Ordering of oblate hard particles between penetrable walls

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Por Paulo Teixeira (Área Departamental de Física / ISEL e Centro de Física Teórica e Computacional / Ciências ULisboa).

In times such as these, when we spend long periods staring at computer screens, it is worthreminding ourselves what is inside most of them: liquid crystal (LC) films bounded bysubstrates that induce a specific alignment - termed anchoring - of the LC molecules.The study of films of LCs made up of elongated particles (not just molecules, but also colloidsor biomaterials), known as calamitic LCs, goes back a long way. Much less is known about LCswhose building blocks are plates or discs, known as discotic LCs (DLCs). Examples of the latterinclude certain compounds consisting of molecules with planar cores, or clay dispersions.Some of these DLCs are semiconductors, others excel as lubricants. They are, therefore, wellworthy of enquiry-We have investigated the structure of a DLC film confined between parallel, flat walls (slitgeometry). In our model, anchoring is tuned by varying the wall penetrability: DLC particles seeeach other as hard oblate ellipsoids (approximated by the hard Gaussian overlap (HGO)potential), but each wall sees a particle as a hard disc of zero thickness and diameter D lessthan or equal to that of the actual particle, σ 0 , embedded inside the particle and locatedhalfway along, and perpendicular to, its minor axis. The resulting anchoring, which can be setindependently at either wall, will be planar (edge-on) for D~0 and homeotropic (face-on) forD~σ 0 ,.We have used both Monte Carlo simulation and density-functional theory, the latterimplemented at the level of Onsager's second-virial approximation with (bulk) Parsons-Leerescaling, for both symmetric (identical walls) and hybrid (non-identical walls) confinement.We found that the theory can in some cases yield semi-quantitatively correct predictions forthe density and order parameter profiles, but is, in general, substantially less accurate than forprolate HGOs.

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