Local order and cluster formation in model fluids with competing interactions: A simulation and theoretical study

In a preliminary study [Phys. Chem. Chem. Phys., 2017, 19, 15247], we have recently documented an elusive mechanism underlying the cluster formation in model fluids with microscopic competing interactions (hard-sphere two-Yukawa). This mechanism consists in a tiny rearrangement of a distant correlation peak in the local density profile. For weak attractions, this peak contributes to the shallow, long-wave oscillation typical of such fluids; as the attraction strengthens, such a portion progressively disengages from the long-range behaviour, and moving backwards takes on the character of a new shell of neighbours, falling beyond the existing ones at shorter distances. This "reversal of trend"-despite its tiny size, in comparison with the overall aspect of the density profile-is shown to precisely occur at the onset of clustering. The scope of the present study is twofold. In the first instance, we positively assess our preliminary finding. To this aim we have studied by Monte Carlo simulations different families of two-Yukawa fluids, under the same conditions investigated in the original paper, namely fixed temperature, high fluid-density and increasingly attractive strength. Apparently, the reversal of trend in spatial correlations sets as a sensitive criterion to identify the clustering threshold, complementing other common indicators, based on the modifications undergone by the low-wavevector peak in the structure factor. Secondly, we document the accuracy of the Hypernetted Chain theory in predicting the spatial rearrangement under scrutiny. This evidence paves the way to an extended investigation of the observed phenomenology by the complementary use of theoretical and simulation tools.