Isotropic–nematic phase transitions in confined mesogenic fluids. The role of substrate anchoring

Abstract

The anchoring of rodlike liquid-crystalline molecules at solid surfaces plays an important role in the design of novel nanotechnological devices such as, for example, biosensors. In this work we investigate the impact of various anchoring scenarios on the isotropic-nematic phase transition of weakly anisometric, prolate mesogens confined to a mesoscopic slit-pore. We employ isostress ensemble Monte Carlo simulations in which the isotropic-nematic transition is driven by applying an external stress in the direction parallel to the substrate plane. The fluid-fluid interaction is described by a Lennard-Jones potential with an orientation-dependent attractive term. Our simulations show that different anchoring scenarios may shift the isotropic-nematic phase transition considerably. We locate this transition through response functions such as an isostress heat capacity, isothermal compressibility, and the Maier-Saupe order parameter. Our results suggest that for the present model system the isotropicnematic transition is likely to be continuous. For the planar anchoring scenario we observe the formation of a new layer of fluid molecules during a structural transformation preceding the isotropicnematic transition.

Nanoconfinement-Induced Structures in Chiral Liquid Crystals

“…For the anisotropy parameters chosen throughout this work [see Section 2, Equation (4)] the aspect ratio of the mesogens was determined to be 1.26 [34], taking the ratios of zeros of ϕ ff ( r ij , û i , û j ) for the side-side ( û i · r̂ ij = 0) and end-end configurations ( | û i · r̂ ij | = 1) of a pair of parallel aligned mesogens ( | û i · û j | = 1) as a definition of their aspect ratio. This rather small value turns out to be advantageous, as it supports fast equilibration of the simulation sample.…”

“…As pointed out in the Appendix of [34], one may employ the hypervirial theorem (see Appendix E.2 of [44]) and write:…”

“…Focusing only on the diagonal components, P xx and P yy , one finds, after some straightforward algebra [34]:…”

Imprinting substrate structures onto a nematic liquid crystal

“…denotes a unit vector. As in our previous work [19][20][21][22] we decompose the intermolecular interaction potential into an isotropic and an anisotropic contribution,…”

“…This procedure follows that proposed earlier by Andrienko et al in their study of topological defects in a liquid crystal. 30 Notice, however, that the computation of eigenvalues and eigenvectors is based upon an already ensemble-averaged Q(x, z) whereas conventionally 19,[31][32][33][34] the instantaneous alignment tensor is used to compute its eigenvalues which are then ensemble-averaged subsequently. However, for reasons explained elsewhere 22 this approach could cause a substantial system size dependence of the local nematic order parameter that needs to be corrected for.…”