713 research outputs found
Intrinsic aging and effective viscosity in the slow dynamics of a soft glass with tunable elasticity
We investigate by rheology and light scattering the influence of the elastic
modulus, , on the slow dynamics and the aging of a soft glass. We show
that the slow dynamics and the aging can be entirely described by the evolution
of an effective viscosity, , defined as the characteristic time
measured in a stress relaxation experiment times . At all time,
is found to be independent of , of elastic perturbations, and
of the rate at which the sample is quenched in the glassy phase. We propose a
simple model that links to the internal stress built up at the
fluid-to-solid transition
Ultraslow dynamics and stress relaxation in the aging of a soft glassy system
We use linear rheology and multispeckle dynamic light scattering (MDLS) to
investigate the aging of a gel composed of multilamellar vesicles. Light
scattering data indicate rearrangement of the gel through an unusual ultraslow
ballistic motion. A dramatic slowdown of the dynamics with sample age
is observed for both rheology and MDLS, the characteristic relaxation time
scaling as . We find the same aging exponent for both
techniques, suggesting that they probe similar physical processes, that is the
relaxation of applied or internal stresses for rheology or MDLS, respectively.
A simple phenomenological model is developed to account for the observed
dynamics.Comment: 8 pages, 4 figures, Submitted to PR
Polyelectrolyte-induced peeling of charged multilamellar vesicles
We study mixtures of charged surfactants, which alone in solution form uni-
and multilamellar vesicles, and oppositely charged polyelectrolytes (PEs). The
phase behavior is investigated at fixed surfactant concentration as a function
of the PE-to-surfactant charge ratio . We find that, for , aggregates
form. Light microscopy and X-ray scattering experiments show that the
isoelectric point plays a crucial role since the morphology and the microscopic
structure of the aggregates are different before () and after the
isoelectric point (). To better understand the dynamics for the formation
of PE/surfactant complexes, we perform light microscopy experiments where we
follow in real-time the effect of a PE solution on one multilamellar vesicle
(MLV). We find that the PE induces a peeling of the bilayers of the MLV one by
one. The peeling is accompanied by strong shape fluctuations of the MLV and
leads ultimately to a pile of small aggregates. This novel phenomenon is
analyzed in detail and discussed in terms of PE-induced tension, and pore
formation and growth in a surfactant bilayer.Comment: to appear in Langmui
Slow dynamics in glassy soft matter
Measuring, characterizing and modelling the slow dynamics of glassy soft
matter is a great challenge, with an impact that ranges from industrial
applications to fundamental issues in modern statistical physics, such as the
glass transition and the description of out-of-equilibrium systems. Although
our understanding of these phenomena is still far from complete, recent
simulations and novel theoretical approaches and experimental methods have shed
new light on the dynamics of soft glassy materials. In this paper, we review
the work of the last few years, with an emphasis on experiments in four
distinct and yet related areas: the existence of two different glass states
(attractive and repulsive), the dynamics of systems very far from equilibrium,
the effect of an external perturbation on glassy materials, and dynamical
heterogeneity
Free radially expanding liquid sheet in air: time- and space-resolved measurement of the thickness field
The collision of a liquid drop against a small target results in the
formation of a thin liquid sheet that extends radially until it reaches a
maximum diameter. The subsequent retraction is due to the air-liquid surface
tension. We have used a time- and space-resolved technique to measure the
thickness field of this class of liquid sheet, based on the grey level
measurement of the image of a dyed liquid sheet recorded using a fast camera.
This method enables a precise measurement of the thickness in the range
, with a temporal resolution equal to that of the
camera. We have measured the evolution with time since impact, , and radial
position, , of the thickness, , for various drop volumes and impact
velocities. Two asymptotic regimes for the expansion of the sheet are
evidenced. The scalings of the thickness with and measured in the two
regimes are those that were predicted in \citet{Rozhkov2004} fort the
short-time regime and \citet{Villermaux2011} for the long time regime, but
never experimentally measured before. Interestingly, our experimental data also
evidence the existence of a maximum of the film thickness at
a radial position corresponding to the crossover of these
two asymptotic regimes. The maximum moves with a constant velocity of the order
of the drop impact velocity, as expected theoretically. Thanks to our
visualization technique, we also evidence an azimuthal thickness modulation of
the liquid sheets.Comment: accepted for publication in Journal of Fluid Mechanic
Origin of the slow dynamics and the aging of a soft glass
We study by light microscopy a soft glass consisting of a compact arrangement
of polydisperse elastic spheres. We show that its slow and non-stationary
dynamics results from the unavoidable small fluctuations of temperature, which
induce intermittent local mechanical shear in the sample, because of thermal
expansion and contraction. Temperature-induced shear provokes both reversible
and irreversible rearrangements whose amplitude decreases with time, leading to
an exponential slowing down of the dynamics with sample age.Comment: published in PRL 97, 238301, 200
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