String formation and demixing in monolayers of dipolar colloidal mixtures

Employing hypernetted chain (HNC) integral equations and a stability analysis we investigate the structure and phase behavior of bidisperse mixtures of dipolar hard spheres with different size ratios s=σ(S)/σ(L) confined to a plane. The dipole moments of the particles are perfectly ordered along an in-plane direction, yielding anisotropic interactions favoring chain formation. Exploring a range of size ratios and compositions, our study predicts a complex interplay between aggregation phenomena, on the one hand, and volume phase transitions, on the other hand. In dilute, strongly asymmetric systems (s = 0.5), our HNC analysis indicates chain formation of the large particles, while the small particles act as a weakly correlated background. According to our fluctuation analysis, this aggregation behavior results in combined condensation-demixing transitions, with a trend towards pure demixing when the concentration of the large particles, c(L), becomes small. In dense systems, the most interesting results are found for intermediate size ratios, s ~ 0.7-0.8. Here we find signatures of a concentration-driven transition from pure chains of large particles (large c(L)) to mixed chains with alternating order of large and small particles (small c(L)). The two regimes are separated by a characteristic "jump" in the HNC non-solution line.