Phase diagram of hard board like colloids from computer simulations

April 19th, 2013 Comments off

Stavros D. Peroukidis, Alexandros G. Vanakaras, S. D. Peroukidis, A G. Vanakaras, Soft Mater, 9, 7419-7423 (2013).  DOI: 10.1039/C3SM51165K.

The rich mesophase polymorhism and the phase sequence of board-like colloids depends critically on their shape anisometry. Implementing extensive Monte Carlo simulations, we calculated the full phase diagram of sterically interacting board-like particles, for a range of experimentally accessible molecular dimensions/anisometries of colloids of this shape. A variety of self organized mesophases including uniaxial and biaxial nematics, smectic, cubatic and columnar phases have been identified. Our results demonstrate clearly that the molecular anisometry influences critically not only the structure and the symmetry of the mesophases but also, and perhaps more interestingly, the phase sequence among them. New classes of phase sequences such as nematic-nematic and, for the first time, a direct transition from a discotic and a biaxial nematic to an orthogonal smectic A phase have been identified. The molecular geometry requirements for such a phase behavior have been located.

Categories: 2013, A. G. Vanakaras, Paper, S.D. Peroukidis Tags:

European Conference on Liquid Crystals, ECLC-2013

February 23rd, 2013 Comments off

 

The purpose of this biennial conference is to bring together the research community working in the interdisciplinary field of Liquid Crystals and soft anisotropic materials. The conference will be held from September 22-27, 2013 in the Paradise Mare Hotel, a five star resort, in Rhodes, Greece.

Please visit the web page of the conference: http://eclc2013.upatras.gr

The organizing committee:

  • George Floudas, University of Ioannina
  • Panayota K. Karahaliou, University of Patras
  • Emmanouil Kriezis, University of Thessaloniki
  • Polykarpos Pissis, NTUA
  • Dimitris Tsiourvas, N.C.S.R “Demokritos”
  • Stavros Peroukidis, University of Patras



Categories: ECLC-2013 Tags:

Light scattering by a metallic nanoparticle coated with a nematic liquid crystal

November 25th, 2012 Comments off

V. Yannopapas, N. Fyttas, V. Kyrimi, E. Kallos, A.G. Vanakaras, D.J. Photinos, PSSA, DOI: 10.1002/pssa.201228489.

We study the optical properties of gold nanoparticles (NPs) coated with a nematic liquid crystal (NLC) whose director field is distributed around the NP according to the anchoring conditions at the surface of the NP. The distribution of the NLC is obtained by minimization of the corresponding Frank free-energy functional whilst the optical response is calculated by the discrete-dipole approximation (DDA). We find, in particular, that the anisotropy of the NLC coating does not affect much the (isotropic) optical response of the NP. However, for strong anchoring of the NLC molecules on the surface of NP, the inhomogeneity of the coating which is manifested by a ring-type singularity (disclination or Saturn ring), produces an enhancement of the extinction cross spectrum over the entire visible spectrum.

Modeling and Computer Simulation Methods for Dendrimers

December 1st, 2011 Comments off

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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Categories: Uncategorized Tags:

Layer-multiple-scattering theory for metamaterials made from clusters of nanoparticles

August 26th, 2011 Comments off

V. Yannopapas and A. G. Vanakaras, Phys. Rev. B, 84, 085119 (2011).

We present a layer-multiple-scattering method of electromagnetic waves for the study of periodic metamaterials formed as a lattice of cavities which are filled by clusters of spherical nanoparticles. Our approach is a three-stage process where we take fully into account all the multiple-scattering events involved: (a) among the spheres of the cluster inside the cavity, (b) between the cluster and the cavity, and (c) among the cavities (containing the clusters) within the metamaterial. As an example, we study the transmission, reflectance, and absorbance spectra of light incident on a finite slab of a $SiO_2$-inverted opal whose voids contain clusters of gold nanoparticles. We find, in particular, that finite slabs of this metamaterial act as highly efficient absorbers over a wide frequency range, from 2–4.5 eV. Also, around the local maxima of the absorbance spectrum, the metamaterial exhibits anomalous dispersion, wherein the real part of the group velocity is opposite to (the real part of) the phase velocity. ©2011 American Physical Society

URL:http://link.aps.org/doi/10.1103/PhysRevB.84.085119
DOI: 10.1103/PhysRevB.84.085119
PACS: 42.70.Qs, 42.25.Bs, 42.50.Gy, 78.67.Pt
Categories: 2011, A. G. Vanakaras, Paper, V. Yannopapas Tags:

Dirac point in the photon dispersion relation of a negative/zero/positive-index plasmonic metamaterial

