26 research outputs found
Colloidal gels—low-density disordered solid-like states
The analogies between colloidal glasses and gels have stimulated an increased effort in recent years to unify the description of the transitions to these disordered solid-like states within a single conceptual framework. Mode coupling theory, which successfully describes the hard sphere colloidal glass transition, has been extended to describe the effect of weak attractions at large volume fractions. By comparison, diffusion limited cluster aggregation models successfully describe the behavior for the other limit, irreversibly aggregated, fractal gels formed at low volume fractions and very large interaction potentials. A theoretical picture, which unifies these two limits, is still the subject of research. In this review, we summarize some of the results obtained to date, aiming to give an overview of our current understanding
New aspects in the phase behaviour of poly-N-isopropyl acrylamide: systematic temperature dependent shrinking of PNiPAM assemblies well beyond the LCST
We investigate the phase behaviour of aqueous dispersions of poly-N-isopropyl acrylamide (PNiPAM) microgels above their lower critical solution temperature (LCST) and find that beyond a well-defined concentration the systems exhibit a peculiar behaviour: the microgels assemble into space-spanning gels that shrink in time while maintaining the shape of the container in which they have been formed. Over a wide range of concentrations this shrinking behaviour is independent of PNiPAM concentration, but systematically depends on temperature in a temperature range significantly exceeding the LCST. The overall shrinking characteristics are consistent with those expected for scaffolds made of materials that exhibit thermal contraction. However, for the PNiPAM assemblies contraction is irreversible and can be as large as 90%. Such characteristics disclose complex interactions between fully collapsed PNiPAM and water well beyond the LCST, the origin of which has yet to be elucidated
Fluctations and noise in time-resolved light scattering experiments : measuring temporally heterogeneous dynamics
We use Time Resolved Correlation (TRC), a recently introduced light
scattering method, to study the dynamics of a variety of jammed, or glassy,
soft materials. The output of a TRC experiment is cI(t,tau), the time series of
the degree of correlation between the speckle patterns generated by the light
scattered at time t and t+tau. We characterize the fluctuations of cI by
calculating their Probability Density Function, their variance as a function of
the lag tau, and their time autocorrelation function. The comparison between
these quantities for a Brownian sample and for jammed materials indicate
unambiguously that the slow dynamics measured in soft glasses is temporally
heterogeneous. The analogies with recent experimental, numerical and
theoretical work on temporal heterogeneity in the glassy dynamics are briefly
discussed.Comment: date de la fin de redaction : 06/09/200
Ultra-long range correlations of the dynamics of jammed soft matter
We use Photon Correlation Imaging, a recently introduced space-resolved
dynamic light scattering method, to investigate the spatial correlation of the
dynamics of a variety of jammed and glassy soft materials. Strikingly, we find
that in deeply jammed soft materials spatial correlations of the dynamics are
quite generally ultra-long ranged, extending up to the system size, orders of
magnitude larger than any relevant structural length scale, such as the
particle size, or the mesh size for colloidal gel systems. This has to be
contrasted with the case of molecular, colloidal and granular ``supercooled''
fluids, where spatial correlations of the dynamics extend over a few particles
at most. Our findings suggest that ultra long range spatial correlations in the
dynamics of a system are directly related to the origin of elasticity. While
solid-like systems with entropic elasticity exhibit very moderate correlations,
systems with enthalpic elasticity exhibit ultra-long range correlations due to
the effective transmission of strains throughout the contact network.Comment: To appear in Soft Matte
Early and Late Posttransplant Lymphoproliferative Disorder After Lung Transplantation-34 Cases From the European PTLD Network
Colloidal fibers as structurant for worm-like micellar solutions
We investigate the rheological properties of a simplified version of a liquid detergent composed of an aqueous solution of the linear alkylbenzene sulphonate (LAS) surfactant, in which a small amount of fibers made of hydrogenated castor oil (HCO) is dispersed. At the concentration typically used in detergents, LAS is in a worm-like micellar phase exhibiting a Maxwellian behavior. The presence of HCO fibers provides elastic properties, such that the system behaves as a simple Zener body, mechanically characterized by a parallel connection of a spring and a Maxwell element. Despite this apparent independence of the contributions of the fibers and the surfactant medium to the mechanical characteristics of the system, we find that the low frequency modulus increases with increasing LAS concentration. This indicates that LAS induces attractive interactions among the HCO fibers, resulting in the formation of a stress-bearing structure that withstands shear at HCO concentrations, where the HCO fibers in the absence of attractive interactions would not sufficiently overlap to provide stress-bearing properties to the system