  Recent computational investigations in polymeric and non-polymeric binary
mixtures have reported anomalous relaxation features when both components
exhibit very different mobilities. Anomalous relaxation is characterized by
sublinear power law behaviour for mean squared displacements, logarithmic decay
in dynamic correlators, and a striking concave-to-convex crossover in the
latters by tuning the relevant control parameter, in analogy with predictions
of the Mode Coupling Theory for state points close to higher-order transitions.
We present Monte Carlo simulations on a coarse-grained model for relaxation in
binary mixtures. The liquid structure is substituted by a three-dimensional
array of cells. A spin variable is assigned to each cell, representing
unexcited and excited local states of a mobility field. Changes in local
mobility (spin flip) are permitted according to kinetic constraints determined
by the mobilities of the neighbouring cells. We introduce two types of cells
(``fast'' and ``slow'') with very different rates for spin flip. This
coarse-grained model qualitatively reproduces the mentioned anomalous
relaxation features observed for real binary mixtures.
