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Workshop Programme

for period 22 - 22 March 2013

One day BLCS/SMTG/INI workshop on the molecular modelling and theory of liquid crystals

22 - 22 March 2013

Timetable

Friday 22 March
08:30-09:00 Registration
09:00-10:40 Analytical and Computational Paths from Molecular Foundations to Continuum Descriptions
10:40-11:00 Morning Coffee
11:00-11:50 Zannoni, C (Università di Bologna)
  Molecular and atomistic simulations of liquid crystals Sem 1
  Liquid crystals (LC) continue to offer a fascinating variety of fundamental physics problems related to molecular organizations in the bulk and their modifications close to interfaces [1]. Here we plan to show some recent results for the simulation of these organizations at molecular and atomistic resolution. At molecular resolution we employ Gay-Berne models and report results for the shape, internal order and chirality of freely suspended low molar mass nematic nanodroplets [2]. We also show that systems as complex as swollen LC elastomers and their deformation in response to the application of an electric field can be simulated [3]. At atomistic level molecular dynamics simulations [4] can now predict actual morphologies and properties in the bulk [4,5] from a specific molecular structure and they can also be useful to analyse NMR data [6,7]. Having validated simulations in the bulk, we also investigate LC close to selected interfaces like hydrogen terminated silicon [1] and crystalline and glassy silica with controlled roughness [8], trying to show how orientational anchoring can be introduced from the microscopic point of view probing the limits of continuum theory on the nanoscale [9].

1 A. Pizzirusso, R. Berardi, L. Muccioli, M. Ricci, C. Zannoni, Chemical Science,3,573(2012) 2 D. Vanzo, M.Ricci, R. Berardi, C.Zannoni, Soft Matter, 8, 11790(2012) 3 G. Skacej, C.Zannoni, PNAS,109,10193(2012) 4 G. Tiberio, L. Muccioli, R. Berardi, C. Zannoni, ChemPhysChem, 10, 125(2009) 5 M.F. Palermo, A. Pizzirusso, L. Muccioli, C. Zannoni, to be submitted (2013) 6 A.Pizzirusso, M.B. Di Cicco, G. Tiberio, L. Muccioli, R. Berardi, C. Zannoni, J.Phys.Chem.B 116,3760(2012) 7 A. C. J. Weber, A. Pizzirusso, L. Muccioli, C. Zannoni, W. L. Meerts, C. A. de Lange, E.E. Burnell, J. Chem. Phys. 136, 174506 (2012) 8 O. M. Roscioni, L. Muccioli, R. G. Della Valle, A. Pizzirusso, M. Ricci, C. Zannoni, to be submitted (2012) 9 M. Ruths, B. Zappone, Langmuir, 28, 8371 (2012).

11:50-12:40 Wilson, M (Durham University)
  Self-assembly of liquid crystalline nanostructures in aqueous solution Sem 1
  Chromonic mesogens are non-conventional amphiphiles, which self-assemble in aqueous solution to form aggregate structures: rods, stacks or layers. At higher concentrations these aggregates can self-organise to form chromonic mesophases. Initial self-assembly is different to that seen in most conventional amphiphiles: it is enthalpically-driven and takes place in the absence of a critical micelle concentration. Subsequent mesophase formation is driven by entropic factors.

There is great interest in chromonic systems as materials for the fabrication of new thin films for biosensors and optical compensators; and also because a better fundamental understanding of chromonic self-assembly is required to control aggregate structure formation in certain classes of drug molecules.

This talk presents results from molecular simulation studies of chromonic self-assembly at different levels of detail. Atomistic molecular dynamics simulations of the dye molecule, sunset yellow, and the drug molecule, disodium cromoglycate, determine for the first time the structure and dynamical properties of chromonic aggregates in aqueous solution. Showing how subtle changes in intermolecular interactions can change the mode of self-assembly. Coarse-grained models, at the level of dissipative particle dynamics (DPD), demonstrate how simulation provides a tool to engineer new nanostructures by exploring the role of molecular shape and interactions in determining the structure of aggregates formed.

