p. pasini and s. Žumer- liquid crystal phases and nano-structures

Download P. Pasini and S. Žumer- Liquid Crystal Phases and Nano-Structures

Post on 10-Oct-2014




0 download

Embed Size (px)


International School of Liquid Crystals 15th Workshop

Liquid Crystal Phases and Nano-StructuresA Workshop to celebrate ten years of the International Liquid Crystal School and Claudio Zannoni 60th birthday E. Majorana Centre for Scientific Culture, Erice 27 October 1 November 2008


Directors of the Workshop: P. Pasini and S. umer

Director of the School: C.Zannoni

Sponsors: Italian Liquid Crystal Society, INSTM-CRIMSON, E4 Computer Engineering

Thermotropic Biaxial Nematics: An Enduring Challenge? Geoffrey Luckhurst School of Chemistry, University of Southampton, Southampton SO17 1BJ, United Kingdom1

Superstructures in nematic colloidsSlobodan umer1,2, Miha Ravnik1, and Brina rnko1 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, SI-1000 Ljubljana, Slovenia; 2 Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia

The discovery of a new liquid crystal phase is an exciting event especially if the phase has been predicted theoretically. In principle, one example of such a phase is the biaxial nematic first predicted by Marvin Freiser1 in 1970. Following his prediction there was much theoretical work but apparently no experimental attempts to find the phase until 1980. Then, in a seminal study, Yu and Saupe discovered the biaxial nematic but for a lyotropic liquid crystal2. Since then although simulations3 of model systems have certainly revealed the existence of the thermotropic biaxial nematic the discovery of a real system has remained elusive. However, many compounds have been claimed to form a biaxial nematic but the evidence was not always strong and when studied with deuterium NMR it was invariably found that the nematic is uniaxial4. In recent years more detailed evidence in support of claims has been presented and the existence of the biaxial nematic seems more convincing. One example of such a claim is for a tetrapode in which four mesogenic groups are tethered by flexible spacers to a central silicon atom5. In addition, it seems that as well as finding the elusive biaxial phase the FTIR studies also determined the four orientational order parameters characterising both the uniaxial and the biaxial nematics5. The identification of the phases was then supported by NMR using a deuteriated calamitic nematogen as a spin probe6. Given the reassuring nature of these investigations we had decided to undertake our own studies. In the first of these the four order parameters were used to test a rather general molecular field theory of the biaxial nematic in which the molecular biaxiality is characterised by two parameters. Attempts to fit the theory to the experimental results proved to be interesting which prompted us to compare the order parameters found for the uniaxial and biaxial nematic phases of a biaxial Gay-Berne mesogen3(b) again with interesting results7. Independently of these theoretical studies we had also determined the phase symmetry with the aid of NMR and a selection of deuteriated spin probes over the entire nematic range of the tetrapode. To help appreciate these NMR results the orientational ordering of the probes in a phase of known biaxiality was determined. Further studies of the tetrapode at low temperatures were inadvertently undertaken and these suggest how some of our puzzling results, both experimental and theoretical, can be understood. The thermotropic biaxial nematic phase would appear to present an enduring challenge at least as far this tetrapode is concerned.M J Freiser, Phys Rev Lett 24, 1041 (1970). L L Yu and A Saupe, Phys Rev Lett 45, 1000 (1980). (a) G R Luckhurst and S Romano, Mol Phys 40, 129 (1980); (b) R Berardi and C Zannoni, J Chem Phys 113, 5971 (2000) 4 G R Luckhurst, Thin Solid Films 393, 40 (2001) 5 K Merkel, A Kocot, J K Vij, R Korlacki, G H Mehl and T Meyer, Phys Rev Lett 93, 237801 (2004). 6 J L Figueirinhas, C Cruz, D Filip, G Feio, A C Ribeiro, Y Frere, T Meyer and G Mehl, Phys Rev Lett 94, 107802 (2005). 7 F Bisi, G R Luckhurst and E G Virga, Phys Rev E 78, 021710 (2008).2 3 1

Our recent theoretical predictions are contrasted with the latest experimental studies of colloidal structures in spatially confined nematic liquid crystals. Modeling is based on phenomenological Landau- de Gennes type free energy where also effects of confinement and external fields are taken into account. The complexity of effective inter-colloidal couplings in a nematic solvent leads to numerous colloidal structures not present in simple liquids. Here we focus on thin nematic films where colloidal particles can share nematic defects. In such a case the coupling is string-like and is therefore much more robust as compared to an interaction based on an array of localized disclinations. These structures can be realized either via -1/2 disclination lines that entangle two or more colloidal particles [1] or via -1 disclination loops with nonsingular cores where topological constrains result in sharing of the corresponding areas of deformed nematic by neighboring particles. The resulting colloidal dimmers, chains, lattices, and braids are the first steps toward assembling of robust nematic colloidal crystals and hierarchical structures [2] that are of potential interest for photonic applications. [1] M. Ravnik, M. karabot, S. umer, U. Tkalec, I. Poberaj, D. Babi, N. Osterman, and I. Muevi, Entangled Nematic Colloidal Dimers and Wires, Phys. Rev. Lett. 99, 247801 (2007) [2] M. karabot, M. Ravnik, S. umer, U. Tkalec, I. Poberaj, D. Babi, I. Muevi, Hierarchical self-assembly of nematic colloidal superstructures, Phys. rev. E 77, 061706 (2008),

