![]() So far the slow movement of the host matrix has been largely ignored, despite representing realistic situations of biological 11, 12, 13, 14, 15, 16 and industrial interest 17, 18, 19, 20, 21, 22, 23. Softness of the immobile particles or interactions among the intruders are known to modify this picture 5, 6, 7, 8, 9, 10. When a critical density of immobile particles is reached, they percolate and the intruder becomes localized 3. Their motion becomes sub-diffusive once the voids are barely interconnected. In the Lorentz gas 3, 4, the prototype model for anomalous transport, point-like intruders move in voids between immobile, randomly-distributed particles. Anomalous behaviour, usually manifested by the presence of sub-diffusivity 1, 2, emerges as a common feature of the dynamics. In the presence of a confining medium, the transport of objects deviates from normal diffusion. 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 demonstrate that the matrix mobility is central for the observed anomalous behaviour. 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. ![]() 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. These are examples of motion through crowded environments, in which the host matrix may retain some glass-like dynamics. 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.
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