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, $G_0$, 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, $\eta_{eff}$, defined as the characteristic time measured in a stress relaxation experiment times $G_0$. At all time, $\eta_{eff}$ is found to be independent of $G_0$, of elastic perturbations, and of the rate at which the sample is quenched in the glassy phase. We propose a simple model that links $\eta_{eff}$ to the internal stress built up at the fluid-to-solid transition

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

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

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 $t_{w}$ is observed for both rheology and MDLS, the characteristic relaxation time scaling as $t_{w}^{\mu}$. We find the same aging exponent $\mu =0.78$ 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

Length scale dependence of dynamical heterogeneity in a colloidal fractal gel

We use time-resolved dynamic light scattering to investigate the slow dynamics of a colloidal gel. The final decay of the average intensity autocorrelation function is well described by $g\_2(q,\tau)-1 \sim \exp[-(\tau/\tau\_\mathrm{f})^p]$, with $\tau\_\mathrm{f} \sim q^{-1}$ and $p$ decreasing from 1.5 to 1 with increasing $q$. We show that the dynamics is not due to a continuous ballistic process, as proposed in previous works, but rather to rare, intermittent rearrangements. We quantify the dynamical fluctuations resulting from intermittency by means of the variance $\chi(\tau,q)$ of the instantaneous autocorrelation function, the analogous of the dynamical susceptibility $\chi\_4$ studied in glass formers. The amplitude of $\chi$ is found to grow linearly with $q$. We propose a simple --yet general-- model of intermittent dynamics that accounts for the $q$ dependence of both the average correlation functions and $\chi$.Comment: Revised and improved, to appear in Europhys. Let