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Timetable (MLCW04)

Liquid Crystal Defects and their Geometry, Active and Solid Liquid Crystals, and Related Systems

Monday 24th June 2013 to Friday 28th June 2013

Monday 24th June 2013
09:00 to 09:35 Registration
09:35 to 09:45 Welcome by Director, Professor John Toland
09:45 to 10:30 Liquid crystal defects, a critical overview
Defects in liquid crystals present an extremely vast set of situations, depending on the nature of the order parameter, elasticity properties, boundary conditions. One distinguishes : defects characteristic of liquid crystalline media with one or two discrete repeat distances (smectic and columnar phases) which can be investigated by pure geometric methods ; defects characteristic of a director order parameter, which employ the Volterra process (for one dimensional defects) and topological methods, those latter providing scope for defects of various dimensionalities ; finally continuous defects whose consideration is essential for a description of defect physical properties. These topics will be confronted with an historical point of view and possible future developments.
10:30 to 11:00 Morning Coffee
11:00 to 12:00 R Kamien (University of Pennsylvania)
Pure and Applied Focal Conics
The epitaxial assembly of toric focal conic domain (TFCD) arrays of smectic-A liquid crystals onto pillar arrays is studied. The 3D nature of the pillar array is crucial to confine and direct the formation of TFCDs on the top of each pillar and between neighboring pillars, leading to highly ordered square and hexagonal array TFCDs persisting deeply into the bulk.
12:00 to 12:45 Exotic defect structures in a strongly confined chiral liquid crystal
We present our numerical results on the structures of a chiral nematic liquid crystal when it is confined in a thin planar cell with strong anchoring on its surfaces. We are interested in the cases where the liquid crystal exhibit a cholesteric blue phase under no confinement, and the cell thickness is of the order of or smaller than the cholesteric pitch. We show various stable exotic defect structures depending on temperature, cell thickness, and the type of anchoring. They include a hexagonal lattice of Skyrmion excitations, and a regular array of ring defects. These exotic defect structures can be regarded as a result of frustrations between bulk blue phase ordering and surface anchoring that does not allow it.
12:45 to 13:30 Lunch at Wolfson Court
14:00 to 14:45 Textures and Topology in Liquid Crystals
The textures of liquid crystals have always been central in identifying mesophases and understanding their properties. They are deeply connected with topology, both of the liquid crystal and of the environment that it lives in, so that controlling and describing topological properties provides a key tool in constructing and understanding complex three-dimensional textures. I will describe two applications of topological ideas to studying mesophases: the general characterisation of three-dimensional textures in nematics -- illustrated through the analysis of torons and the Hopf fibration in confined cholesterics -- and the formation of elegant knots and links around colloidal particles representing non-orientable surfaces.
14:45 to 15:30 Nematic braids: Effects of chirality and confinement
Nematic disclinations can form stable braids when they are stabilized by a confining geometry, chirality, or by interplay of both effects. These are stable or metastable topologically diverse defect structures in the nematic ordering field. Based on the synergy of our theoretical and numerical, approaches we are able to characterize geometries and properties of disclination loops forming braids by winding numbers, lengths, knot or link types, and self-linking numbers. We focus our attention to selected nematic braids of the lowest complexity: knotted 2D colloidal crystals, opal structures permeated by nematics, and knots in cholesteic drops. With this overview I would like to show how topology and geometry enables the assembling of complex soft materials. [1] S. ?opar and S. Žumer, Nematic Braids: Topological Invariants and Rewiring of Disclinations, Phys. Rev. Lett. 106, 177801 (2011). [2] S. ?opar, and S. Žumer, Quaternions and hybrid nematic disclinations, Proc. R. Soc. A 469, 1471 (2013). [3] U. Tkalec, M. Ravnik, S. ?opar, S. Žumer and I. Muševi?, Reconfigurable Knots and Links in Chiral Nematic Colloids, Science 333, 62 (2011). [4] S. ?opar, N. A. Clark, M. Ravnik and S. Žumer, Elementary building blocks of nematic disclination networks in densely packed 3D colloidal lattices, Soft Matter, DOI: 10.1039/C3SM50475A (2013).
15:30 to 16:00 Afternoon Tea
16:00 to 17:00 T Lubensky (University of Pennsylvania)
Rigidity, Zero Modes, States of Self Stress, and Surface Phonons in Periodic and Diluted Periodic Networks near their Instability Limit
Frames consisting of nodes connected pairwise by rigid rods or central-force springs, possibly with preferred relative angles controlled by bending forces, are useful models for systems as diverse as architectural structures, crystalline and amorphous solids, sphere packings and granular matter, networks of semi-flexible polymers, and protein structure. The rigidity of these networks depends on the average coordination number z of the nodes: If z is small enough, the frames have internal zero-frequency modes, and they are "floppy"; if z is large enough, they have no internal zero modes and they are rigid. The critical point separating these two regimes occurs at a rigidity threshold, which corresponds closely to what is often referred to as the isostatic point, that for central forces in d-dimensions occurs at coordination number zc = 2d. At and near the rigidity threshold, elastic frames exhibit unique and interesting properties, including extreme sensitivity to boundary conditions, power-law scaling of elastic moduli with (z- zc), and diverging length and time scales.

