Nanorings in planar confinement: the role of repulsive surfaces on the formation of lacuna smectics

Abstract

We study the structure and liquid-crystalline phase behaviour of a model of confined non-convex circular soft-repulsive nanorings in a planar slit geometry using molecular-dynamics simulation. The separation distance between the structureless parallel soft-repulsive walls is made large enough to allow for the formation of a distinct bulk phase in the central region of the box which is in coexistence with the adsorbed fluid thus allowing the analysis of single-wall effects. As the density of the particles is increased, the fluid adsorbs (wets) onto the planar surfaces leading to the formation of well-defined smectic-A layers with a spacing proportional to the diameter of the rings. An analysis of the nematic order parameter at distances perpendicular to the surface reveals that the particles in each layer exhibit anti-nematic behaviour and planar (edge-on) anchoring relative to the short symmetry axis of the rings. This behaviour is in stark contrast to the behaviour observed in convex disc-like particles that have the tendency to form nematic (discotic) structures with homeotropic (face-on) anchoring. The smectic phases formed by nanorings in the bulk and under confinement are characterised by the formation of low-density layered liquid-crystalline states with large voids, referred to here as lacuna smectic phases. In contrast to what is typically found for confined liquidcrystalline systems involving convex particles, no apparent biaxiality is found for nanorings in planar confinement. We argue that formation of the low-density lacuna smectic layers with planar anchoring is a consequence of the non-convex shape of the circular rings that allow for interpenetration between the particles as observed for nanorings under bulk conditions [C. Avendaño, G.

Ordering of Oblate Hard Particles between Hybrid Penetrable Walls

“…Avendaño et al . 15 simulated soft-repulsive rings between parallel, soft-repulsive walls, with results that are in stark contrast to the behavior of convex DLCs. Finally, other numerical studies of confined DLCs have employed the popular Gay–Berne (GB) model and more complex wall–particle interactions.…”

Ordering of oblate hard particles between symmetric penetrable walls

“…Hard walls were revealed to induce homeotropic (face-on) anchoring and stabilise the N phase, whereas adsorbent walls promoted planar (edge-on) alignment and a more disordered bulk. Avendaño et al [20] have reported what is, to our knowledge, the only computer simulation of a non-convex DLC composed of softrepulsive rings between parallel, soft-repulsive walls. These tend to align edge-on, forming low-density smectic layers with anti-nematic order and no biaxiality, in stark contrast to the behaviour of convex DLCs.…”

Engineering porous two-dimensional lattices via self-assembly of non-convex hexagonal platelets

“…[21][22][23][24][25][26] Self-assembly of particles with non-convex shapes has also been used for the design and fabrication of highly open structures as in most cases their shapes, even at their most efficient possible packing attained, hinder the formation of closed-packed structures. [27][28][29] For example, ring-shaped [30][31][32][33][34][35] wireframe, 36 and branched particles [37][38][39][40] exhibit the formation of three-and quasi-two-dimensional highly open porous structures when the particle geometry is highly non-convex. Examples of non-convex particles confined in 2D have also been studied experimentally and in simulations.…”