  Dynamical heterogeneities in a colloidal fluid close to gelation are studied
by means of computer simulations. A clear distinction between some fast
particles and the rest, slow ones, is observed, yielding a picture of the gel
composed by two populations with different mobilities. Analyzing the statics
and dynamics of both sets of particles, it is shown that the slow particles
form a network of stuck particles, whereas the fast ones are able to move over
long distances. Correlation functions show that the environment of the fast
particles relaxes much faster than that of the slow ones, but at short times
the bonds between fast particles are longer lived due to the flexibility of
their structure. No string-like motion is observed for the fast particles, but
they occupy preferential sites in the surface of the structure formed by the
slow ones.
