Nematic liquid crystals of bifunctional patchy spheres.

Anisotropic interactions can bring about the formation, through self-assembly, of semi-flexible chains, which in turn can give rise to nematic phases for suitable temperatures and concentrations. A minimalist model constituted of hard cylinders decorated with attractive sites has been already extensively studied numerically. Simulation data shows that a theoretical approach recently proposed is able to properly capture the physical properties of these self-assembly-driven liquid crystals. Here, we investigated a simpler model constituted of bifunctional Kern-Frenkel hard spheres which does not possess steric anisotropy but which can undergo a istropic-nematic transition as a result of their self-assembly into semi-flexible chains. For this model we compare an accurate numerical estimate of isotropic-nematic phase boundaries with theoretical predictions. The theoretical treatment, originally proposed for cylinder-like particles, has been greatly simplified and its predictions are in good agreement with numerical results. Finally, we also assess a crucial, and not obvious, hypothesis used in the theory, i.e. the ability of the Onsager trial function to properly model particle orientation in the presence of aggregation, that has not been properly checked yet.

Self-assembly of mesogenic bent-core DNA nanoduplexes.

Short cylinder-like DNA duplexes, comprising 6 to 20 base pairs, self-assemble into semi-flexible chains, due to coaxial stacking interactions between their blunt ends. The mutual alignment of these chains gives rise to macroscopically orientationally ordered liquid crystal phases. Interestingly, experiments show that the isotropic-nematic phase boundary is sequence-dependent. We perform all-atom simulations of several sequences to gain insights into the structural properties of the duplex and correlate the resulting geometric properties with the observed location of the isotropic-nematic phase boundary. We identify in the duplex bending the key parameter for explaining the sequence dependence, suggesting that DNA duplexes can be assimilated to bent-core mesogens. We also develop a coarse-grained model for the different DNA duplexes to evaluate in detail how bending affects the persistence length and excluded volume of the aggregates. This information is fed into a recently developed formalism to predict the isotropic-nematic phase boundary for bent-core mesogens. The theoretical results agree with the experimental observations.

Self-assembly-driven nematization.

The anisotropy of attractive interactions between particles can favor, through a self-assembly process, the formation of linear semi-flexible chains. In the appropriate temperatures and concentration ranges, the growing aspect ratio of the aggregates can induce formation of a nematic phase, as recently experimentally observed in several biologically relevant systems. We present here a numerical study of the isotropic-nematic phase boundary for a model of bifunctional polymerizing hard cylinders, to provide an accurate benchmark for recent theoretical approaches and to assess their ability to capture the coupling between self-assembly and orientational ordering. The comparison indicates the importance of properly modeling excluded volume and orientational entropy and provides a quantitative confirmation of some theoretical predictions.