Splay-induced order in systems of hard tapers

Abstract

The main objective of this work is to clarify the role that taper-shaped elongated molecules, i.e., molecules with one end wider than the other, can play in stabilizing orientational order. The focus is exclusively on entropy-driven self-organization induced by purely excluded volume interactions. Drawing an analogy to RM734 (4-[(4-nitrophenoxy)carbonyl]phenyl-2,4-dimethoxybenzoate), which is known to stabilize ferroelectric nematic (${\text{N}}_{\text{F}}$) and nematic splay (${\text{N}}_{\text{S}}$) phases, and assuming that molecular biaxiality is of secondary importance, we consider monodisperse systems composed of hard molecules. Each molecule is modeled using six collinear tangent spheres with linearly decreasing diameters. Through hard-particle, constant-pressure Monte Carlo simulations, we study the emergent phases as functions of the ratio between the smallest and largest diameters of the spheres (denoted as $d$) and the packing fraction ($\ensuremath{\eta}$). To analyze global and local molecular orderings, we examine molecular configurations in terms of nematic, smectic, and hexatic order parameters. Additionally, we investigate the radial pair distribution function, polarization correlation function, and the histogram of angles between molecular axes. The last characteristic is utilized to quantify local splay. The findings reveal that splay-induced deformations drive unusual long-range orientational order at relatively high packing fractions ($\ensuremath{\eta}>0.5$), corresponding to crystalline phases. When $\ensuremath{\eta}<0.5$, only short-range order is affected, and in addition to the isotropic liquid, only the standard nematic and smectic-A liquid crystalline phases are stabilized. However, for $\ensuremath{\eta}>0.5$, apart from the ordinary nonpolar hexagonal crystal, three additional frustrated crystalline polar blue phases with long-range splay modulation are observed: antiferroelectric splay crystal (${\text{Cr}}_{\text{S}}{\text{P}}_{\text{A}}$), antiferroelectric double-splay crystal (${\text{Cr}}_{\text{DS}}{\text{P}}_{\text{A}}$), and ferroelectric double-splay crystal (${\text{Cr}}_{\text{DS}}{\text{P}}_{\text{F}}$). Finally, we employ Onsager-Parsons-Lee local density functional theory to investigate whether any sterically induced (anti)ferroelectric nematic or smectic-A type of ordering is possible for our system, at least in a metastable regime.