  Transitions in structural heterogeneity of colloidal depletion gels formed
through short-range attractive interactions are correlated with their dynamical
arrest. The system is a density and refractive index matched suspension of 0.20
volume fraction poly(methyl methacyrlate) colloids with the non-adsorbing
depletant polystyrene added at a size ratio of depletant to colloid of 0.043.
As the strength of the short-range attractive interaction is increased,
clusters become increasingly structurally heterogeneous, as characterized by
number-density fluctuations, and dynamically immobilized, as characterized by
the single-particle mean-squared displacement. The number of free colloids in
the suspension also progressively declines. As an immobile cluster to gel
transition is traversed, structural heterogeneity abruptly decreases.
Simultaneously, the mean single-particle dynamics saturates at a localization
length on the order of the short-range attractive potential range. Both
immobile cluster and gel regimes show dynamical heterogeneity. Non-Gaussian
distributions of single particle displacements reveal enhanced populations of
dynamical trajectories localized on two different length scales. Similar
dependencies of number density fluctuations, free particle number and dynamical
length scales on the order of the range of short-range attraction suggests a
collective structural origin of dynamic heterogeneity in colloidal gels.