This talk will explore elastic and mechanical properties and mode structures of model periodic and diluted periodic lattices, such as the square and kagome lattices with central-force springs, that are just on verge of mechanical instability, and 4-coordinated lattices in two and three dimensions that are stabilized by bending forces. It will discuss the origin and nature of zero modes of these structures under both periodic (PBC) and free boundary conditions (FBC), and it will derive general conditions under which (a) the zero modes under the two boundary conditions are essentially identical and (b) under which zero modes do not appear in the periodic spectrum but do appear as surface Rayleigh waves in the free spectrum. In the former situation, lattices are generally in a type of critical state that admits states of self-stress in which there can be tension in bars with zero force on any node, and distortions away from that state give rise to surface modes under free boundary conditions whose degree of penetration into the bulk diverges at the critical state. This general phenomenon also occurs in sub-isostatic lattices like the honeycomb lattice. The talk will also explore diluted 4-coordinated lattices as models for networks of semi-flexible polymers, discuss the special properties that result when constituent polymers adopt strictly straight configurations.

17:00 to 18:00 Welcome wine reception
Tuesday 25th June 2013
09:00 to 09:45 Topological colloids and particle-induced defects in liquid crystals
We fabricate colloidal particles with nontrivial surface topology, including colloidal handlebodies, knots, links, Mobius strings, and Seifert surfaces. When introduced into a nematic liquid crystal with a uniform far-field director, these particles induce three-dimensional director fields and topological defects around them. We use director switching by electric fields, laser tweezing of defects, and local photothermal melting of the liquid crystal to promote transformations among many stable and metastable particle-induced director configurations and then explore them by means of a direct label-free three-dimensional nonlinear optical imaging. This reveals many peculiarities of the interplay between topologies of colloidal surfaces, director fields, and defects, which we find being in agreement with Gauss-Bonnet and Poincaré-Hopf index theorems. The lecture will conclude with a brief discussion of how these findings may lay the groundwork for new types of topology-di ctated elastic self-assembly in liquid crystals and experimental study of low-dimensional topology.
09:45 to 10:30 Colloidal Doping of Cholesterics and Blue Phases
At the interface between a cholesteric phase and an isotropic fluid, an array of defects arises. This creates for embedded colloidal particles a free energy landscape that depends on the ratio of colloid size to cholesteric pitch and on the strength of anchoring at the colloid-cholesteric interface. I shall outline recent experiments and simulations that probe this landscape. A somewhat similar situation arises when colloids are added to blue phases, for which the defect array is not interfacial but permeates the bulk phase. Preliminary simulations suggest a variety of structures, many metastable, with interesting transitions inducible by external fields and/or flow.
10:30 to 11:00 Morning Coffee
11:00 to 11:45 Modelling actomyosin droplets, and their consequences for cell motility
We present a lattice Boltzmann study of the dynamics of an actomyosin droplet, described in terms of a continuum model which follows the time evolution of actomyosin density, actin polarisation and flow. This analysis offers a simple representation of a “cell extract”, which is a highly simplified system used in vitro to understand cell dynamics, and which essentially only comprises the actin cytoskeleton and an enclosing cell membrane. In the absence of polymerization and depolymerization processes (‘treadmilling’), the dynamics of our actomyosin droplet arises solely from the contractile motion of myosin motors; this should be unchanged when polarity is inverted. Our results suggest that motility can arise in the absence of treadmilling, by spontaneous symmetry breaking (SSB) of polarity inversion symmetry. This motility mode driven by myosin contractility alone may be relevant to cell motion in three dimensions, where frictional forces, which are crucial to convert actin polymerisatino into motion, are likely to be modest. We also show of active droplets crawling on a substrate, when both treadmilling and contractility are taken into account. Our droplets can adopt a number of morphologies and motility modes found experimentally in cells, such as lamellipodia, pseudopodia and oscillatory cell motion.
