Convergence of the iterative process for the diagrammatic expansion as related to liquid structure and freezing

Like aggregation from unlike attraction: stripes in symmetric mixtures of cross-attracting hard spheres

Self-assembly of colloidal particles into striped phases is at once a process of relevant technological interest-just think about the possibility to realise photonic crystals with a dielectric structure modulated along a specific direction-and a challenging task, since striped patterns emerge in a variety of conditions, suggesting that the connection between the onset of stripes and the shape of the intermolecular potential is yet to be fully unravelled. Hereby, we devise an elementary mechanism for the formation of stripes in a basic model consisting of a symmetric binary mixture of hard spheres that interact via a square-well cross attraction. Such a model would mimic a colloid in which the interspecies affinity is of longer range and significantly stronger than the intraspecies interaction. For attraction ranges shorter enough than the particle size the mixture behaves like a compositionally-disordered simple fluid. Instead, for wider square-wells, we document by numerical simulations the existence of striped patterns in the solid phase, where layers of particles of one species are interspersed with layers of the other species; increasing the attraction range stabilises the stripes further, in that they also appear in the bulk liquid and become thicker in the crystal. Our results lead to the counterintuitive conclusion that a flat and sufficiently long-ranged unlike attraction promotes the aggregation of like particles into stripes. This finding opens a novel way for the synthesis of colloidal particles with interactions tailored at the development of stripe-modulated structures.

Structural stability of simple classical fluids: universal properties of the lyapunov-exponent measure

A threshold for the stability of the solution of integral equations for the pair correlation function of a classical fluid can be determined from the Floquet matrix for the iterative form of the integral equation. Correspondingly, a measure of the structural stability of the fluid, analogous to the Lindemann ratio for a solid, is provided by the Lyapunov exponent lambda that is related to the perturbed dynamics. The behavior of lambda as a function of density, temperature, interatomic potential, and closure relations for the integral equation, is analyzed and discussed. In analogy with the Lindemann parameter, we find-for the hypernetted-chain-type closures-that lambda(T/T(inst)) is "quasiuniversal," i.e., very weakly dependent on the interaction potential, up to a temperature T/T(inst) approximately 5, where T(inst) is the stability-threshold temperature. We show how this result connects the Lyapunov exponent measure of the pair structure with the equation of state of the fluid.