  The increase of the viscosity, which is observed in attractive colloidal
systems by varying the temperature or the volume fraction, can be related to
the formation of structures due to particle aggregation. In particular we have
studied the non trivial dependence of the viscosity from the temperature and
the volume fraction in the copolymer-micellar system L64. The comparison of the
experimental data with the results of numerical simulations in a simple model
for gelation phenomena suggests that this intriguing behavior can be explained
in terms of cluster formation and that this picture can be quite generally
extended to other attractive colloidal systems.