11:45 to 12:30 Polar Active LCs: Non-equilibrium steady states and fluctuations
12:30 to 13:30 Lunch at Wolfson Court
14:00 to 14:45 J Yeomans (University of Oxford)
Active Nematics
Active systems, such as the cytoskeleton and bacterial suspensions, provide their own energy and hence operate out of thermodynamic equilibrium. Continuum models describing active systems are closely related to those describing liquid crystal hydrodynamics, together with an additional ‘active’ stress term. We discuss how the behaviour of the active continuum models depends on model parameters, such as the strength of the activity and the liquid crystal tumbling parameter, and we compare our results to recent experiments on cytoskeletal gels.
14:45 to 15:30 Z Dogic (Brandeis University)
Hierarchical active matter: from extensible bundles to active gels, streaming liquid crystals and motile emulsions
The emerging field of active matter promises an entirely new category of materials, with highly sought after properties such as autonomous motility and internally generated flows. In this vein, I will describe recent experiments that have focused on reconstituting dynamical structures from purified biochemical components. In particular I will describe recent advances that include: (1) assembly of a minimal model of synthetic cilia capable of generating periodic beating patterns, and conditions under which they exhibit metachronal traveling waves, (2) study of 2D active nematic liquid crystals whose streaming flows are determined by internal fractures and self-healing as well as spontaneous unbinding and recombination of oppositely charged disclination defects, (3) reconstitution of active gels characterized by highly tunable and controllable spontaneous internal flows, and (4) assembly of active emulsions in which aqueous droplets spontaneously crawl when in contact with a hard wall.
15:30 to 16:00 Afternoon Tea
16:00 to 16:45 J Toner (University of Oregon)
Rice, Locusts and Chemical Waves: A Hydrodynamic Theory of Polar Active Smectics
We present a hydrodynamic theory of polar active smectics, by which we mean active striped systemsactive systems, both with and without number conservation. For the latter, we find quasi long-ranged smectic order in $d=2$ and long-ranged smectic order in $d=3$. In $d=2$ there is a Kosterlitz-Thouless type phase transition from the smectic phase to the ordered fluid phase driven by increasing the noise strength. For the number conserving case, we find that giant number fluctuations are greatly suppressed by the smectic order; that smectic order is long-ranged in $d=3$; and that nonlinear effects become important in $d=2$.
17:00 to 21:00 Walk to Grantchester, pub dinner* (either on Tues or Thu)
Wednesday 26th June 2013
09:00 to 09:45 V Vitelli (Universiteit Leiden)
Chiral symmetry breaking in confined nematics
We analyze the complex nematic textures and defect structures that result from the competition between topological constraints and the elasticity of nematic liquid crystals confined in droplets with handles stabilized against surface-tension-driven instabilities using a yield-stress material as outer fluid. We uncover a surprisingly persistent twisted configuration of the nematic director inside the droplets when tangential anchoring is established at their boundaries, which we explain after considering the influence of saddle-splay on the elastic free energy. For toroidal droplets, we find that the saddle-splay energy screens the twisting energy resulting in a spontaneous breaking of mirror symmetry; the chiral twisted state persists for aspect ratios as large as ~20. For droplets with additional handles, two additional -1 surface defects per handle are generated in regions with local saddle geometry.
