Session B50: Focus Session: Nano to Mesoscale Structures in Ordered Systems: Liquid Crystalline Structure and Interactions

Molecular Assembly and Liquid Crystal Properties of a Near-IR Absorbing Dye

The molecules of the near-IR absorbing dye IR-806 spontaneously assemble in water at very low concentrations, forming a liquid crystal phase at room temperature when the concentration is above 0.6 wt\%. Unlike most chromonic liquid crystal systems, macroscopic phase separation between the isotropic and liquid crystal phases is not observed. Also unlike most chromonic liquid crystal systems, the absorption spectrum of IR-806 changes dramatically as the concentration increases and molecular assembly proceeds. Analysis of the absorption spectra provides evidence of an isodesmic assembly process at an extremely low concentration, followed by a second non-isodesmic assembly process at a higher concentration just before the liquid crystal phase appears.   

Studies on Sunset Yellow Chromonic liquid crystals by Polarized Raman and X-ray Scattering

Sunset Yellow FCF (SSY) molecules aggregate into columns in water and form chromonic liquid crystalline phases. Nematic SSY is aligned both in a flat capillary and in the magnetic field with columns pointing perpendicular to the long axis of capillary and perpendicular to the magnetic field direction, respectively. Temperature and concentration dependence of order parameters, both $<$P$_{200}>$ and $<$P$_{400}>$, are calculated based on polarized Raman measurement. The scission energy, E, determined from the Arrhenius thermal evolution of the longitudinal correlation length, is found to be around 4.3k$_{B}$T in the nematic N phase based on x-ray measurement. Flow behavior of 1.1 M nematic SSY chromonic solution under steady shear is predicted using the order parameters measured and the aspect ratios of columns.   

Photoisometric liquid crystals integrated with plasmonic nano-structures

Optically switching photoisometric liquid crystals can be used for a variety of applications. However, the dependence of the efficiency of photoisometric processes on optical fluence requires power levels too high for many practical applications. Recently it has been shown that symmetry breaking in deterministic~aperiodic plasmonic nano-structures boosts the efficiency of nonlinear processes by producing significant spatial field localization. This technology has the potential to dramatically enhance the performance of photoisometric liquid crystal devices. In this work, we combine photoisometric liquid crystals with lithographically fabricated periodic and aperiodic plasmonic nano-particle arrays in microfluidic channels to enhance their all optical switching properties. Using rigorous analytical multiple scattering methods we engineer particle arrays for near field enhancement at a control wavelength and the diffraction properties of a probe beam at another. Optical pump probe measurements of retardance and absorption are used to characterize the liquid crystal's structure factor as function pump power. The effects of plasmonic particle arrays on the switching dynamics on the photoisometric liquid crystals are also experimentally explored.   

Elasticity of lyotropic chromonic liquid crystal Sunset Yellow probed by magnetic Frederiks transition

By using director reorientation in the magnetic field, we determine the concentration and temperature dependencies of the splay $K_1$, twist $K_2$, and bend $K_3$ elastic constants (normalized by the anisotropy of the diamagnetic susceptibility) for a nematic lyotropic chromonic liquid crystal (LCLC) Sunset Yellow. In a sharp contrast to thermotropic liquid crystals, the Frederiks effects in LCLC show a hysteresis, which is more pronounced at high concentration and low temperatures. We attribute the hysteresis to the changes in self-assembled structure of LCLC aggregates under the influence of field-imposed deformations.   

Electric Field Induced Stable Micro Rotor in Nematic Liquid Crystal Drops Constrained on Thin Cellulosic Fibers

We directly visualize the response of nematic liquid crystal drops of toroidal topology constrained on thin fibers, suspended in air, to an AC applied electric field \textbf{E}. This new localized liquid crystal system can exhibit non-trivial point defects, which may become energetically unstable against expanding into ring disclinations depending on the fiber constraining geometries. The director anchoring tangential near the fiber surface and homeotropic at the air interface, making a hybrid shell distribution that in turn causes a ring of disclination line around the main axis of the fiber at the center of the droplet. Upon application of \textbf{E}, the disclination ring first expands and slightly moves along the fiber main axis, followed by the appearance of a stable ``spherical particle'' orbiting around the fiber at the center of the liquid crystal drop. The rotation speed of this particle was found to vary linearly with the applied voltage. This constrained liquid crystal geometry seems to meet the essential requirements in which soliton like particles can develop and exhibit stable orbiting in three dimensions upon application of an external electric field. This is another example of a soft energy transducer system which allows, at the micro scale, the transfer in a continuous way of electrical to mechanical energy.   

