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.

The rheological response, in particular the non-linear response, to oscillatory shear is experimentally investigated in colloidal glasses. The glasses are highly concentrated binary hard-sphere mixtures with relatively large size disparities. For a size ratio of 0.2, a strong reduction of the normalized elastic moduli, the yield strain and stress and, for some samples, even melting of the glass to a fluid is observed upon addition of the second species. This is attributed to the more efficient packing, as indicated by the shift of random close packing to larger total volume fractions. This leads to an increase in free volume which favours cage deformations and hence a loosening of the cage. Cage deformations are also favoured by the structural heterogeneity introduced by the second species. For a limited parameter range, we furthermore found indications of two-step yielding, as has been reported previously for attractive glasses. In samples containing spheres with more comparable sizes, namely a size ratio of 0.38, the cage seems less distorted and structural heterogeneities on larger length scales seem to become important. The limited structural changes are reflected in only a small reduction of the moduli, yield strain and stress.Comment: 9 pages, 8 figures, accepted in Soft Matte

We investigate, using simultaneous rheology and confocal microscopy, the time-dependent stress response and transient single-particle dynamics following a step change in shear rate in binary colloidal glasses with large dynamical asymmetry and different mixing ratios. The transition from solid-like response to flow is characterised by a stress overshoot, whose magnitude is linked to transient superdiffusive dynamics as well as cage compression effects. These and the yield strain at which the overshoot occurs vary with the mixing ratio, and hence the prevailing caging mechanism. The yielding and stress storage are dominated by dynamics on different time and length scales, the short-time in-cage dynamics and the long-time structural relaxation respectively. These time scales and their relation to the characteristic time associated with the applied shear, namely the inverse shear rate, result in two different and distinct regimes of the shear rate dependencies of the yield strain and the magnitude of the stress overshoot.