Nanoconfinement-Induced Structures in Chiral Liquid Crystals

Abstract

We employ Monte Carlo simulations in a specialized isothermal-isobaric and in the grand canonical ensemble to study structure formation in chiral liquid crystals as a function of molecular chirality. Our model potential consists of a simple Lennard-Jones potential, where the attractive contribution has been modified to represent the orientation dependence of the interaction between a pair of chiral liquid-crystal molecules. The liquid crystal is confined between a pair of planar and atomically smooth substrates onto which molecules are anchored in a hybrid fashion. Hybrid anchoring allows for the formation of helical structures in the direction perpendicular to the substrate plane without exposing the helix to spurious strains. At low chirality, we observe a cholesteric phase, which is transformed into a blue phase at higher chirality. More specifically, by studying the unit cell and the spatial arrangement of disclination lines, this blue phase can be established as blue phase II. If the distance between the confining substrates and molecular chirality are chosen properly, we see a third structure, which may be thought of as a hybrid, exhibiting mixed features of a cholesteric and a blue phase.

Defect topologies in chiral liquid crystals confined to mesoscopic channels

“…48 The nematic phase is characterized by a global director  n and a scalar nematic order parameter S. For |ε 3 | > 0, more complex ordered phases form that are dominated by helical structures such as a cholesteric phase and blue phases. 33 These structures can be analyzed quantitatively by determining the local director field  n(r), the associated order-parameter field S(r), and the pitch p of the helix/helices forming under favorable thermodynamic conditions and suitable choices for ε 3 0.…”

“…To circumvent the problem with improperly chosen dimensions of the simulation cell, Melle et al 33 have placed the liquid crystal between the planar surfaces of a slit-pore with degenerate planar anchoring 57 of the mesogens. Suppose, the thermodynamic state has been chosen such that a cholesteric phase would form.…”

“…This approach works, in principle, also for blue phases consisting of more complex helical structures than the cholesteric phase. 33 Experimentally, one can control p by considering binary mixtures in which a small amount of chiral material is immersed into an achiral (nematic) host phase. Depending on the concentration of chiral material, a chiral nematic phase forms characterized by some pitch (see p. 284-285 of Ref.…”

“…This is not so for the model system employed in this and previous studies by some of us. 33,34 Our model consists of a Lennard-Jones core with superimposed orientation dependent dispersion attraction combined with a certain handedness of these interactions. In fact, as we showed earlier, this model is capable of forming blue phases II and III besides a cholesteric phase.…”

“…In fact, as we showed earlier, this model is capable of forming blue phases II and III besides a cholesteric phase. 33 In particular, the blue phase II could be identified unambiguously through its characteristic double-twist helices arranged in a simple-cubic lattice and the tetrahedral arrangement of its disclination lines. For colloids placed in the cholesteric phase of this model, disclination lines near the colloid's surface could be observed in excellent qualitative agreement with lattice Boltzmann studies 35 and even with experimental data 36 both of which focus on length scales much larger than those characteristic of our model.…”