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.

We investigate the structural, dynamical, and viscoelastic properties of colloid-polymer mixtures at intermediate colloid volume fraction and varying polymer concentrations, thereby tuning the attractive interactions. Within the examined range of polymer concentrations, the samples varied from fluids to gels. In the liquid phase, an increasing correlation length of the density fluctuations when approaching the gelation boundary was observed by static light scattering and microscopy, indicating clustering and formation of space-spanning networks. Simultaneously, the correlation function determined by dynamic light scattering decays completely, indicating the absence of dynamical arrest. Clustering and formation of transient networks when approaching the gelation boundary is supported by significant changes in the viscoelastic properties of the samples. Upon increasing the polymer concentration beyond the gelation boundary, the rheological properties changed qualitatively again, now they are consistent with the formation of colloidal gels. Our experimental results, namely, the location of the gelation boundary as well as the elastic (storage) and viscous (loss) moduli, are compared to different theoretical models. These include consideration of the escape time as well as predictions for the viscoelastic moduli based on scaling relations and mode coupling theories.

The history dependence of the glasses formed from flow-melted steady states by a sudden cessation of the shear rateγ is studied in colloidal suspensions, by molecular dynamics simulations, and modecoupling theory. In an ideal glass, stresses relax only partially, leaving behind a finite persistent residual stress. For intermediate times, relaxation curves scale as a function ofγt, even though no flow is present. The macroscopic stress evolution is connected to a length scale of residual liquefaction displayed by microscopic mean-squared displacements. The theory describes this history dependence of glasses sharing the same thermodynamic state variables, but differing static properties.PACS numbers: 64.70. P-83.50.-v Materials are often produced by solidification from the melt, involving nonequilibrium quenches. This imprints a history-dependent microstructure that strongly affects macroscopic material properties. One example is residual stresses [1,2]: if particle configurations cannot fully relax to equilibrium, some of the stresses, arising in the presence of flow in the melt, persist in the solid.Small glass droplets (known as Prince Rupert's drops or Dutch tears since the 17th century) vividly display the effects of residual stresses [3]: they withstand the blow of a hammer onto their main body, but explode when the slightest damage is inflicted upon their tail (releasing the frozen-in stress network). Today, safety glass and "Gorilla glass" covers for smartphones are deliberately pre-stressed during production to strengthen them. A theoretical understanding of residual stresses and their microscopic origins is however still not achieved.We seek to understand generic mechanisms by which residual stresses arise. A convenient starting point is to investigate the stress relaxation σ(t) following the cessation of shear flow of rateγ, from a well-defined nonequilibrium stationary state (NESS). Such "mechanical quenches" are ubiquitous in soft matter, where pre-shear is applied to "rejuvenate" the otherwise ill-defined glassy state [4][5][6][7]. For these systems, the soft-glassy rheology model (SGR) [8] predicts asymptotic power laws that imply the relaxation of stresses to zero [9]. In the following, we will reserve the term residual stress to describe a finite, persistent stress remaining in the (ideal) glass even at arbitrarily large times after the cessation of flow.In addition to macroscopic rheology, we investigate the evolution of the microscopic dynamics as characterized by the waiting-time dependent mean-squared displacements (MSD). The latter reveal the dynamical shrinkage of shear-fluidized regions after cessation, and phenomena akin to, yet different from the intensely studied aging dynamics after thermal quenches [10,11].Experiments on a variety of colloidal suspensions, together with molecular-dynamics (MD) simulations, provide a coherent qualitative picture that can be rationalized by mode-coupling theory of the glass transition (MCT) [12] within the integration-through-transients (ITT) formalis...

Orientation and rupture of fractal colloidal gels during start-up of steady shear flow J. Rheol. 49, 657 (2005) AbstractThe transient response of model hard sphere glasses is examined during the application of steady rate start-up shear using Brownian dynamics simulations, experimental rheology and confocal microscopy. With increasing strain, the glass initially exhibits an almost linear elastic stress increase, a stress peak at the yield point and then reaches a constant steady state. The stress overshoot has a nonmonotonic dependence with Peclet number, Pe, and volume fraction, u, determined by the available free volume and a competition between structural relaxation and shear advection. Examination of the structural properties under shear revealed an increasing anisotropic radial distribution function, g(r), mostly in the velocity-gradient (xy) plane, which decreases after the stress peak with considerable anisotropy remaining in the steady-state. Low rates minimally distort the structure, while high rates show distortion with signatures of transient elongation. As a mechanism of storing energy, particles are trapped within a cage distorted more than Brownian relaxation allows, while at larger strains, stresses are relaxed as particles are forced out of the cage due to advection. Even in the steady state, intermediate super diffusion is observed at high rates and is a signature of the continuous breaking and reformation of cages under shear. V C 2016 The Society of Rheology.

Starlike PEP-PEO block copolymer micelles offer the possibility to investigate the phase behavior and interactions of regular star polymers (ultrasoft colloids). Micellar functionality f can be smoothly varied by changing solvent composition (interfacial tension). Structure factors obtained by small-angle neutron-scattering can be quantitatively described in terms of an effective potential for star polymers. The experimental phase diagram reproduces to a high level of accuracy the predicted liquid-solid transition. Whereas for intermediate f a bcc phase is observed, for high f the formation of a fcc phase is preempted by glass formation.