12:40-14:00 Lunch and Posters
14:00-14:20 Stokes, C (University of Manchester)
  Simulations of bent core molecules using molecular dynamics Sem 1
  The nematic phase is normally uniaxial – i.e. there is just one distinct optical axis. In such a phase one set of molecular axes are aligned but the other axes are orientationally disordered. In 1970, however, Freiser showed that a biaxial nematic phase was theoretically possible, in which all three molecular axes are aligned. This phase has since been observed experimentally, predicted to exist theoretically for various particle models and has been seen in simulation studies. Such a phase would have three distinct optical axis and there are possible applications to liquid crystal displays, should a suitable material be found. In this talk I would like to present the results of simulation studies on purely repulsive bent-core models (V-shaped particles). In the limit of very long, thin arms, such shapes have been predicted to exhibit a biaxial nematic phase for bend angles in the region of 110o. In order to test these predictions and also to explore the system’s pha se behaviour at pressures at which the nematic phase is unstable, we have run molecular dynamics simulations for one-component systems, binary mixtures and higher order mixtures (4 – 6 components). In all case the runs started from the isotopic phase and the system, was then slowly compressed, so any phase observed had formed spontaneously. We explored the effects of varying the arm lengths and bend angles of these particles on the phase behaviour. For a one-component system, only particles with bend angles greater than ca. 130o spontaneously formed ordered, equilibrated phases. Typically the phase sequence was isotropic →uniaxial nematic→biaxial smectic A (except for very straight particles in which the smectic phase was uniaxial). No biaxial nematic phase was observed for the arm-lengths simulated. It is possible that if the smectic phase could be destabilized, a biaxial nematic might form in its place. Such destabilization might occur in mixtures. Binary mixtures of bent cores, however, were found.
14:20-14:40 Greco, C (Università degli Studi di Padova)
  A molecular model for the electroclinic effect in nematic liquid crystals Sem 1
  The electroclinic effect (ECE) is an electro-optical effect that consists in a tilt of the optical dielectric tensor of a liquid crystal (LC) material upon application of an external electric field. The tilt is linear in the electric filed, and the proportionality coefficient, the electroclinic coefficient, is a property of the LC material. Originally observed in the orthogonal smectic-A phase made of chiral molecules (smectic-A* phase)[1], the ECE effect was subsequently also measured in the helix-unwound nematic N* phase [2], thus demonstrating that smectic layering is not essential for its appearance. Recently it was also measured in nematic LCs made of non-chiral molecules, upon imposition of an external mechanical twist [3]. The origin of the ECE in nematics is not obvious, and different molecular/environmental contributions have been proposed over the years. We have developed a molecular model for the ECE in nematics. Based on the molecular and phase symmetry, we hav e obtained expressions of the EC coefficient as a function of the relevant molecular properties (dipole moment, polarizability). The use of an atomistic representation of the molecular shape, charges and polarizability allows us to analyze the relationship between the ECE and the molecular structure. We will present some examples and discuss the role of the molecular and phase chirality.

[1] Garoff. S.; Meyer R. B. Phys. Rev. Lett. 1977, 38, 848-851. [2] Li, Z.; Petschek, R. G.; Rosenblatt, C. Phys. Rev. Lett. 1989, 62, 796-799. [3] Basu, R.; Pendery, J. S.; Petschek, R.G.; Lemieux, R. P.; Rosenblatt, C. Phys. Rev. Lett. 2011, 107, 237804: 1-4.

14:40-15:00 Ferreiro, C (University of Bristol)
  Random packing of mixtures of hard rods and spheres Sem 1
  Random packing of mixtures of hard spherocylinders and hard spheres are studied for $d \approx L$, where $L$ is the length of the spherocylinder and $d$ the diameter of the spheres. Packing fractions of mixtures of hard spherocylindres with aspect ratios $ 4 < L/D < 20$ and how this packing fraction changes with different concentrations of spheres are investigated. The results obtained are compared with observations of mixtures of sterically stabilized rod-like particles and spheres. These colloidal suspensions of rod-like particles are made with sepiolite clays and have previously been found to show liquid crystal behaviour [1]. The addition of spheres to a colloidal rod suspension affects the isotropic-nematic phase transition that occurs in pure suspensions. One might expect that the addition of spheres would lead to a wide phase gap between rod-rich and sphere-rich phases [2]. An experimental study did find some evidence for nematic rod domains with spheres accumulating at their grain boundaries [3]. Very easily however these systems got stuck in gel states consisting of (bundles of) rods trapping spheres [3]. Rather than trying to calculate equilibrium phase diagrams, we are therefore investigating random close packed configurations which may resemble the experimental observations more closely.

References [1] Z. X. Zhang and J. S. van Duijneveldt, J. Chem. Phys., 124, 154910 (2006). [2] Henk N.W. Lekkerkerker and Remco Tuinier, Colloids and the Depletion Interaction, Springer, 2011. [3] N. Yasarawan and J.S. van Duijneveldt, Soft Matter, 6, 353-362 (2010).

15:00-15:40 Afternoon Tea
15:40-16:00 Teixeira, P (ISEL and Universidade de Lisboa)
  The phase behaviour of shape-changing spheroids Sem 1
  Low-molecular-weight liquid crystals are typically modelled as collections of either hard rods or hard discs. However, small,flexible molecules known as tetrapodes also exhibit liquid crystalline phases, including the elusive biaxial nematic phase [1,2]. This is a consequence of the interplay between conformational and packing entropies: the molecules are able to adopt an anisometric stable conformation that allows then to pack more efficiently into orientationally ordered mesophases. Previous theoretical studies of such systems have been presented [3], but in order to capture the essential physics of the process, we introduce a minimal model which permits a clear detailed analysis. In our model a particle can exist in one of two states, corresponding to a prolate and an oblate spheroid. The energies of these two states differ by a prescriamount ε, and the two conformers are in chemical equilibrium. The interactions between the particles are described by the Gaussian Overlap Model [4] and we investigate the phase behaviour using a second-virial (Onsager) approach, which has been successfully applied to binary mixtures of plate-like and rod-like particles [5]. Depending on conditions these mixtures may exhibit biaxial nematic phases and N+--N– co-existence. We use both bifurcation analysis and a numerical minimisation of the free energy to show that, in the L2 approximation: (u) there is no stable biaxial phase even for ε=0 (although there is a metastable biaxial phase in the same density range as the stable uniaxial phase); (ii) the isotropic-to-nematic transition is into either one of two degenerate uniaxial phases, rod-rich or disc-rich.