Evolutionary strategies for solving extremely complicated NMR spectra of solutes in nematic solventsC.A. de Lange Laser Centre, Vrije Universiteit De Boelelaan 1081, 1081 HV Amsterdam The Netherlands E.E. Burnell Chemistry Department, University of Britisch Columbia 2036 Main Mall, Vancouver (B.C.) V6T 1Z1 Canada W.L. Meerts Molecular- and Biophysics Group, Institute for Molecules and Materials Radboud Universiteit Nijmegen, P.O. Box 9010, 6500 GL Nijmegen The Netherlands The complexity of NMR spectra of solutes in partially ordered solvents such as liquid crystals increases rapidly with the number of spins. Spectra of simple solutes with sufficient symmetry and containing not too many spins (typically ~ 6) are readily analysed. The analysis of larger spin systems is more difficult, and often impossible. In this contribution the application of a general automated Evolutionary Algorithm to solving the highly complex proton NMR spectrum of the twelve-spin system pentane will be presented. This molecule interconverts rapidly among several symmetry-unrelated conformations on the NMR time scale. The interpretation of the spectral parameters that are obtained from the analysis requires the use of a model to connect relative orientational orders in symmetry-unrelated conformers.

RECENT ADVANCES IN THE DYNAMICS OF CONFINED NEMATIC LIQUID CRYSTALS WITH DEFECTS AND BOUNDARY NON-HOMOGENEITIES A. F. Martins* and A. Vron Depart. Materials Science, The New University of Lisbon, 2829-516, Caparica, Portugal. (* e-mail: asfm@fct.unl.pt)

AbstractIn spite of intense research and significant progress in the last thirty years, some aspects of the dynamics of real nematic liquid crystals confined between parallel plates (LCD geometry) are still not clear. In particular, local non-homogeneities in the boundary alignment of the molecules may have a significant effect on the electric field driven orientational dynamics that has not been recognized until recently [1]. This effect will be discussed in the present communication. We present 2-D computer simulations of the electric field driven director dynamics in thin nematic samples confined between parallel plates, in the presence of topological defects or boundary or volume non-homogeneities in the sample. Deuterium NMR spectra directly computed from the numerical results are good agreement with the experimental data [2]. We propose that in addition to the well known collective modes of reorientation (observed simultaneously over the whole sample), a qualitatively different mode of reorientation, that we may call progressive mode, exists and may dominate the director reorientation. This mode has now been observed in thin samples of low-molecular mass liquid crystals [2] as well as in polymeric systems [3]. The progressive mode originates in the local non-homogeneities and then propagates through the whole sample building up a periodic pattern. Its deuterium NMR signature a doublet with time-dependent intensity and constant splitting [1,2] - is clearly distinct from that of collective modes - a doublet with time-dependent splitting and constant intensity [4-6]. [1] A.F. Martins, A. Vron, Thin Solid Films (2008), doi: 10.1016/j.tsf.2008.09.052. [2] G.R. Luckhurst, A. Sugimura, B.A. Timimi and H. Zimmermann, Liquid Crystals 32, 1389 (2005). [3] A.F. Martins et al., to be published. [4] A.F. Martins, P.Esnault and F. Volino, Phys. Rev. Lett. 57, 1745 (1986). [5] A. Vron, A.E. Gomes, C.R. Leal, J. vd Klink, A.F. Martins, Mol. Cryst. Liquid Cryst. 331, 499 (1999). [6] A.F. Martins, A.E. Gomes, A. Polimeno, L. Orian, Phys. Rev. E 62, 2301 (2000).

International workshop, Liquid Crystal Phases and Nanostructures, Erice, Italy, 27 Oct.-1 Nov. 2008

Nematic fluctuations and semi-soft elasticity in liquid crystal elastomersMartin opi1,2 and Andrej Petelin21

An atomistic model for the elastic constants of nematicsMirko Cestari, Alessandro Bosco and Alberta Ferrarini Dipartimento di Scienze Chimiche, Universit di Padova, via Marzolo 1, 35131, Padova, Italy present address: SISSA, via Beirut 2-4, Trieste, Italy e-mail: alberta.ferrarini@unipd.it Several molecular models for the elastic constants of nematics have been presented, since the early work of Nehring and Saupe,1 and have


View more >