Enhanced density fluctuations in fluid systems with competing interactions

The effects of competing interactions on the liquid-vapor transition in continuous systems have been studied by considering a simple hard-core model fluid with a two-Yukawa tail potential which is attractive at a short distance and repulsive at a long distance. We find that the size of the temperature-density region where the system experiences large density fluctuations without undergoing phase separation is greatly increased with respect to a fluid with a purely attractive tail potential. Moreover, in this region one observes a remarkable enhancement of the short-range correlations up to distances that can be more than one order of magnitude larger than the particle size. These effects are related with the occurrence of clouds of strongly correlated particles as a consequence of the competition between the attractive and the repulsive part of the interaction.