July 21st, 2011 Comments off

V. Yannopapas and A. Vanakaras, Phys. Rev. B., 84, 045128, (2011).

We report on the emergence of a Dirac point in the dispersion relation of a plasmonic metamaterial. It is realized as a three-dimensional crystal (cubic or orthorhombic) whose lattice sites are decorated by aggregates of gold nanoparticles embedded in a high-index dielectric material. The Dirac-type dispersion lines of the photon modes are not a result of diffraction as in photonic crystals but due to subwavelength features and emerge from the gapless transition from a negative to a positive index band. The Dirac point is manifested as a dip in the spectrum of light transmittance through a finite slab of the metamaterial; however, transmittance does not decrease diffusively but exponentially due to the inherent losses of gold in the given spectral regime. ©2011 American Physical Society

URL: http://link.aps.org/doi/10.1103/PhysRevB.84.045128
DOI: 10.1103/PhysRevB.84.045128
Categories: 2011, A. G. Vanakaras, V. Yannopapas Tags:

Molecular simulation of hierarchical structures in bent-core nematics

May 28th, 2011 Comments off
S. D. Peroukidis, A. G. Vanakaras, and D. J. Photinos, Phys. Rev. E,  84, 010702(R)  (2011).

Abstract: The structure of nematic liquid crystals formed by bent-core mesogens (BCMs) is studied in the context of Monte Carlo simulations of a simple molecular model that captures the symmetry, shape and flexibility of achiral BCMs. The results indicate the formation of (i) clusters exhibiting local smectic order, orthogonal or tilted, with strong in-layer polar correlations and anti-ferroelectric juxtaposition of successive layers and (ii) large homochiral domains through the helical arrangement of the tilted smectic clusters, whilst the orthogonal clusters produce achiral (untwisted) nematic states.

 

e-print
suppl. inf.

84, 010702(R) (2011)

Categories: 2011, Paper Tags:

Polymer Brushes on Periodically Nanopatterned Surfaces

April 28th, 2011 Comments off
A. Koutsioubas and A. G. Vanakaras, Langmuir, 24 (23), 13717–13722 (2008).   DOI: 10.1021/la802536v e-print

Abstract: Structural properties of polymer brushes tethered on a periodically nanopatterned substrate are investigated by computer simulations. The substrate consists of an alternating succession of two different types of equal-width parallel stripes, and the polymers are end-tethered selectively on every second stripe. Three distinct morphologies of the nanopatterned brush have been identified, and their range of stability has been determined in terms of a single universal parameter that combines the grafting density, the polymer length, and the stripe width. We propose scaling relations for the average brush height and for the architectural properties of the outer surface of the nanopatterned brush under good solvent conditions. Our analysis provides guidelines for fabricating well-defined and tunable nanopatterned polymeric films. Copyright © 2008 American Chemical Society

e-print_la2008

Categories: 2008, Paper Tags:

Liquid crystalline phases and demixing in binary mixtures of shape-anisometric colloids

May 30th, 2010 Comments off

S. D. Peroukidis, A. G. Vanakaras and  D. J. Photinos, J. Mat. Chem.20, 10495-10502, (2010).

Abstract. A theoretical model of shape-anisometric particles embedded in a cubic lattice is formulated for binary mixtures combining rod-like, plate-like and spherical particles. The model aims at providing a tool for the prediction and interpretation of complex phase behavior in a variety of liquid crystalline colloids, biological and macromolecular systems. Introducing just repulsive interactions among the particles, a rich variety of phase structures and multiphasic equilibria is obtained, including isotropic, nematic, lamellar and columnar phases, demixing into phases of the same or different symmetries and structural microsegregation of the different species of the mixture within the same phase.
This article is part of the collection: Modelling of Materials (c) http://pubs.rsc.org | doi:10.1039/C0JM01692F

e-print JMC2010

Categories: 2010, Paper Tags:

Extending the Maier-Saupe theory to cybotactic nematics

May 9th, 2010 Comments off

S. Droulias, A.G. Vanakaras, D.J. Photinos,  Liquid Crystals37:6, 969-976, (2010).

Abstract: A theory of thermotropic nematic liquid crystals in which molecules form internally ordered clusters is presented. The formulation is based on the same mean field approximation and form of the anisotropic potential used in the Maier-Saupe theory. A uniaxial nematic and two macroscopically isotropic phases are predicted. One of the isotropic phases consists of thermodynamically stable clusters with internal orientational order. The nematic phase shows cybotactic order throughout its range of stability. This order persists above the transition temperature, either as a stable feature or as a pretransitional effect. The values of the order parameter and of the entropy change at the nematic to isotropic phase transition depend on the size of the clusters. The Maier-Saupe theory is obtained in the limit of extremely large or extremely small clusters. URL: http://dx.doi.org/10.1080/02678292.2010.488819

e-print_LC2010

Categories: 2010, Paper Tags: