188 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
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
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
Hierarchical cross-linking in physical alginate gels: a rheological and dynamic light scattering investigation
We investigate the dynamics of alginate gels, an important class of
biopolymer-based viscoelastic materials, by combining mechanical tests and
non-conventional, time-resolved light scattering methods. Two relaxation modes
are observed upon applying a compressive or shear stress. Dynamic light
scattering and diffusive wave spectroscopy measurements reveal that these modes
are associated with discontinuous rearrangement events that restructure the gel
network via anomalous, non-diffusive microscopic dynamics. We show that these
dynamics are due to both thermal activation and internal stress stored during
gelation and propose a scenario where a hierarchy of cross-links with different
life times is responsible for the observed complex behavior. Measurements at
various temperatures and sample ages are presented to support this scenario.Comment: To appear in Soft Matte
Glassy dynamics and dynamical heterogeneity in colloids
Concentrated colloidal suspensions are a well-tested model system which has a
glass transition. Colloids are suspensions of small solid particles in a
liquid, and exhibit glassy behavior when the particle concentration is high;
the particles are roughly analogous to individual molecules in a traditional
glass. Because the particle size can be large (100 nm - 1000 nm), these samples
can be studied with a variety of optical techniques including microscopy and
dynamic light scattering. Here we review the phenomena associated with the
colloidal glass transition, and in particular discuss observations of spatial
and temporally heterogeneous dynamics within colloidal samples near the glass
transition. Although this Chapter focuses primarily on results from
hard-sphere-like colloidal particles, we also discuss other colloidal systems
with attractive or soft repulsive interactions.Comment: Chapter of "Dynamical heterogeneities in glasses, colloids, and
granular media", Eds.: L. Berthier, G. Biroli, J-P Bouchaud, L. Cipelletti
and W. van Saarloos (Oxford University Press, to appear), more info at
http://w3.lcvn.univ-montp2.fr/~lucacip/DH_book.ht
Microscopic dynamics and failure precursors of a gel under mechanical load
Material failure is ubiquitous, with implications from geology to everyday
life and material science. It often involves sudden, unpredictable events, with
little or no macroscopically detectable precursors. A deeper understanding of
the microscopic mechanisms eventually leading to failure is clearly required,
but experiments remain scarce. Here, we show that the microscopic dynamics of a
colloidal gel, a model network-forming system, exhibit dramatic changes that
precede its macroscopic failure by thousands of seconds. Using an original
setup coupling light scattering and rheology, we simultaneously measure the
macroscopic deformation and the microscopic dynamics of the gel, while applying
a constant shear stress. We show that the network failure is preceded by
qualitative and quantitative changes of the dynamics, from reversible particle
displacements to a burst of irreversible plastic rearrangements
Adaptive Speckle Imaging Interferometry: a new technique for the analysis of microstructure dynamics, drying processes and coating formation
We describe an extension of multi-speckle diffusing wave spectroscopy adapted
to follow the non-stationary microscopic dynamics in drying films and coatings
in a very reactive way and with a high dynamic range. We call this technique
"Adaptive Speckle Imaging Interferometry". We introduce an efficient tool, the
inter-image distance, to evaluate the speckle dynamics, and the concept of
"speckle rate" (SR, in Hz) to quantify this dynamics. The adaptive algorithm
plots a simple kinetics, the time evolution of the SR, providing a non-invasive
characterization of drying phenomena. A new commercial instrument, called
HORUS(R), based on ASII and specialized in the analysis of film formation and
drying processes is presented.Comment: 11 pages, 4 figure
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