30 research outputs found
Interface instability in shear banding flow
We report on the spatio-temporal 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
spatio-temporal dynamics of the interface are indentified along the stress
plateau. We build a model based on the flow symetry which qualitatively
describes the observed patterns
Time scales in shear banding of wormlike micelles
Transient stress and birefringence measurements are performed on wormlike micellar solutions that "shear band", i.e. undergo flow-induced coexistence of states of different viscosities along a constant stress "plateau". Three well-defined relaxation times are found after a strain rate step between two banded flow states on the stress plateau. Using the Johnson-Segalman model, we relate these time scales to three qualitatively different stages in the evolution of the bands and the interface between them: band destabilization, reconstruction of the interface, and travel of the fully formed interface. The longest timescale is then used to estimate the magnitude of the (unknown) "gradient" terms that must be added to constitutive relations to explain the history independence of the steady flow and the plateau stress selection
Taylor-like vortices in the 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.Comment: 5 pages, 4 figure
Recent experimental probes of shear banding
Recent experimental techniques used to investigate shear banding are
reviewed. After recalling the rheological signature of shear-banded flows, we
summarize the various tools for measuring locally the microstructure and the
velocity field under shear. Local velocity measurements using dynamic light
scattering and ultrasound are emphasized. A few results are extracted from
current works to illustrate open questions and directions for future research.Comment: Review paper, 23 pages, 11 figures, 204 reference
Phenomenology and physical origin of shear-localization and shear-banding in complex fluids
We review and compare the phenomenological aspects and physical origin of
shear-localization and shear-banding in various material types, namely
emulsions, suspensions, colloids, granular materials and micellar systems. It
appears that shear-banding, which must be distinguished from the simple effect
of coexisting static-flowing regions in yield stress fluids, occurs in the form
of a progressive evolution of the local viscosity towards two significantly
different values in two adjoining regions of the fluids in which the stress
takes slightly different values. This suggests that from a global point of view
shear-banding in these systems has a common physical origin: two physical
phenomena (for example, in colloids, destructuration due to flow and
restructuration due to aging) are in competition and, depending on the flow
conditions, one of them becomes dominant and makes the system evolve in a
specific direction.Comment: The original publication is available at http://www.springerlink.co
Flow birefringence and stress optical law of viscoelastic solutions of cationic surfactants and sodium salicylate
The optical and rheological properties of different viscoelastic solutions of
surfactant are studied in order to gather experimental data used to
calculate the value of the stress optical coefficient C. Three surfactants
of the same family (CTAB) have been chosen; they differ by the length of
the hydrocarbon chain; it concerns the
dodecyltrimethylammonium bromide (C15H34BrN or DoTAB),
the myristyl trimethyl ammonium bromide (C17H38BrN or MyTAB),
and the hexadecyltrimethylammonium bromide (C19H42BrN or CTAB).
Different parameters like the temperature of the solution and the salinity
of the solvent have been made to vary.
Flow birefringence experiments and rheological
measurements are performed on these solutions in order to study the
dependence of the extinction angle χ, of the birefringence intensity
and of the shear stress with the shear rate
. These data are used to check the stress optical law which
turns out to be valid in a wide range of shear rates. The stress optical
coefficient C is then computed: it is found to vary with the salinity of the
solvent and the temperature of the solution for a given surfactant.
Then, for all solutions of this work the variations of C are related
to the variations of the polarizability anisotropy and the persistence length
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