09:45 to 10:30 Hyperbolic geometry in liquid crystalline interfaces
Fluid interfaces, such as soap films, liquid droplets, or lipid membranes, are known to give rise to several special geometries, whose complexity and beauty continue to fascinate us, as observers of the natural world, and challenge us as scientists. Here I show that a special class of surfaces of constant negative Gaussian curvature can be obtained in fluid interfaces equipped with an orientational ordered phase. These arise in various soft and biological materials, such as nematic liquid crystals, cytoskeletal assemblies, or hexatic colloidal suspensions. The purely hyperbolic morphology originates from the competition between surface tension, that reduces the area of the interface at the expense of increasing its Gaussian curvature, and the orientational elasticity of the ordered phase, that in turn suffers for the distortion induced by the underlying curvature.
10:30 to 11:00 Morning Coffee
11:00 to 11:45 CD Modes (Rockefeller University)
Avenues to Active Shape Control in Nematic Solid Sheets
Much recent progress has been made in the study of nematic solids, both glassy and elastomeric in connection with these materials' remarkable coupling of the nematic texture and liquid crystalline properties to bulk elasticity and deformations. This talk will present a survey of the mechanisms and machineries that have been recently developed to allow for active control of many aspects of the shape of a thin sheet or shell of nematic solid: extrinsic bending, the blueprinting of intrinsic curvature and shape, switchable pores, and more. These examples, in addition to providing a rich tool box for potential device design, will also elucidate the connection between the topology of the nematic director field and the geometry of elastic deformations in this model system.
11:45 to 12:30 E Virga (Università degli Studi di Pavia)
Curvature potentials for defects on nematic shells
Nematic shells are thin films of nematic liquid crystal deposited on rigid colloidal particles, which can be manufactured in different shapes and guises. The two-dimensional order tensor that describes the local organization of liquid crystal molecules, which tend to lie parallel to the colloids' surface, vanishes whenever no orientation is prevailing on average. The points where this takes place are called defect, as they lack order. The lectuer will review recent work concerned with the interaction between defects and the underlying surface. In particular, arguments will be offered that identify appropiate geometric potentials, depending on the shell's curvatures, which either promote or hamper defects, attract or repel them.
12:30 to 13:30 Lunch at Wolfson Court
14:00 to 14:45 Liquid Crystal elastomers: microstructures and active shape control
Liquid crystal elastomers are highly responsive materials thanks to their ability to switch between symmetry-related states of spontaneous distortion. We will review methods to predict their macroscopic response to applied loads and fields using the technique of quasi-convexification. In addition, we will discuss the possibilities of inducing prescribed shape changes through imprinted patterns of director orientations and/or localized actuation.
14:45 to 15:30 K Urayama (Kyoto Institute of Technology)
Stretching of Polydomain and Monodomain Nematic Elastomers: Experimental Study
We review our experimental studies on the stretching driven director-rotation of polydomain nematic elastomers (PNEs) and monodomain NEs (MNEs). The texture of PNEs primarily depends on the preparation state, i.e., whether the cross-links are introduced in the high-temperature isotropic or low-temperature nematic state. The director in the isotropic-genesis PNEs can be rotated at unusually low energy cost by external fields. As a result, the isotropic-genesis PNEs can be largely deformed at small tensile force and moderate electric field strength. In contrast, the nematic genesis PNEs do not show such soft response due to the memory of the initial random director configuration. We observe the remanent of the initial director configuration even in the highly stretched state. The formation of stripe patterns has been known as a typical phenomenon of mechanical instability for the MNEs stretched normally to the initial global director. We introduce a different type of mechanical instability observed in our recent experiments for monodomain NEs.
