Phase behaviour of liquid-crystal monolayers of rod-like and plate-like particles

Phys. Chem. Chem. Phys.

et al. 2016

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The effect of out-of-plane orientational freedom on the orientational ordering properties of a monolayer of hard ellipsoids is studied using the Parsons-Lee scaling approach and replica exchange Monte Carlo computer simulation. Prolate and oblate ellipsoids exhibit very different ordering properties, namely, the axes of revolution of prolate particles tend to lean out, while those of oblate ones prefer to lean into the confining plane. The driving mechanism of this is that the particles try to maximize the available free area on the confining surface, which can be achieved by minimizing the cross section areas of the particles with the plane. In the lack of out-of-plane orientational freedom the monolayer of prolate particles is identical to a two-dimensional hard ellipse system, which undergoes an isotropic-nematic ordering transition with increasing density. With gradually switching on the out-of-plane orientational freedom the prolate particles lean out from the confining plane and destabilisation of the in-plane isotropic-nematic phase transition is observed. The system of oblate particles behaves oppositely to that of prolates. It corresponds to a two-dimensional system of hard disks in the lack of out-of-plane freedom, while it behaves similar to that of hard ellipses in the freely rotating case. Solid phases can be realised by lower surface coverage due to the out-of-plane orientation freedom for both oblate and prolate shapes.

Section: Introductionmentioning

confidence: 71%

Effect of orientational restriction on monolayers of hard ellipsoids

Varga

Phys. Chem. Chem. Phys.

et al. 2016

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“…Various phase diagrams were obtained for different values of the two parameters that describe the particle shape: the largest particle aspect ratio κ 1 and the particle biaxiality θ. This study is an extension of our previous work in which monolayers of uniaxial rod-like and plate-like particles were analysed [34].…”

Section: Discussionmentioning

confidence: 99%

Effect of shape biaxiality on the phase behavior of colloidal liquid-crystal monolayers

Gonzalez-Pinto

Phys. Chem. Chem. Phys.

et al. 2015

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We extend our previous work on monolayers of uniaxial particles [J. Chem. Phys. 140, 204906 (2014)] to study the effect of particle biaxiality on the phase behavior of liquid-crystal monolayers.Particles are modelled as board-like hard bodies with three different edge lengths σ 1 ≥ σ 2 ≥ σ 3 , and use is made of the restricted-orientation approximation (Zwanzig model). A density-functional formalism based on the fundamental-measure theory is used to calculate phase diagrams for a wide range of values of the largest aspect ratio (κ 1 = σ 1 /σ 3 ∈ [1, 100]). We find that particle biaxiality in general destabilizes the biaxial nematic phase already present in monolayers of uniaxial particles.While plate-like particles exhibit strong biaxial ordering, rod-like ones with κ 1 > 21.34 exhibit reentrant uniaxial and biaxial phases. As particle geometry is changed from uniaxial-to increasingly biaxial-rod-like, the region of biaxiality is reduced, eventually ending in a critical-end point. For κ 1 > 60, a density gap opens up in which the biaxial nematic phase is stable for any particle biaxiality. Regions of the phase diagram where packing-fraction inversion occurs (i.e. packing fraction is a decreasing function of density) are found. Our results are compared with the recent experimental studies on nematic phases of magnetic nanorods.

“…More recently, Martínez-Ratón et al [314] used a fundamental-measure density functional to study a monolayer of particles consisting of hard uniaxial boards of sizes L × σ × σ with centres lying on a flat surface. The aim was to study the existence of a phase transition from a uniaxial to a biaxial nematic phase for both prolate (L > σ) and oblate (L < σ) geometries.…”

Section: Polydisperse Systemsmentioning

confidence: 99%

Hard-body models of bulk liquid crystals

Mederos

J. Phys.: Condens. Matter

2014

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Hard models for particle interactions have played a crucial role in the understanding of the structure of condensed matter. In particular, they help to explain the formation of oriented phases in liquids made of anisotropic molecules or colloidal particles and continue to be of great interest in the formulation of theories for liquids in bulk, near interfaces and in biophysical environments. Hard models of anisotropic particles give rise to complex phase diagrams, including uniaxial and biaxial nematic phases, discotic phases and spatially ordered phases such as smectic, columnar or crystal. Also, their mixtures exhibit additional interesting behaviours where demixing competes with orientational order. Here we review the different models of hard particles used in the theory of bulk anisotropic liquids, leaving aside interfacial properties and discuss the associated theoretical approaches and computer simulations, focusing on applications in equilibrium situations. The latter include one-component bulk fluids, mixtures and polydisperse fluids, both in two and three dimensions, and emphasis is put on liquid-crystal phase transitions and complex phase behaviour in general.