298 research outputs found
Spatio-temporal dynamics of wormlike micelles under shear
Velocity profiles in a wormlike micelle solution (CTAB in D2O) are recorded
using ultrasound every 2 s after a step-like shear rate into the shear-banding
regime. The stress relaxation occurs over more than six hours and corresponds
to the very slow nucleation and growth of the high-shear band. Moreover,
oscillations of the interface position with a period of about 50 s are observed
during the growth process. Strong wall slip, metastable states and transient
nucleation of three-band flows are also reported and discussed in light of
previous experiments and theoretical models.Comment: 4 pages, 5 figures, submitted to Phys.Rev.Let
Oscillations of a solid sphere falling through a wormlike micellar fluid
We present an experimental study of the motion of a solid sphere falling
through a wormlike micellar fluid. While smaller or lighter spheres quickly
reach a terminal velocity, larger or heavier spheres are found to oscillate in
the direction of their falling motion. The onset of this instability correlates
with a critical value of the velocity gradient scale
s. We relate this condition to the known complex rheology of wormlike
micellar fluids, and suggest that the unsteady motion of the sphere is caused
by the formation and breaking of flow-induced structures.Comment: 4 pages, 4 figure
Thermodynamics of the glassy state: effective temperature as an additional system parameter
A system is glassy when the observation time is much smaller than the
equilibration time. A unifying thermodynamic picture of the glassy state is
presented. Slow configurational modes are in quasi-equilibrium at an effective
temperature. It enters thermodynamic relations with the configurational entropy
as conjugate variable. Slow fluctuations contribute to susceptibilities via
quasi-equilibrium relations, while there is also a configurational term.
Fluctuation-dissipation relations also involve the effective temperature.
Fluctuations in the energy are non-universal, however. The picture is supported
by analytically solving the dynamics of a toy model.Comment: 5 pages, REVTEX. Phys. Rev. Lett, to appea
Velocity profiles in shear-banding wormlike micelles
Using Dynamic Light Scattering in heterodyne mode, we measure velocity
profiles in a much studied system of wormlike micelles (CPCl/NaSal) known to
exhibit both shear-banding and stress plateau behavior. Our data provide
evidence for the simplest shear-banding scenario, according to which the
effective viscosity drop in the system is due to the nucleation and growth of a
highly sheared band in the gap, whose thickness linearly increases with the
imposed shear rate. We discuss various details of the velocity profiles in all
the regions of the flow curve and emphasize on the complex, non-Newtonian
nature of the flow in the highly sheared band.Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
Ehrenfest relations at the glass transition: solution to an old paradox
In order to find out whether there exists a thermodynamic description of the
glass phase, the Ehrenfest relations along the glass transition line are
reconsidered. It is explained that the one involving the compressibility is
always satisfied, and that the one involving the specific heat is principally
incorrect.
Thermodynamical relations are presented for non-ergodic systems with a
one-level tree in phase space. They are derived for a spin glass model, checked
for other models, and expected to apply, e.g., to glass forming liquids. The
second Ehrenfest relation gets a contribution from the configurational entropy.Comment: 4 pages revtex, to appear in Phys. Rev. Let
Linear and nonlinear rheology of wormlike micelles
Several surfactant molecules self-assemble in solution to form long,
cylindrical, flexible wormlike micelles. These micelles can be entangled with
each other leading to viscoelastic phases. The rheological properties of such
phases are very interesting and have been the subject of a large number of
experimental and theoretical studies in recent years. We shall report on our
recent work on the macrorheology, microrheology and nonlinear flow behaviour of
dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium
Tosilate). This system forms elongated micelles and exhibits strong
viscoelasticity at low concentrations ( 0.9 wt%) without the addition of
electrolytes. Microrheology measurements of have been done using
diffusing wave spectroscopy which will be compared with the conventional
frequency sweep measurements done using a cone and plate rheometer. The second
part of the paper deals with the nonlinear rheology where the measured shear
stress is a nonmonotonic function of the shear rate . In
stress-controlled experiments, the shear stress shows a plateau for
larger than some critical strain rate, similar to the earlier
reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a
signature of mechanical instability in the form of shear bands. We have carried
out extensive experiments under controlled strain rate conditions, to study the
time-dependence of shear stress. The measured time series of shear stress has
been analysed in terms of correlation integrals and Lyapunov exponents to show
unambiguously that the behaviour is typical of low dimensional dynamical
systems.Comment: 15 pages, 10 eps figure
Structure factor of polymers interacting via a short range repulsive potential: application to hairy wormlike micelles
We use the Random Phase Approximation (RPA) to compute the structure factor,
S(q), of a solution of chains interacting through a soft and short range
repulsive potential V. Above a threshold polymer concentration, whose magnitude
is essentially controlled by the range of the potential, S(q) exhibits a peak
whose position depends on the concentration. We take advantage of the close
analogy between polymers and wormlike micelles and apply our model, using a
Gaussian function for V, to quantitatively analyze experimental small angle
neutron scattering profiles of semi-dilute solutions of hairy wormlike
micelles. These samples, which consist in surfactant self-assembled flexible
cylinders decorated by amphiphilic copolymer, provide indeed an appropriate
experimental model system to study the structure of sterically interacting
polymer solutions
Thermodynamic picture of the glassy state
A picture for thermodynamics of the glassy state is introduced. It assumes
that one extra parameter, the effective temperature, is needed to describe the
glassy state. This explains the classical paradoxes concerning the Ehrenfest
relations and the Prigogine-Defay ratio. As a second part, the approach
connects the response of macroscopic observables to a field change with their
temporal fluctuations, and with the fluctuation-dissipation relation, in a
generalized non-equilibrium way.Comment: Proceedings of the Conference "Unifying Concepts in Glass Physics",
ICTP, Trieste, 15 - 18 September 199
Ecological-economic assessment of the effects of freshwater flow in the Florida Everglades on recreational fisheries
This research develops an integrated methodology to determine the economic value to anglers of recreational fishery ecosystem services in Everglades National Park that could result from different water management scenarios. The study first used bio-hydrological models to link managed freshwater inflows to indicators of fishery productivity and ecosystem health, then link those models to anglers\u27 willingness-to-pay for various attributes of the recreational fishing experience and monthly fishing effort. This approach allowed us to estimate the foregone economic benefits of failing to meet monthly freshwater delivery targets. The study found that the managed freshwater delivery to the Park had declined substantially over the years and had fallen short of management targets. This shortage in the flow resulted in the decline of biological productivity of recreational fisheries in downstream coastal areas. This decline had in turn contributed to reductions in the overall economic value of recreational ecosystem services enjoyed by anglers. The study estimated the annual value of lost recreational services at 11.88 per AF in November to 41.54 per AF. Linking anglers\u27 recreational preference directly to a decision variable such as water delivery is a powerful and effective way to make management decision
Vorticity Banding During the Lamellar-to-Onion Transition in a Lyotropic Surfactant Solution in Shear Flow
We report on the rheology of a lamellar lyotropic surfactant solution
(SDS/dodecane/pentanol/water), and identify a discontinuous transition between
two shear thinning regimes which correspond to the low stress lamellar phase
and the more viscous shear induced multi-lamellar vesicle, or ``onion'' phase.
We study in detail the flow curve, stress as a function of shear rate, during
the transition region, and present evidence that the region consists of a shear
banded phase where the material has macroscopically separated into bands of
lamellae and onions stacked in the vorticity direction. We infer very slow and
irregular transformations from lamellae to onions as the stress is increased
through the two phase region, and identify distinct events consistent with the
nucleation of small fractions of onions that coexist with sheared lamellae.Comment: 10 pages, 10 figure
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