15:30 to 16:00 Afternoon Tea & Poster Session
16:00 to 16:15 M Warner & T Wilkinson
I-CAMP Welcome
16:00 to 17:30 Posters/Discussions/Free
16:15 to 16:45 I-CAMP Overview & Logistics INI 1
16:45 to 17:30 Liquid Crystals History INI 1
17:30 to 18:15 T Wilkinson & T Lubensky & J Ball & E Terentjev & T Sluckin
PANEL Discussion - From history to future of liquid crystals
19:30 to 22:00 Conference Dinner at Lucy Cavendish College
20:00 to 22:00 I-CAMP get-together party in the Eagle pub (DNA structure)
Thursday 27th June 2013
09:00 to 09:45 Hard and soft elasticity of polydomain liquid crystal elastomers
Liquid crystal elastomers are solids that resist deformation with an elastic modulus appropriate for rubber. However, some special large deformations that induce the liquid-crystal order to rotate though the elastomer can be imposed at almost zero stress. These deformations are said to be soft. Soft elasticity is now well understood in monodomain liquid crystal elastomers (in which the liquid crystal order points in the same direction throughout the sample) but such samples are rather difficult to prepare, especially in anything but a thin film. If a liquid crystal polymer is simply crosslinked to make an elastomer, the results will be a polydomain. We model each domain of a polydomain as a small monodomain and ask whether such polydomains exhibit global soft elasticity. We show that the fabrication history of the sample is critical. Elstomers crosslinked in an isotropic state then cooled to a nematic state whilst a solid have global soft modes while those cooled to a nematic state then crosslinked do not. The addition of smectic phases allows more complicated fabrication histories with a variety of soft and hard behaviour. Finally, I will consider whether any of these polydomains can be used to build actuators.
09:45 to 10:30 N Uchida (Tohoku University)
Topological Defects in Nematic Elastomers, Gels and Membranes
We review the structure and mechanical properties of topological defects in nematic elastomers and membranes. Nematic elastomers crosslinked in the isotropic phase (isotropic-genesis polydomains) exhibit anomalously soft mechanical response, which is explained by anisotropic orientational correlation in the polydomain state. In contrast, nematic-genesis polydomains show large mechanical stress due to memory of initial defect distributions. Orientation-curvature coupling in nematic membranes causes a correlation pattern similar to that of isotropic-genesis polydomains, which results in low-energy defected states and anomalously slow ordering kinetics. The notion of relative orientational correlation function is introduced to characterize the multi-defect states in these and other materials.
10:30 to 11:00 Morning Coffee
11:00 to 11:45 Fluctuations and nonlinear elasticity of nematic elastomers
In this talk, I will first discuss how to describe elastic deformations in systems with partial translational orders and orientational orders. I will then discuss how to incorporate the effects of thermal and quenched fluctuations into elasticity theory. Finally I will address the anomalous elasticity of ideal nematic elastomers in presence of thermal as well as quenched fluctuations.
11:45 to 12:30 J Adams (University of Surrey)
Modelling Smectic Liquid Crystal Elastomers
Liquid crystal elastomers (LCEs) are rubbery materials that composed of liquid crystalline polymers (LCPs) crosslinked into a network. The rod-like mesogens incorporated into the LCPs are have random orientations in the high temperature isotropic phase, but can adopt the canonical liquid crystalline phases as the temperature is lowered. In this talk I will describe some modelling work of the layered smectic phase of LCEs.

