Abstract
ABSTRACTMonte Carlo simulations in the isothermal-isobaric ensemble are used to investigate the formation of an ordered, biaxial nematic phase in a binary mixture of thermotropic liquid crystals. The orientational dependence of the interaction between molecules of each pure component is the same as in the well-known Maier-Saupe model; each pure component of the mixture is therefore capable of forming a uniaxial nematic phase. For the interaction between molecules of different components, we use the same Maier-Saupe model but change the sign of the coupling constant. As a consequence a T-shaped arrangement of these molecules is energetically favoured. The formation of the biaxial phase occurs in two steps. At higher temperatures T, one of the components forms a uniaxial nematic phase whereas the other is in a quasi two-dimensional restricted isotropic liquid state. We develop a simple theoretical model to understand the high degree of (ostensible) nematic order in the latter. At lower T, the second component becomes nematic and then the entire mixture of the two compounds has biaxial symmetry. The biaxial nematic phase does not demix into domains rich in molecules of one or the other species.
Highlights
Liquid crystals are a intriguing realisation of what is commonly referred to as ‘complex fluids’ [1,2]
In a biaxial nematic phase, a second symmetry axis exists besides the nematic director
We focus on binary mixtures of two liquid-crystalline compounds that are capable of forming nematic phases under favourable thermodynamic conditions
Summary
Liquid crystals are a intriguing realisation of what is commonly referred to as ‘complex fluids’ [1,2]. To realise thermotropic liquid crystals of biaxial symmetry experimentally, one can ‘glue’ together rod- and disc-like molecules as demonstrated by Hunt et al [11]. There are studies of mixtures of rodand disc-like mesogens that deny the existence of a thermodynamically stable biaxial nematic phase [23,24] In both of these studies, it is surmised that the reason for the absence of a stable biaxial nematic phase could be the improperly chosen isotropic interaction potential between the different components that stabilises a mixed state insufficiently. This is concluded because a competition between the demixing of the mixture and the formation of a biaxial nematic phase has been observed. In the Appendix, we provide the derivation of rotation tensors required for the analysis of twodimensional orientation distribution functions (odf)
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