Synthesis and investigations of mesogen encapsulated gold nanoparticles

Nematic mesogen encapsulated gold nanoparticles with defined size and shapes are currently of great interest for a wide range of applications for electro-optical device or metamaterials. However the synthesis of most of the materials reported so far is quite cumbersome. Thus there is the need for new routes to synthesize more advanced compounds. A suitable strategy could be based on functionalizing the organic shell. In this contribution we report a new method to prepare gold nanoparticles with a bifunctional capping agent enabling control over their size and also act as a linking group for the connection with the mesogenic groups. The result showed monolayer coated gold nanoparticles without any co-ligands and lower the isotropic point. Physical and optical properties of the nematic gold nanoparticles have been characterized by HRTEM, EDS, NMR, DSC, TGA, XRD, and OPM. The results show stable nematic mesophase formation at room temperature. The investigate system displays a typical nematic schlieren texture, and a decreased viscosity when compared to other LC Au NPs. Structure properties relationships will be discussed and the materials will be compared to earlier research.   

Nonlinear Diffusion on a Sphere

We are interested in describing orientation of non-spherical particles such as liquid crystal molecules, nanoparticles and colloidal particles due to interactions with each other and with external fields. Since the orientation of a rod-like particle corresponds to a point on a unit sphere, the time evolution of the orientational distribution function corresponds to nonlinear diffusion on a sphere. We use a direct cell-based method to solve the Smoluchowski equation describing the behavior. We construct the Voronoi tessellation on the sphere, and regularize it. We then use a finite volume method to compute the particle density in the cells. Numerical results show the time evolution of the orientational probability density function. These results can describe the behavior of nanorod suspensions in electric fields.   

Elastic Torque on a Ferromagnetic Disk within a Nematic Liquid Crystal

An aspherical particle suspended in a nematic liquid crystal will impose an orientationally dependent energy due to coupling to the nematic elasticity. This energy depends strongly on the anchoring conditions on the surface of the inclusion, its shape, as well as the proximity of other boundary conditions on the fluid such as those set by the container. To study these properties, ferromagnetic nickel disks with homeotropic surface anchoring were suspended in the liquid crystal 4-cyano-4'-pentylbiphenyl (5CB) in a planar cell. The disks, 300 nm in thickness and 10 $\mu$m in diameter, possess a permanent magnetic moment confined to the disk's plane. In the absence of any external torque the disks align with the normal to their faces parallel to the director. Rotating of the disks from this preferred orientation creates an elastic deformation that is manifested by an opposing torque. Balancing this torque with the torque from an external magnetic field for various angles of rotation, we have mapped out the orientationally dependent energy. Over a large range of angles the torque shows a linear dependence as predicted by an electrostatic analogy.   

Anomalous diffusion of colloidal particles in a nematic liquid crystal

We explore the Brownian motion of colloidal microspheres in a nematic liquid crystal within time scales below 100ms that was not accessible in previous experiments. Our experimental results point towards an apparent sub-diffusion of the colloids with a mean square displacement MSD $\propto t^{1/2}$. For longer time scales, the particles exhibit normal diffusion with two anisotropic diffusion constants parallel and perpendicular to the nematic director $\bf n$ [1]. The nonlinear effect vanishes when the host is heated up to the isotropic phase; therefore the subdiffusive behavior can be attributed to the coupling of slow director fluctuations of the nematic with the colloidal particle dynamics. We also discuss the role of finite accuracy of measurements. \\[4pt] [1] J. C. Loudet, P. Hanusse and P. Poulin, Science 306, (2004).   

Colloids at a Chiral Liquid Crystal-Isotropic Liquid Interface

Whilst the behavior of particles trapped at liquid-liquid interfaces is relatively well understood the behavior as one of the phases begins to break translational symmetry is almost completely unexplored. Here the particles seed defects in the partially ordered liquid and new, effective, particle-particle interactions are induced. We use a chiral (cholesteric) liquid crystal which has a characteristic length scale, the pitch length, similar to the particle size. Our system consists of particles with planar anchoring which are trapped at an interface between the liquid crystal and an isotropic liquid (silicone oil which induces homeotropic anchoring). The creation of the cholesteric ``fingerprint'' texture allows the deformation of the cholesteric around a particle to be easily visualized. This allows us to determine the nature of the defects created and their symmetries. We have clear trends for the distribution of particles with respect to the interface as a function of particle size. Inspired by computer simulations we study the position of small particles (diameter $<$ pitch length) within the fingerprint texture. The behavior of the unadorned interface between the chiral liquid crystal and the oil is also explored.   

