We introduce a new scheme for investigating temporally heterogeneous dynamics, which is termed time-resolved correlation (TRC). TRC is applied to data obtained by diffusing wave spectroscopy probing the slow dynamics of a strongly aggregated colloidal gel. Other examples of TRC data, collected for different jammed materials in single and multiple scattering, are provided to demonstrate the wide range of applicability of this method. In all cases we find evidence that the slow dynamics results from a series of discrete steps rather than from a continuous motion, suggesting temporal heterogeneities to be a general feature of slow dynamics in jammed systems.

We report experiments on hard-sphere colloidal glasses that show a type of shear banding hitherto unobserved in soft glasses. We present a scenario that relates this to an instability due to shear-concentration coupling, a mechanism previously thought unimportant in these materials. Below a characteristic shear rate γ(c) we observe increasingly nonlinear and localized velocity profiles. We attribute this to very slight concentration gradients in the unstable flow regime. A simple model accounts for both the observed increase of γ(c) with concentration, and the fluctuations in the flow.

We study the flow of concentrated hard-sphere colloidal suspensions along smooth, nonstick walls using cone-plate rheometry and simultaneous confocal microscopy. In the glass regime, the global flow shows a transition from Herschel-Bulkley behavior at large shear rate to a characteristic Bingham slip response at small rates, absent for ergodic colloidal fluids. Imaging reveals both the "solid" microstructure during full slip and the local nature of the "slip to shear" transition. Both the local and global flow are described by a phenomenological model, and the associated Bingham slip parameters exhibit characteristic scaling with size and concentration of the hard spheres.

In supercooled molecular fluids or concentrated colloids and grains, the dynamics slow down markedly with no distinct structural changes as the glass 1 or the jamming 2 transition is approached. There is now ample evidence that structural relaxation in glassy systems can only occur through correlated rearrangements of particle 'blobs' of size ξ (refs 3-7), leading to dynamics that are heterogeneous both in time and in space. On approaching these transitions, ξ grows in glass-formers 6-8 , colloids 3,4,9 and driven granular materials 10 alike, strengthening the analogies between the glass and the jamming transitions and providing a possible explanation for the slowing down of the dynamics. However, little is known yet on the behaviour of dynamical heterogeneity very close to dynamical arrest. Here, we measure in colloids the maximum of a 'dynamical susceptibility', χ * , that quantifies the temporal fluctuations of the dynamics, the growth of which is usually associated with that of ξ (ref. 11). We find that χ * initially increases with particle volume fraction, but drops markedly very close to jamming. We show that this behaviour results from the competition between the growth of ξ and the reduced particle displacements associated with rearrangements in very dense suspensions, unveiling a richer-than-expected scenario.Dynamical heterogeneity is a key ingredient in many of the most advanced attempts to understand and rationalize the glass and the jamming transitions. The recent observation of a critical-like growth of temporal and spatial dynamical fluctuations in a two-dimensional athermal system approaching jamming 10 , similar to that hypothesized for glass-formers 12 , has raised hope that the glass and the jamming transitions may be unified, calling at the same time for further, tighter experimental verifications. Here, we investigate temporal dynamical heterogeneity in a three-dimensional thermal system, concentrated colloidal suspensions close to the maximum packing fraction. Temporal and spatial dynamical heterogeneity are usually closely related: the former can be quantified by a 'four-point dynamical susceptibility' χ 4 (the variance of a time-resolved correlation function describing the system relaxation), the amplitude of which is proportional to ξ 3 (refs 11,13,14). Surprisingly, we find that very close to jamming, temporal and spatial dynamical heterogeneity decouple: whereas ξ continuously grows with volume fraction, the amplitude of temporal fluctuations drops sharply close to the maximum packing fraction. These findings challenge current scenarios where the slowing down of the dynamics on approaching jamming is accompanied by enhanced temporal fluctuations of the dynamics.The dynamics of colloidal hard spheres slows down markedly close to ϕ = ϕ g ≈ 0.58; beyond ϕ g , ultraslow relaxations on short length scales are still observed 3 , until dynamics freeze at the maximum (random) packing fraction, ϕ max . We study concentrated suspensions of polyvinyl chloride xenospheres 15 suspended in dioctyl p...

