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Deck the Walls with Anisotropic Colloids in Nematic Liquid Crystals.
Nematic liquid crystals (NLCs) offer remarkable opportunities to direct colloids to form complex structures. The elastic energy field that dictates colloid interactions is determined by the NLC director field, which is sensitive to and can be controlled by boundaries including vessel walls and colloid surfaces. By molding the director field via liquid-crystal alignment on these surfaces, elastic energy landscapes can be defined to drive structure formation. We focus on colloids in otherwise defect-free director fields formed near undulating walls. Colloids can be driven along prescribed paths and directed to well-defined docking sites on such wavy boundaries. Colloids that impose strong alignment generate topologically required companion defects. Configurations for homeotropic colloids include a dipolar structure formed by the colloid and its companion hedgehog defect or a quadrupolar structure formed by the colloid and its companion Saturn ring. Adjacent to wavy walls with wavelengths larger than the colloid diameter, spherical particles are attracted to locations along the wall with distortions in the nematic director field that complement those from the colloid. This is the basis of lock-and-key interactions. Here, we study ellipsoidal colloids with homeotropic anchoring near complex undulating walls. The walls impose distortions that decay with distance from the wall to a uniform director in the far field. Ellipsoids form dipolar defect configurations with the colloid's major axis aligned with the far field director. Two distinct quadrupolar defect structures also form, stabilized by confinement; these include the Saturn I configuration with the ellipsoid's major axis aligned with the far field director and the Saturn II configuration with the major axis perpendicular to the far field director. The ellipsoid orientation varies only weakly in bulk and near undulating walls. All configurations are attracted to walls with long, shallow waves. However, for walls with wavelengths that are small compared to the colloid length, Saturn II is repelled, allowing selective docking of aligned objects. Deep, narrow wells prompt the insertion of a vertical ellipsoid. By introducing an opening at the bottom of such a deep well, we study colloids within pores that connect two domains. Ellipsoids with different aspect ratios find different equilibrium positions. An ellipsoid of the right dimension and aspect ratio can plug the pore, creating a class of 2D selective membranes
Lassoing saddle splay and the geometrical control of topological defects
Systems with holes, such as colloidal handlebodies and toroidal droplets,
have been studied in the nematic liquid crystal (NLC) 4-cyano-4'-pentylbiphenyl
(5CB): both point and ring topological defects can occur within each hole and
around the system, while conserving the system's overall topological charge.
However, what has not been fully appreciated is the ability to manipulate the
hole geometry with homeotropic (perpendicular) anchoring conditions to induce
complex, saddle-like deformations. We exploit this by creating an array of
holes suspended in an NLC cell with oriented planar (parallel) anchoring at the
cell boundaries. We study both 5CB and a binary mixture of bicyclohexane
derivatives (CCN-47 and CCN-55). Through simulations and experiments, we study
how the bulk saddle deformations of each hole interact to create novel defect
structures, including an array of disclination lines, reminiscent of those
found in liquid crystal blue phases. The line locations are tunable via the NLC
elastic constants, the cell geometry, and the size and spacing of holes in the
array. This research lays the groundwork for the control of complex elastic
deformations of varying length scales via geometrical cues in materials that
are renowned in the display industry for their stability and easy
manipulability.Comment: 9 pages, 7 figures, 1 supplementary figur
Simple Estimation of X- Trion Binding Energy in Semiconductor Quantum Wells
A simple illustrative wave function with only three variational parameters is
suggested to calculate the binding energy of negatively charged excitons (X-)
as a function of quantum well width. The results of calculations are in
agreement with experimental data for GaAs, CdTe and ZnSe quantum wells, which
differ considerably in exciton and trion binding energy. The normalized X-
binding energy is found to be nearly independent of electron-to-hole mass ratio
for any quantum well heterostructure with conventional parameters. Its
dependence on quantum well width follows an universal curve. The curve is
described by a simple phenomenological equation.Comment: 8 pages, 3 Postscript figure
Elasticity-Dependent Self-assembly of Micro-Templated Chromonic Liquid Crystal Films
We explore micropatterned director structures of aqueous lyotropic chromonic
liquid crystal (LCLC) films created on square lattice cylindrical-micropost
substrates. The structures are manipulated by modulating the LCLC mesophases
and their elastic properties via concentration through drying. Nematic LCLC
films exhibit preferred bistable alignment along the diagonals of the micropost
lattice. Columnar LCLC films, dried from nematics, form two distinct director
and defect configurations: a diagonally aligned director pattern with local
squares of defects, and an off-diagonal configuration with zig-zag defects. The
formation of these states appears to be tied to the relative splay and bend
free energy costs of the initial nematic films. The observed nematic and
columnar configurations are understood numerically using a Landau-de Gennes
free energy model. Among other attributes, the work provide first examples of
quasi-2D micropatterning of LC films in the columnar phase and lyotropic LC
films in general, and it demonstrates alignment and configuration switching of
typically difficult-to-align LCLC films via bulk elastic properties.Comment: 9 pages; 9 figures; accepted for publication in Soft Matte
Elastocapillary driven assembly of particles at free-standing smectic-A films
Colloidal particles at complex fluid interfaces and within films assemble to
form ordered structures with high degrees of symmetry via interactions that
include capillarity, elasticity, and other fields like electrostatic charge.
Here we study microparticle interactions within free-standing smectic-A films,
in which the elasticity arising from the director field distortion and
capillary interactions arising from interface deformation compete to direct the
assembly of motile particles. New colloidal assemblies and patterns, ranging
from 1D chains to 2D aggregates, sensitive to the initial wetting conditions of
particles at the smectic film, are reported. This work paves the way to
exploiting LC interfaces as a means to direct spontaneously formed,
reconfigurable, and optically active materials.Comment: 8 pages, 6 figures. Supplementary Materials: 3 pages, 3 figure
Tunable colloid trajectories in nematic liquid crystals near wavy walls
The ability to dictate colloid motion is an important challenge in fields
ranging from materials science to living systems. Here, by embedding energy
landscapes in confined nematic liquid crystals, we design a versatile platform
to define colloidal migration. This is achieved by placing a wavy wall, with
alternating hills and wells, in nematic liquid crystals, to impose a smooth
elastic energy field with alternating splay and bend distortions. This domain
generates (meta) stable loci that act as attractors and unstable loci that
repel colloids over distances large compared to the colloid radius. Energy
gradients in the vicinity of these loci are exploited to dictate colloid
trajectories. We demonstrate several aspects of this control, by studying
transitions between defect configurations, propelling particles along well
defined paths and exploiting multistable systems to send particles to
particular sites within the domain. Such tailored landscapes have promise in
reconfigurable systems and in microrobotics applicationsComment: 13 pages, 8 figure
Near-field spectroscopy of a gated electron gas: a direct evidence for electrons localization
The near-field photoluminescence of a gated two-dimensional electron gas is
measured. We use the negatively charged exciton, formed by binding of an
electron to a photo-excited electron-hole pair, as an indicator for the local
presence of charge. Large spatial fluctuations in the luminescence intensity of
the negatively charged exciton are observed. These fluctuations are shown to be
due to electrons localized in the random potential of the remote ionized
donors. We use these fluctuations to image the electrons and donors
distribution in the plane.Comment: 10 pages, 5 figures, to be published in PR
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