of shearing at the stress plateau, flow birefringence experiments were undertaken with a rheo-optical device working with polarized light propagating parallel to the vorticity axis of the Couette. In the plateau regime, the flow is clearly inhomogeneous. Within the gap of a cell, two phases of very different birefringence are observed in the steady state of shearing, as well as after cessation of the shearing.

Towards a continuous microfluidic rheometer

“…The second one is as a mixture made of CetylPyridinium Chloride (CpCl) and NaSal diluted at 6 wt% in a solution of NaCl at 0.5 mol/L in water. This system is known to form wormlike micelles (Berret et al 1994(Berret et al , 1997.…”

Oscillating Viscosity in a Lyotropic Lamellar Phase under Shear Flow

“…They involve mesoscopic scales (typically about a micron), they appear in homogeneous flow, and are thus characteristic of complex fluids. They may correspond to a phase transition existing at thermal equilibrium (such as the isotropic/nematic transition) [4][5][6] or to a textural change (nematics [7,8] or smectics [9,10]). In the latter situation, the shear flow affects the density of defects and induces, in the case of lamellar phases, the formation of compact multilamellar vesicles [11].…”

Flow Velocity Profiles and Shear Banding Onset in a Semidilute Wormlike Micellar System under Couette Flow

“…When WM fluids are submitted to a steady flow, they can show shear banding 1,3 that is a transition between a homogeneous and a non-homogeneous state of flow 4,5 where macroscopic bands bearing different shear rate coexist in the fluid. In most of the cases, shear bands develop in the velocity gradient direction (gradient banding), although vorticity bands have also been observed (vorticity banding).…”