Soft Matter Theory & Simulations Group

AG Vanakaras, DJ Photinos, Liquid Crystals 45 (13-15), 2184-2196, (2018).

Abstract: Molecular dynamics simulations of selected members of the cyano-biphenyl (CB) series of dimers (CBnCB) have been set up using atomistic detail for the description of the alkyl spacer and coarse-grained representation of the CB end-segments. Detailed results are presented for the CB7CB dimers, showing an isotropic fluid phase and two nematic phases. The positional and orientational correlation functions extracted from the simulations are used to elucidate the structure of the low-temperature nematic phase. Polar molecular ordering is clearly identified along a direction undergoing helical twisting at right angles to the helical axis, with a constant pitch of about of 8 nm. The local ordering of the various molecular segments is calculated and found to be in excellent agreement with experimental NMR measurements. Key findings of the simulation are shown to be correctly predicted by the theoretical model of the polar-twisted nematic (N_PT) phase [A.G. Vanakaras, D.J. Photinos, Soft Matter. 12 (2016) 2208–2220]. The complete failure of the usual twist bend model (N_TB) to account for these findings is demonstrated.

A. Kumar, A. G. Vanakaras, D. J. Photinos, J. Phys. Chem. B, 121(47), 10689-10703 (2017).

Abstract The potential of mean torque governing the orientational ordering of prochiral solutes in the two nematic phases (N and N_X) formed by certain classes of symmetric achiral bimesogens is formulated and used for the analysis of existing NMR measurements on solutes of various symmetries dissolved in the two phases. Three distinct attributes of the solvent phase, namely polarity of the orientational ordering, chirality of the constituent molecules, and spatial modulation of the local director, are identified as underlying three possible mechanisms for the generation of chiral asymmetry in the low temperature nematic phase (N_X). The role and quantitative contribution of each mechanism to enantiotopic discrimination in the N_X phase are presented and compared with the case of the conventional chiral nematic phase (N*). It is found that polar ordering is essential for the appearance of enantiotopic discrimination in small rigid solutes dissolved in the N_X phase and that such discrimination is restricted to solutes belonging to the point group symmetries C_s and C_2v.

An international school on “Modeling and Computer Simulation Methods for Dendrimers” will be held in the University of Patras, Department of Materials Science from 12 to 15 December 2011

Updated information and program here

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A. G. Vanakaras, D.J. Photinos and E. T. Samulski, Phys. Rev. E, 57(5), R4875-R4878 (1998).

Abstract: We show through explicit molecular modeling how tilt may be induced in layered mesophases and propagate across the-layers, using a concrete representation of flexible tail-core-tail calamitic mesogens in conjunction with-the variational cluster expansion. The results demonstrate that spontaneous symmetry breaking observed in smectic-liquid crystals-the tilt of the director relative to the layers-can be induced by excluded. volume interactions, both in the synclinic and in the anticlinic configurations.

e-print PRE_r1998

A. G. Vanakaras, S. C. McGrother, G. Jackson and D. J. Photinos, Molec.Cryst. Liq. Cryst., 323, 199 (1998).

Abstract: The possibility of using H-bonding interactions to promote the stabilization of phase biaxiality in nematic binary mixts. of oblate and prolate thermotropic mesogens is discussed. Onsager’s theory of the isotropic-nematic transition was extended to allow for such selective assocns. among unlike species and it is used to calc. the phase diagram of binary mixts. consisting of hard spherocylinders and cut spheres. Directional, short-ranged attractions between rods and disks strongly stabilize the biaxial nematic mixt. against demixing and suggest that interactions of H-bonding type may provide an efficient mechanism for sustaining phase biaxiality in binary mixts. of real thermotropic nematogens. Preliminary Monte Carlo simulations designed to test such predictions are discussed.

e-print MCLC98

A. G. Vanakaras and D.J. Photinos, Molec.Cryst. Liq. Cryst., 299, 65-71 (1997).

Abstract: We use the variational cluster approximation to study the relative thermodynamic stability of the spatially uniform phases of binary mixtures of hard rods and discs. The factors promoting the stability of the biaxial nematic phase are identified and discussed. The results suggest that a stable thermotropic nematic biaxial mixture cannot be obtained from molecules of the sizes and electric dipole interaction strengths commonly encountered in real calamitic and discotic thermotropic phases.

e-print_MCLC1997

A. G. Vanakaras and D.J. Photinos, Mol. Phys., 85(6), 1089-1104 (1995).

Abstract: A theory for the nematic-isotropic (N-I) phase transition of prolate uniaxial molecules with longitudinal dipole moments is presented. The theory is based on the variational cluster expansion, truncated after the two-molecule term, and is implemented for polar hard spherocylinders with and without attractions, and for polar linear arrays of Lennard-Jones interaction centres. We find that the dipole interactions substantially shift the N-I transition temperature and strongly promote antiparallel molecular association, but have a weak effect on the order parameters, the pressure, and the N-I coexistence densities. The effect of dipoles on phase stability is very sensitive to their position within the molecular frame. Off-centre dipoles are shown to give rise to phase re-entrance according to the sequence N-I-N on heating at constant density. The theory does not predict a stable ferroelectric nematic phase.

e-print mol_phys95

• Biaxial Nematic Displays (BIND), EU-FP7 / ITC-1-3.2 / STREP-CP-FP-INFSO.
• Functional LC Dendrimers: Synthesis of New Materials, Resource for New Applications (DENDREAMERS), Marie Curie Actions, EU-FP7-PEOPLE-ITN.
• Self–organized Nanomaterials for tailored optical and electrical properties (NANOGOLD), EU-FP7-NMP-2008-2.2-2

Extending the Maier-Saupe theory to cybotactic nematics
S. Droulias, A. G. Vanakaras, D. J. Photinos,  arXiv:1002.2398v1 [cond-mat.soft] (2010).

Two-Dimensional Ordered Porous Patterns by Molecular Design

Langmuir, in press (2010)

Symmetries and allignement of biaxial nematic liquid crystals

Biaxial Nematics: symmetries, order domains and field-induced phase transitions

Molecular Modeling of Liquid Crystalline Self-Organization of Fullerodendrimers: Columnar to Lamellar Phase Transitions Driven by Temperature and/or Concentration Changes

Thermotropic biaxial liquid crystals: Spontaneous or field stabilized?