Smectic liquid crystal elastomers have highly anisotropic mechanical behaviour. This arises in side chain smectic-A systems because the smectic layers behave as if they are embedded in the rubber matrix [1] (the same cannot be said of main chain smectic systems). The macroscopic mechanical behaviour of these solids is sensitive to the buckling of the layers, so it is a multiscale problem. A coarse grained free energy that includes the fine-scale buckling of the layers has been developed [2], which enables continuum modelling of these systems. I will describe how this continuum model, when augmented with an additional energy term describing layer buckling and other effects such as finite chain extension, can be used to model deformation of smectic-A elastomers in different experimentally accessible geometries.

Modelling smectic-C elastomers, with their tilted director, present a bigger challenge to calculating their coarse grained energy. The constraint placed on the director by the layer normal results in some unusual properties of their soft modes such as negative Poisson ratio. I will describe the geometry of these deformation modes in smectic-C elastomers [3].

[1] C. M. Spillmann et al, Phys. Rev. E 82, 031705, (2010). [2] J. Adams, S. Conti and A. DeSimone, Mathematical Models and methods in Applied Sciences, 18, 1 (2008). [3] A. W. Brown and J. M. Adams, Phys. Rev. E, 85, 011703 (2012) .

12:30 to 13:30 Lunch at Wolfson Court
14:00 to 14:45 Joint Lecture with Dynamics of Suspensions, Gels, Cells and Tissues (CFMW01)
Growth and instabilities of healthy and cancerous tissues
Joanny, J-F (Institut Curie)
Thursday 27 June 2013, 14:00-14:45
14:45 to 15:30 Joint Lecture with Dynamics of Suspensions, Gels, Cells and Tissues (CFMW01)
Reconfigurable assemblies of active, auto-chemotactic gels
Balazs, A (University of Pittsburgh)
Thursday 27 June 2013, 14:45-15:30
15:30 to 16:00 Afternoon Tea
16:00 to 16:45 Joint Lecture with Dynamics of Suspensions, Gels, Cells and Tissues (CFMW01)
Microscopic simulation of active gels: The controlling role of end detachment
Head, D (University of Leeds)
Thursday 27 June 2013, 16:45-17:15

Phase transitions and solitons in self-propelled particles: kinetic theory and diagrammatic approach
Ihle, T (North Dakota State University)
Thursday 27 June 2013, 17:15-17:45

17:00 to 21:00 Walk to Grantchester, pub dinner* (either on Tues or Thu)
Friday 28th June 2013
09:00 to 09:45 M Ravnik (University of Ljubljana)
Three-dimensional liquid crystalline superstructures for photonics (** joint talk with I-CAMP)
Liquid crystalline materials allow for formation of complex optical and photonic patterns –the superstructures- within the bulk of the material or by adding colloidal inclusions. Memory [1], self-assembly [2], topology [3], and material flow [4] can serve as guiding and controlling mechanism of the structures, offering possible competitive advantages to be used in complex optics and photonics. Here, we present three dimensional colloidal and bulk liquid crystal superstructures, as recently achieved by numerical modelling and experiments. Central to the superstructures are complex conformations of topological defects, as they can bind, stabilise, or distort the structure. We show that 3D colloidal crystals can be assembled from elastic dipoles of spherical beads in nematic liquid crystals [5] or via inherently inhomogeneous order profiles in bulk and confined cholesteric blue phases [6]. Colloidal crystals are generalised to close-packed colloidal lattices, which we show can serve as natural templates for defect networks [7]. Finally, photonic bands are calculated for selected structures and possible defects in the structure are discussed [8].

[1] T. Araki, M. Buscaglia, T. Bellini, H. Tanaka, Nature Materials 10, 303 (2011). [2] P. Poulin, H. Stark, T. C. Lubensky, D. A. Weitz, Science 275, 1770 (1997). [3] U. Tkalec, M. Ravnik, S. Copar, S. Zumer I. Musevic, Science 333, 62 (2011). [4] A. Sengupta, U. Tkalec, M. Ravnik, J.M. Yeomans, C. Bahr, S. Herminghaus, Phys. Rev. Lett. 110, 048303 (2013). [5] A. Nych, U. Ognysta, M. Škarabot, M. Ravnik, S. Žumer, I. Muševič, Nature Comm. 4, 1489 (2013). [6] M. Ravnik, G. P. Alexander, J. M. Yeomans, S. Zumer, Proc. Natl. Acad. Sci. USA 108, 5188 (2011). [7] S. Copar, N.A. Clark, M. Ravnik, S. Zumer, Elementary building blocks of densely packed 3D colloidal lattice entangled by nematic disclinations, accepted in Soft Matter. [8] M. Stimulak and M. Ravnik, Photonic bands in blue phase colloidal crystals, to be submitted;