Deformable Colloids in the Presence of a Liquid Crystal

Spherical colloidal particles immersed in a liquid crystal experience a non-uniform pressure, as well as directional interactions among one another due to the defects they induce in the surrounding liquid crystal. Here, we use a lattice-Boltzmann algorithm to investigate the behavior of initially circular, 2D deformable colloids placed in a nematic liquid crystal. The colloidal particles, which are represented using a bead-spring model, are of a sufficient physical size to ensure that the anchoring of the liquid crystal molecules on their surface has a significant impact on the background liquid crystal. We present the resulting equilibrium particle shapes for a range of surface elasticities, and investigate the interaction between pairs of particles.   

Multistable alignment of nematic liquid crystals on patterned surfaces

A nematic in contact with a substrate patterned to promote a spatially-varying easy axis experiences a large elastic distortion adjacent to the surface and relaxes to a uniform state in the bulk. The bulk ordering may be thought of as an effective easy axis which, unlike conventional surface treatments, can be easily controlled by adjusting the geometry of the pattern. In this work, the behavior of a nematic film confined between substrates periodically patterned with rectangles is examined analytically. It is shown that multiple stable configurations exist and the effective azimuthal anchoring energy may be arbitrarily controlled by changing the aspect ratio of the rectangles. The various effects of flexoelectricity and saddle-splay elasticity, both important because of large spatial gradients in molecular orientation near the surface, are also considered. Prospects for applications of these surfaces in electrooptic devices such as displays are discussed.   

Direct Nanomechanical Measurement of an Anchoring Transition in a Nematic Liquid Crystal Subject to Hybrid Anchoring Conditions

A Surface Forces Apparatus was used to measure the normal force between two solid curved surfaces confining a film of nematic liquid crystal (5CB, 4'-$n$-pentyl-4-cyanobiphenyl) under hybrid planar-homeotropic anchoring conditions. Upon reduction of the surface separation $D$, we measured an increasingly repulsive force in the range $D$ = 35-80 nm, reaching a plateau in the range $D$ = 10-35 nm, followed by a short-range oscillatory force at $D$ $<$ 5 nm. The oscillation period was comparable to the cross-sectional diameter of the liquid crystal molecule and characteristic of a configuration with the molecules parallel to the surfaces. These results show that the director field underwent a confinement-induced transition from a splay-bend distorted configuration at large $D$, which produces elastic repulsive forces, to a uniform planar configuration with broken homeotropic anchoring, which does not produce additional elastic forces as $D$ is decreased. These findings, supported by measurements of the birefringence of the confined film at different film thicknesses, provide the first direct visualization of an anchoring transition at the nanometer scale.   

Unified theory of chiral smectic A monolayers and $\pi$-wall defects

Monodisperse suspensions of the rodlike chiral \textit{fd} viruses are condensed into one rod length thick colloidal monolayers of aligned rods by depletion forces. Twist deformations of the molecules are expelled to the monolayer edge as in a chiral smectic A (Sm-A*) liquid crystal, and a cholesteric (Ch) region forms at the edge. Coalescence of two such isolated monolayers results in a cholesteric wall sandwiched between two regions of aligned \textit{fd} viruses, dubbed $\pi$-wall defects. Based on the analogy of Sm-A* with superconductors, we develop a unified theory of the $\pi$-wall defects and the monolayer edge structure. Our model yields the molecular tilt profiles, the local thickness change, and the crossover from Sm-A*-to-Ch behavior across the monolayer and the $\pi$-wall. These allow us to determine the line tension as a function of the depletant polymer concentration and the chirality of the viruses, in agreement with experiment.   

Molecular Tilt on Monolayer-Protected Nanoparticles

We present a simple Ginzburg-Landau model to describe the order of ligands coating small metal nanoparticles (NPs). Two dimensionless parameters are introduced: a preferential tilt angle and a ratio epsilon between the energy cost due to spatial variations in the tilt of the coating molecules and that of the van der Waals interactions which favors uniform tilt. Even for the ground state, topological defects are present due to the topology of the NPs. The ground state for spherical particles is an ordered bipolar defective texture (B) for small epsilon and an untilted phase (U) for large epsilon. Octahedral particles have an additional phase (6V) at small epsilon characterized by the presence of six topological defects.