  We demonstrate that an extended picture of kinetic constraints in
glass-forming liquids is sufficient to explain dynamic anomalies observed in
dense suspensions of strongly attracting colloidal particles. We augment a
simple model of heterogeneous relaxation with static attractions between
facilitating excitations, in a way that mimics the structural effect of
short-ranged interparticle attractions. The resulting spatial correlations
among facilitated and unfacilitated regions give rise to new relaxation
mechanisms that account for non-monotonic dependence of relaxation times on
attraction strength as well as logarithmic decay of density correlations in
time. These unusual features are a simple consequence of spatial segregation of
kinetic constraints, suggesting an alternative physical perspective on
attractive colloids than that suggested by mode-coupling theory. Based on the
behavior of our model, we predict a crossover from super-Arrhenius to Arrhenius
temperature dependence as attractions become dominant at fixed packing
fraction.
