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- Very low surface tension liquid–vapour interfaces of patchy colloidsPublication . Teixeira, PauloThe liquid–vapour interface of a model of one-component patchy colloids is re-visited. The model consistsof hard spheres decorated with short-ranged attractive sites (‘patches’) of different types on their surfaces.In an earlier paper [A. Oleksy and P.I.C. Teixeira, Phys. Rev. E 91 (2015) 012301], the density-functionaltheory (DFT) form of Wertheim’s first-order perturbation theory (TPT1) was used to study a realisationof this model with two patches of type A and nine patches of type B (2A9B colloids), which exhibit re-entrant liquid–vapour coexistence curves and very low-density liquid phases. Here, it is shown that thenon-monotonic temperature dependence of the surface tension and interface thickness in this modelcan be qualitatively reproduced by a local DFT, where the spatial non-uniformity is introduced throughthe square gradient of the density multiplied by a density-dependent prefactor. This simpler theory isthen applied to colloids decorated with two patches of type A and ten patches of type B (2A10B colloids),and interactions chosen so as to exhibit a closed-loop phase diagram when only AA and AB bonds arepresent [N.G. Almarza et al., J. Chem. Phys. 137 (2012) 244902]. In this case, the surface tension goesto zero at both upper and lower critical points, with a maximum at some intermediate temperature. AsBB attractions are gradually switched on, the lower critical point disappears and the surface tension vstemperature curve acquires a very deep minimum at a temperature lower than that of the maximum. Onfurther increasing the strength of the BB attractions, both the minimum and the maximum disappear andthe surface tension becomes a monotonically decreasing function of the temperature, as in atomic fluids.The very low surface tension at the minimum for weak BB attractions, combined with the ability to formphysical gels, suggests that 2AnB colloids might be exploited as temperature-controlled surfactants andfoam stabilizers.
- Phase behaviour of pure and mixed patchy colloids - theory and simulationPublication . Teixeira, Paulo; Tavares, JoseWe review the phase behaviour of pure and mixed patchy colloids, as revealed (mostly) by theory and computer simulation. These experimentally-realisable systems are excellent models for investigating the general problem of the interplay between (equilibrium) phase transitions and self-assembly in soft condensed matter. We focus on how liquid vapour condensation can be pre-empted by the formation of different types of aggregates, in particular rings, which we argue is relevant to the criticality of empty fluids and network fluids, and possibly also of dipolar fluids. In this connection we also discuss percolation and gelation in pure and mixed patchy colloids. Finally, we describe the rich phase behaviour of (mostly binary) patchy colloid mixtures.
- How patchy can one get and still condense? The role of dissimilar patches in the interactions of colloidal particlesPublication . Tavares, Jose; Teixeira, Paulo; Gama, MargaridaWe investigate the influence of strong directional, or bonding, interactions on the phase diagram of complex fluids, and in particular on the liquid-vapour critical point. To this end we revisit a simple model and theory for associating fluids which consist of spherical particles having a hard-core repulsion, complemented by three short-ranged attractive sites on the surface (sticky spots). Two of the spots are of type A and one is of type B; the interactions between each pair of spots have strengths [image omitted], [image omitted] and [image omitted]. The theory is applied over the whole range of bonding strengths and results are interpreted in terms of the equilibrium cluster structures of the coexisting phases. In systems where unlike sites do not interact (i.e. where [image omitted]), the critical point exists all the way to [image omitted]. By contrast, when [image omitted], there is no critical point below a certain finite value of [image omitted]. These somewhat surprising results are rationalised in terms of the different network structures of the two systems: two long AA chains are linked by one BB bond (X-junction) in the former case, and by one AB bond (Y-junction) in the latter. The vapour-liquid transition may then be viewed as the condensation of these junctions and we find that X-junctions condense for any attractive [image omitted] (i.e. for any fraction of BB bonds), whereas condensation of the Y-junctions requires that [image omitted] be above a finite threshold (i.e. there must be a finite fraction of AB bonds).
- Interplay between self-assembly and condensation in models with asymmetric patchesPublication . Tavares, Jose; Teixeira, Paulo