Intermediate range order and structure in colloidal dispersions with competing interactions.

Colloidal dispersions with a short-range attraction and long-range repulsion can exhibit an intriguing intermediate range order, manifested in scattering experiments as a low-q peak in the structure factor. Monte Carlo simulations are performed on fluids that exhibit intermediate range order to explicitly determine its connection to a possible state of microphase separation, equilibrium clustering. This is accomplished by decomposing the structure factor into cluster-cluster, monomer-monomer, and cross-correlations that cannot be extracted from experimental scattering patterns. Our simulation results indicate that the intermediate range order arises from either monomeric or cluster species, depending on solution conditions, and reflects the presence of a preferred length scale that is not trivially related to the interparticle potential. Further, criteria are established to define monomer, cluster, and percolated states in these systems that facilitate further studies. Combining scattering techniques with simulations provides an effective method for identifying clustered states in complex fluids.