References: [1] K. Merkel et al., Phys. Rev. Lett. 93, 237801 (2004). [2] J. L. Figueirinhas et al., Phys. Rev. Lett. 94, 107802 (2005). [3] A. G. Vanakaras et al., Mol. Cryst. Liq. Cryst. 362, 67 (2001). [4] B. J. Berne and P. Pechukas, J. Chem. Phys. 56, 4213 (1972). [5] P. J. Camp et al., J.

16:00-16:20 Cheung, D (University of Warwick)
  Simulation of armoured and swollen vesicles Sem 1
  Polymer vesicles, fluid filled polymer sacs, have attracted much attention for applications such as drug delivery vehicles, miniature chemical reactors, or as synthetic, minimal cells. In these applications the vehicles may undergo significant changes in osmotic pressure, pH, or concentration, which may lead to vesicle rupture or collapse. In order to avoid this a number of possible strategies may be used to stabilise vesicles against changes in external environment. In this talk I will discuss some recent simulation work studying two of these - armouring and swelling.

Recently it has been shown that polymer vesicles may be coated with a layer of colloidal particles that armour these, in a similar manner to some biological systems. Simple Monte Carlo simulations were used to reproduce the packing patterns seen in these experimental systems and to study the effect of surface charge density on the self-assembly [1]. Dissipative particle dynamics simulations were used to study the swelling of a polymer bilayer when exposed to small hydrophobic molecules. Above a critical density of hydrophobic molecules the bilayer undergoes a morphological transition characterised by the formation of a bud within the bilayer, consistent with experimental observations of polymer vesicles [2].

[1] R Chen, DJG Pearce, S Fortuna, DL Cheung, and SAF Bon, J Am Chem Soc, 133, 2151 (2011) [2] CDJ Parmenter, R Chen, DL Cheung, and SAF Bon, Soft Matter, in press

16:20-16:40 Gibaud, T (École Normale Supérieure)
  Reconfigurable self-assembly through chiral control of interfacial tension Sem 1
  The interfacial tension between molecular species in self-assembling systems plays a crucial role in determining the physical properties of the mesoscopic assemblages. The predominant method for controlling interfacial tension is the addition of surfactant molecules, which preferentially adsorb onto the interface and modify the interactions between the two phases. Using a model colloidal membrane (Fig.1) composed of chiral, rod-like fd-viruses, I will present a new method for controlling interfacial tension which does not require additional surfactant components, but instead utilizes the intrinsic chirality of the constituent rods. I will demonstrate that chirality can be used to continuously tune the interfacial tension of a membrane and to drive a dramatic phase transition from two-dimensional membranes to one-dimensional twisted ribbons. Using a wide variety of microscopic techniques, this transition is characterized over length-scales, ranging from nanometers to microns. Finally, using optical forces we demonstrate that malleable chiral assemblages can easily be moved, stretched, attached to each other, and transformed between multiple polymorphic states, thus enabling precise assembly and sculpting of highly adaptable materials with complex topologies
16:40-17:00 Buzza, M (University of Hull)
  Self-Assembly of Two Dimensional Colloidal Alloys Sem 1
  We study the self-assembly of mixed monolayers of hydrophobic and hydrophilic colloidal particles adsorbed at oil/water interfaces both experimentally and theoretically. Experimentally, we find that by tuning the interactions, composition and packing geometry of the mixed monolayer, a rich variety of two-dimensional super-lattice and cluster structures are formed which are stabilised by strong electrostatic interactions mediated through the oil phase [1,2]. The 2D structures obtained are in excellent agreement with zero temperature lattice sum calculations, indicating that the self-assembly process can be effectively controlled [1-3]. Monte Carlo simulations further reveal that the melting behaviour of these super-lattice structures proceeds via a multi-stage process, with melting temperatures that have a very strong and non-monotonic dependence on composition [3].

[1] A.D. Law, D.M.A. Buzza, T.S. Horozov, Phys. Rev. Lett., 106, 128302 (2011) [2] A.D. Law, M. Auriol, D. Smith, T.S. Horozov, D.M.A. Buzza, submitted [2] A.D. Law, T.S. Horozov, D.M.A. Buzza, Soft Matter, 7, 8923 (2011)

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