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

Abstract

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.

Effect of orientational restriction on monolayers of hard ellipsoids

“…The isotropic-nematic phase transition of hard ellipses corresponds to a planar nematic-biaxial nematic phase transition of the oblate ellipsoids. Here we note that our present work can be considered as an extension of our previous studies done for monolayers of uniaxial and biaxial hard particles using restricted-orientation approximation [39,40].…”

Synthesis and assembly of colloidal cuboids with tunable shape biaxiality

“…Moreover, the formation of mesophases was further rationalized in terms of the particle shape biaxiality θ which can be calculated as 33 : where two aspect ratios are κ 1 = L / T and κ 2 = W / T , respectively. θ varies from −1 (uniaxial platelike geometry), via 0 (i.e., L / W = W / T , perfect biaxiality), to 1 (uniaxial rodlike geometry) 34 . Specifically, rodlike colloids with less noticeable biaxial shape ( θ > 0.05) were stabilized in SmAu phase, as they can rotate freely along the L -axis because of the relatively small energy barriers for their rotation within layers.…”

“…This is largely due to the limited access to high-quality colloids with the inherent biaxial shape. Currently available biaxial colloids include bent silica rod 28 , goethite 29 , hectorite 30 , and lead carbonate boardlike particles 31 with unique D 2h symmetry 32 – 34 , of which goethite particles are mostly studied. These goethite colloids, however, show very broad polydispersity (up to 55%) in size and shape.…”

Self-assembly of freely-rotating polydisperse cuboids: unveiling the boundaries of the biaxial nematic phase

“…6 The stability of this N B phase was ascribed to the particles' quasi self-dual shape, a geometry in between oblate and prolate, and to their significant size dispersity, which hinders the formation of the Sm phase. This key work has reignited recent interest, sparking numerous theoretical, experimental and computer simulation studies on hard board-like particles (HBPs) [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and other biaxial geometries. 24,[28][29][30][31][32] Theoretical and computational studies on monodisperse systems have suggested that self-dual-shaped particles exhibit a higher tendency to form biaxial nematics.…”