We present a comprehensive study of the slip and flow of concentrated colloidal suspensions using cone-plate rheometry and simultaneous confocal imaging. In the colloidal glass regime, for smooth, non-stick walls, the solid nature of the suspension causes a transition in the rheology from Herschel-Bulkley (HB) bulk flow behavior at large stress to a Bingham-like slip behavior at low stress, which is suppressed for sufficient colloid-wall attraction or colloid-scale wall roughness. Visualization shows how the slip-shear transition depends on gap size and the boundary conditions at both walls and that partial slip persist well above the yield stress. A phenomenological model, incorporating the Bingham slip law and HB bulk flow, fully accounts for the behavior. Microscopically, the Bingham law is related to a thin (sub-colloidal) lubrication layer at the wall, giving rise to a characteristic dependence of slip parameters on particle size and concentration. We relate this to the suspension's osmotic pressure and yield stress and also analyze the influence of van der Waals interaction. For the largest concentrations, we observe non-uniform flow around the yield stress, in line with recent work on bulk shear-banding of concentrated pastes. We also describe residual slip in concentrated liquid suspensions, where the vanishing yield stress causes coexistence of (weak) slip and bulk shear flow for all measured rates.

We investigate the time-dependent rheology and slip behaviour of colloidal gels formed under polymerinduced depletion attraction. The shape of the flow curves at low applied shear rates is suggestive of slip, which we confirm using confocal imaging. Time-dependent linear viscoelastic measurements show an unexpected drop of the elastic modulus below the viscous one after a critical time. We present a dynamic phase diagram characterizing the dependence of slip on polymer concentration and colloid volume fraction. Confocal imaging links slip to the restructuring of clusters with time, which leads to a reduction of the number of contacts between the colloidal network and the rheometer surfaces. Such behaviour is shear rate dependent and correlated to changes in the gel structure, which changed from independent small aggregates at high shear rates to percolated clusters at low shear rates.

Dispersions of platelets in the nematic phase are submitted to large amplitude oscillatory shear flow and probed by high temporal resolution small angle x-ray scattering. The response displays rich dynamic and structural behavior. Under small amplitude deformations we observe an elastic response, while structurally symmetry is broken: a preferential direction of deformation is selected which induces off-plane orientation of the platelets. We associate the elastic responses with the tilting director of the platelets towards the flow direction at all strain amplitudes. At large strain amplitudes there is a yielding transition between elastic and plastic deformation, accompanied by a flipping of the director. At intermediate strain amplitudes the director has a rich dynamic behavior, illustrating the complex motion of platelets in shear flow. These observations are confirmed by steady-shear flow reversal experiments, which underline the unique character of sheared nematic platelet dispersions. DOI: 10.1103/PhysRevLett.109.246001 PACS numbers: 83.85.Hf, 83.80.Xz, 82.70.Dd Liquid crystals form a unique class of materials due to the combination of highly ordered structure with low mechanical modulus. This is most apparent for dispersions of colloidal rods, where early work of Zocher [1] and Bawden et al.[2] exemplified ordering for very low volume fractions of rods. These observations led Onsager to his seminal thermodynamic description of the relation between crowding and structure [3], used in a variety of fields. The coupling between structure and mechanical properties is rich and nonlinear, although the mechanical modulus of such colloidal dispersions can be very low. There exists a good level of understanding of the interplay between steady shear flow and the different elastic contributions of the nematic structure, as well as the complex response of the director field, which describes the average orientation of the anisotropic particles [4][5][6]. For this reason sheared nematics are benchmark systems for complex flow studies. Even so, there is a dearth of studies on the viscoelastic character of colloidal liquid crystals. Moreover, the flow behavior of the nematic phase of colloidal platelets is poorly studied, although they are the most ubiquitous colloids in nature and have many practical applications [7].In this Letter we report the link between the structural and mechanical response of nematic colloidal platelets to large amplitude oscillatory shear flow (LAOS) and steady shear flow reversals. Colloidal platelets, especially clays, often form gels at high concentrations instead of a nematic phase. Although this complicates a fundamental understanding [8], it does result in strong shear-thinning effects due to sheared-induced breakup of these gels [9]. Shear thinning of charge-stabilized systems in the isotropic phase, as studied with in situ small angle x-ray scattering (SAXS), is interpreted in terms of a shear-dependent effective volume fraction due to the aligning platelets [10] that can cause Taylor...

We investigate the aging behavior of glassy suspensions of nearly hard-sphere colloids submitted to a constant shear stress. For low stresses, below the yield stress, the system is subject to creep motion. As the sample ages, the shear rate exhibits a power-law decrease with time with exponents that depend on the sample age. We use a combination of rheological experiments with time-resolved photon correlation spectroscopy to investigate the time evolution of the sample dynamics under shear on various time and length scales. Long-time light-scattering experiments reveal the occurrence of microscopic rearrangement events that are linked with the macroscopic strain deformation of the sample. Dynamic time sweep experiments indicate that while the internal microscopic dynamics slow down continuously with waiting time, the storage and loss moduli are almost constant after a fast, weak decrease, resembling the behavior of quenched systems with partially frozen-in stresses.