Azimuthal instability of the interface in a shear banded flow by direct visual observation.

The stability of the shear banded flow of a Maxwellian fluid is studied from an experimental point of view using rheology and flow visualization with polarized light. We show that the one-layer homogeneous flow cannot sustain shear rates corresponding to the end of the stress plateau. The high shear rate branch is not found and the shear stress oscillates at the end of the plateau. An azimuthal instability appears: the shear induced band becomes unstable and the interface between the two bands undulates in time and space with a period τ, a wavelength λ and a wave vector k parallel to the direction of the tangential velocity.

Taylor-like vortices in shear-banding flow of giant micelles.

Using flow visualizations in Couette geometry, we demonstrate the existence of Taylor-like vortices in the shear-banding flow of a giant micelles system. We show that vortices stacked along the vorticity direction develop concomitantly with interfacial undulations. These cellular structures are mainly localized in the induced band and their dynamics is fully correlated with that of the interface. As the control parameter increases, we observe a transition from a steady vortex flow to a state where pairs of vortices are continuously created and destroyed. Normal stress effects are discussed as potential mechanisms driving the three-dimensional flow.

Shear thickening in three surfactants of the alkyl family CnTAB: small angle neutron scattering and rheological study.

Three surfactants of the alkyl family only differing by the length of the aliphatic tail and mixed with sodium salicylate are studied in equilibrium and under flow using rheology and small angle neutron scattering. All three undergo the shear thickening transition associated with the emergence and growth of the so-called shear induced phase or SIS. The rheology in light and heavy water are at first compared, and the influence of the deuterated solvent on the transition characteristics is examined. Small-angle neutron scattering (SANS) experiments are performed on nonflowing solutions in order to find the average cross section radius Ri and the local morphology of the micelles. These data are fitted with two models: a first one that is valid for rigid monodisperse cylindrical particles to get Ri, and a second one that is suitable for semiflexible micelles which shall lead to the contour length L of the micelles. Under flow, scattering experiments are performed over a shear rate range covering three flow regimes. In the first one, prior to the shear thickening transition, the patterns are isotropic; during the last two, corresponding to the existence of the SIS, the scattering figures gradually lose their circular symmetry for an elongated elliptic shape characteristic of an anisotropic medium. The average orientation of the micelles is quantified by the anisotropy factor Af which turns out to be of the same order of magnitude for the three surfactants.

Local velocity measurements in heterogeneous and time-dependent flows of a micellar solution.

We present and discuss the results of pointwise velocity measurements performed on a viscoelastic micellar solution made of cetyltrimethylammonium bromide and sodium salicylate in water, respectively, at the concentrations of 50 and 100 mmol. The sample is contained in a Couette device and subjected to flow in the strain controlled mode. This particular solution shows shear banding and, in a narrow range of shear rates at the right end of the stress plateau, apparent shear thickening occurs. Time-dependent recordings of the shear stress in this range reveal that the flow has become unstable and that large sustained oscillations of the shear stress and of the first normal stresses difference emerge and grow in the flow. Local pointwise velocity measurements clearly reveal a velocity profile typical of shear banding when the imposed shear rate belongs to the plateau, but also important wall slip in the entire range of velocity gradients investigated. In the oscillations regime, the velocity is recorded as a function of time at a fixed point close to the rotor of the Couette device. The time-dependent velocity profile reveals random fluctuations but, from time to time, sharp decreases much larger than the standard deviation are observed. An attempt is made to correlate these strong variations with the stress oscillations and a correlation coefficient r is computed. However, the small value found for the coefficient r does not allow us to draw a final conclusion as concerns the correlation between stress oscillations and velocity fast decreases.

Interface instability in shear-banding flow.

We report on the spatiotemporal dynamics of the interface in shear-banding flow of a wormlike micellar system (cetyltrimethylammonium bromide and sodium nitrate in water) during a start-up experiment. Using the scattering properties of the induced structures, we demonstrate the existence of an instability of the interface between bands along the vorticity direction. Different regimes of spatiotemporal dynamics of the interface are identified along the stress plateau. We build a model based on the flow symmetry which qualitatively describes the observed patterns.

Temporal oscillations of the shear stress and scattered light in a shear-banding--shear-thickening micellar solution.

The results of optical and rheological experiments performed on a viscoelastic solution (cetyltrimethylammonium bromide + sodium salicylate in water) are reported. The flow curve has a horizontal plateau extending between two critical shear rates characteristic of heterogeneous flows formed by two layers of fluid with different viscosities. These two bands which also have different optical anisotropy are clearly seen by direct observation in polarized light. At the end of the plateau, apparent shear thickening is observed in a narrow range of shear rates; in phase oscillations of the shear stress and of the first normal stress difference are recorded in a shearing device operating under controlled strain. The direct observation of the annular gap of a Couette cell in a direction perpendicular to a plane containing the vorticity shows that the turbidity of the whole sample also undergoes time dependent variations with the same period as the shear stress. However no banding is observed during the oscillations and the flow remains homogeneous.

Rayleigh scattering and flow birefringence measurement in colloidal solutions.

A unique method for measuring the birefringence intensity of colloidal solutions which become anisotropic under the action of orienting flow is described. When a laser beam crosses a liquid-containing asymmetrical particle orientated by the flow, the scattered intensity in a direction perpendicular to the wave vector of the incident light shows nodes and antinodes. The distance between two antinodes is related to the phase difference between the eigenpolarizations of the medium and thus to the birefringence of the medium.

Flow birefringence, stress optical rule and rheology of four micellar solutions with the same low shear viscosity.

The flow birefringence and the rheological properties of four viscoelastic solutions having nearly the same zero shear viscosity and subjected to shear flows are investigated in the linear and non-linear domains. The surfactant used for the samples is the cetyltrimethylammonium chloride in water at the concentration of 100 mmol/l with an organic salt, the sodium salicylate. The low shear viscosity curve versus the salt concentration is non-monotonic and has two maxima separated by a minimum forming four domains in which the salt concentration is chosen. For the two solutions belonging to the inner branch, i.e. between the two maxima, a simple Maxwellian behaviour is observed and shear banding occurs as confirmed by the flow birefringence pictures. Contrary to the results of P. Fisher (1996) where the unstable flow regime is restricted to the first decreasing part of the low shear viscosity curve of a cetylpyridinium chloride solution, we show that shear banding exits in a wider domain of the salt concentration.

Insight in shear banding under transient flow.

The rheo-optical behavior of a viscoelastic solution of a surfactant subjected to transient shear flows is reported. A steplike shear rate is suddenly imposed and we record the transient physical characteristics of the liquid, i.e., the shear stress sigma, the transmitted intensity I(t), and the scattering pattern. At the inception of the flow the shear stress shows an important overshoot followed by damped oscillations which also appear in the intensity profile. Then a sigmoïdal relaxation process brings the liquid in its final steady state. During this first phase, the diffusion pattern due to enhanced concentration fluctuations can also be observed. A fine anisotropic layer appears near the moving wall shortly after the inception of the flow, but its width starts to increase significantly well after the end of the relaxation process. When the laser beam travels through this band a diffraction pattern can be observed indicating that it is formed by small subbands, the characteristic width of which is in the order of 100 micro m.