09:45 to 10:30 C Modes (Rockefeller University)
Photo-Actuation in LC Glass (**joint talk with I-CAMP)
10:30 to 11:00 Morning Coffee
11:00 to 11:30 Photo-elastomers (**joint talk with I-CAMP)
11:30 to 11:45 Conclusions
Conclusions for Liquid Crystal Defects and their Geometry, Active and Solid Liquid Crystals, and Related Systems workshop.
12:15 to 13:30 Lunch at Wolfson Court
14:00 to 14:45 D Broer ([Eindhoven University of Technology])
Responsive Liquid Crystal Polymer Networks & Hydogels (**joint talk with I-CAMP)
In-situ photopolymerization of liquid crystalline (LC) monomers has proven to be a valuable technique for the formation of well-ordered polymer networks. Their anisotropic properties led to a variety of applications in optics, electronics and mechanics. The use of light to initiate polymerization enables lithographic approaches for patterning. The LC behaviour enables formation of complex morphologies on molecular level. Controlling the director profile of an LC network film in transversal direction gives geometrical morphing upon minor changes in order parameter. Examples of suited profiles of molecular orientation are twisted or splayed director configurations tied up in the polymer network. Reversible order parameter changes can be induced by a variety of means. It can be simply induced by temperature changes resulting in gradients in thermal expansion over the cross-section of the film. But more sophisticated and of interest for applications is a light induced change as a result of the E-Z isomerization of a built-in azo group. When the LC polymer networks are confined in the x-y plane of the film, e.g. by strong adhesion to a high modulus substrate, deformation takes place into the third dimension forming surface topographies.

Rather than composing the netyworks of covalent bonds alone, one can chose to replace some bonds by secondary interactions such as hydrogen bridges, thus providing responsive molecularly organized hydrogels. We applied the H-bridge based dimerization of benzoic acid to form nematic LC acrylate monomers. By a controlled and reversible rupture of the H-bridges mechanical responses can be initiated. Photopolymerization of smectic LC monomers lock in structures of different length scales. The first length scale is the resolution of lithography, and goes down to a few micrometers. The second length scale is set by the spacing of smectic layers and is typically a few nanometers. The third length scale is the intermolecular distance in the layers, usually around 1 nm or below. By modifying smectic molecules with H-bridges the smectic periodicity can break-up into separated layers with a well-defined spacing, also in the nanometer range. The nanopores form by breaking the H-bridges at elevated temperatures or by contact with an alkaline solution. The integrity of the film is maintained by copolymerizing with fully covalent smectic crosslinkers. By making this crosslinker photosensitive the pore size can be regulated by UV light.

14:45 to 15:30 Kinetics of Light-Induced Patterning in Liquid Crystal Elastomers (**joint talk with I-CAMP)
Liquid crystal elastomers doped with molecules that change conformation on absorption of a photon show very large changes photoelastic response. This can be investigated by recording a holographic grating in the material with the use of two crossed UV laser beams. Angular dependence of the diffraction efficiency in the vicinity of the Bragg peak can be analyzed using a numerical model that takes into account the propagation of writing beams and rate equations for the local concentration of the absorbing trans conformer, and spatial distribution of the trans and cis conformers can be obtained. Stress dependence on the irradiation time can be analyzed in a similar way. Strongly nonlinear relationship between the concentration of the cis isomers of the azomesogens and the refractive index modification of the material, which is characteristic for the phase transition region, results in non-monotonous time dependence of the diffraction efficiency of a probe beam. From this effect the sensitivity of the nematic transition temperature on the molar fraction of the cis isomers is determined. The relation between the cis isomer molar fraction and nematic order also provides a possibility for recording hidden holograms, which can be made visible by cooling the sample from the paranematic to the nematic phase.
15:30 to 16:00 Afternoon Tea
16:00 to 16:45 Liquid Crystal Elastomers and Light (**joint talk with I-CAMP)
Liquid crystal elastomers are solid liquid crystals; they combine elasticity with orientational order. Mechanical strain therefore changes liquid crystalline order and the optical properties of these materials. Conversely, light can change the orientational order, and give rise to mechanical forces and changes in shape. Light-matter interactions in LCEs therefore involve a broad range of unusual phenomena, which raise a number of intriguing questions. I will present the results of some experiments probing such phenomena, and discuss the underlying physics.
University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons