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

C Avendano and G Jackson and HH Wensink, MOLECULAR PHYSICS, 116, 2901-2910 (2018).

DOI: 10.1080/00268976.2018.1484950

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 liquid-crystalline 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. Avendano, G. Jackson, E.A. Muller and F.A. Escobedo, Proc. Natl. Acad. Sci. U.S.A. 113, 9699 (2016); H.H. Wensink and C. Avendano, Phys. Rev. E 94, 062704 (2016). GRAPHICS .

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