  By means of molecular dynamics, we study the structure and the dynamics of a
microscopic model for colloidal gels at low volume fractions. The presence of
directional interactions leads to the formation of a persistent interconnected
network at temperatures where phase separation does not occur. We find that
large scale spatial correlations strongly depend on the volume fraction and
characterize the formation of the persistent network. We observe a pre-peak in
the static structure factor and relate it to the network structure. The slow
dynamics at gelation is characterized by the coexistence of fast collective
motion of the mobile parts of the network structure (chains) with large scale
rearrangements producing stretched exponential relaxations. We show that, once
the network is sufficiently persistent, it induces slow, cooperative processes
related to the network nodes. We suggest that this peculiar glassy dynamics is
a hallmark of the physics of colloidal gels at low volume fractions.
