  One of the most interesting phenomena in the soft-matter realm consists in
the spontaneous formation of super-molecular structures (microphases) in
condition of thermodynamic equilibrium. A simple mechanism responsible for this
self-organization or pattern formation is based on the competition between
attractive and repulsive forces with different length scales in the microscopic
potential, typically, a short-range attraction against a longer-range
repulsion.
  We analyse this problem by simulations in 2D fluids. We find that, as the
temperature is lowered, liquid-vapor phase separation is inhibited by the
competition between attraction and repulsion, and replaced by a transition to
non-homogeneous phases. The structure of the fluid shows well defined
signatures of the presence of both intra- and inter-cluster correlations.
  Even when the competition between attraction and repulsion is not so strong
as to cause microphase formation, it still induces large density fluctuations
in a wide region of the temperature-density plane. In this large-fluctuation
regime, pattern formation can be triggered by a weak external modulating field.
