  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 collec- tives 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, 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.
