Abstract. We investigate stresses and particle motion during the start up of flow in a colloidal dispersion close to arrest into a glassy state. A combination of molecular dynamics simulation, mode coupling theory and confocal microscopy experiment is used to investigate the origins of the widely observed stress overshoot and (previously not reported) super-diffusive motion in the transient dynamics. A link between the macro-rheological stress versus strain curves and the microscopic particle motion is established. Negative correlations in the transient auto-correlation function of the potential stresses are found responsible for both phenomena, and arise even for homogeneous flows and almost Gaussian particle displacements.

Concentrated hard-sphere suspensions and glasses are investigated with rheometry, confocal microscopy, and Brownian dynamics simulations during start-up shear, providing a link between microstructure, dynamics, and rheology. The microstructural anisotropy is manifested in the extension axis where the maximum of the pair-distribution function exhibits a minimum at the stress overshoot. The interplay between Brownian relaxation and shear advection as well as the available free volume determine the structural anisotropy and the magnitude of the stress overshoot. Shear-induced cage deformation induces local constriction, reducing in-cage diffusion. Finally, a superdiffusive response at the steady state, with a minimum of the time-dependent effective diffusivity, reflects a continuous cage breakup and reformation. DOI: 10.1103/PhysRevLett.108.098303 PACS numbers: 82.70.Dd, 64.70.pv, 83.10.Mj, 83.80.Hj The fundamental understanding of the relation between microscopic structure, dynamics, and flow properties in complex yield stress materials is a challenging and open problem with widespread applications in metals, plastics, paints, slurries, etc.[1]. The glassy state, ubiquitous in natural, biological, and synthetic systems, also poses a frontier question in condensed matter physics. Colloidal glasses, associated with both phenomena, have received a lot of attention as model systems that may shed light on the glass transition and the flow of complex materials. Above a certain volume fraction, hard spheres (HS) form glasses characterized by a suppressed long-time diffusion [2,3] and a yield stress behavior [4]. Steady and oscillatory shear experiments show that, beyond a critical yield strain, they flow due to shear-induced cage breaking and irreversible out-of-cage particle rearrangements [5,6] with the stress and the structural relaxation rate increasing sublinearly with the shear rate [4,7], while slip and shear banding phenomena are detected at low rates [8,9]. Although the vast majority of studies investigate steady shear, the transient response encompasses the underlying mechanisms for shear melting and may provide insight on the glass state at rest.Step rate experiments and simulations performed on systems such as polymers [10], nanocomposites [11], metallic glasses [12], soft colloids [13], and colloidal gels [14] show an initial stress increase, often followed by a stress overshoot before the steady state is reached. For HS suspensions, mode coupling theory, molecular dynamics simulations, and confocal microscopy [15,16] indicate a relation of the stress overshoot with a superdiffusive particle motion attributed to negative correlations in the stress autocorrelation function. Beyond the mean-field-type approach of mode-coupling theory [17], however, a complete understanding linking the local microscopic structure and particle displacements at the level of the cage with macroscopic rheology during yielding is still lacking.Here, we investigate the start-up flow of model HS glasses and supercooled liquids usi...

SynopsisThe depletion attraction, induced upon addition of a nonadsorbing polymer to a colloidal solution, can lead to gel formation at sufficiently high polymer concentrations, which corresponds to strong attractive interactions. We have investigated the nonlinear rheological response, in particular the yielding, of colloidal gels with an intermediate volume fraction and variable interparticle attraction. Two distinct yielding processes are observed in both oscillatory experiments, namely, dynamic strain sweeps and transient experiments, here step rate, creep, and recovery tests. The first yielding process occurs at strains similar to the range of the interparticle potential and is interpreted as the breaking of bonds, which destroys the particle network and leads to individual clusters. The process of bond breaking is successfully modeled as the escape of a particle from the potential well of its nearest neighbor. The second yield point occurs at larger strains and is related to the deformation and fragmentation of clusters, consistent with the observed dependence of the yield strain on attraction. Both yield stresses exhibit a power-law dependence on attraction strength in agreement with observations of other systems and theoretical predictions. Furthermore, the observed two-step yielding reveals similarities, and also differences, to the rheology of attractive colloidal glasses.

Many natural and industrial processes rely on constrained transport, such as proteins moving through cells, particles confined in nanocomposite materials or gels, individuals in highly dense collectives and vehicular traffic conditions. These are examples of motion through crowded environments, in which the host matrix may retain some glass-like dynamics. Here we investigate constrained transport in a colloidal model system, in which dilute small spheres move in a slowly rearranging, glassy matrix of large spheres. Using confocal differential dynamic microscopy and simulations, here we discover a critical size asymmetry, at which anomalous collective transport of the small particles appears, manifested as a logarithmic decay of the density autocorrelation functions. We demonstrate that the matrix mobility is central for the observed anomalous behaviour. These results, crucially depending on size-induced dynamic asymmetry, are of relevance for a wide range of phenomena ranging from glassy systems to cell biology.