Active nematics are out-of-equilibrium liquid crystal fluids composed of rod-like subunits, which can generate large-scale, self-driven flows. In this emerging field of active nematics, new methods are needed to investigate and potentially control phase structure and dynamics. The use of complex engineered surfaces using microfabrication is an excellent way to control local orientation directors, taking advantage of the interplay between surface curvatures and topological defects. Epoxy-based lithography represents a simple and appealing approach, using low cost, minimal materials and a time efficient process. In this manuscript, we discuss methods for optimized fabrication protocols using negative and positive tone epoxy-based photoresists to create microfluidic devices for active matter. Arrays of curved objects and submerged topographies can be used to generate a variety of liquid crystal defect configurations not typically observed on unconfined planar surfaces.
Liquid crystal (LC) phase transition dynamics can be used as a powerful tool to control the assembly of dispersed nanoparticles. Tailored mesogenic ligands can both enhance and tune particle dispersion in the liquid crystal phase to create liquid crystal nano-composites - a novel type of material. Soft nanocomposites have recently risen to prominence for their potential usage in a variety of industrial applications such as photovoltaics, photonic materials, and the liquid crystal laser. Our group has developed a novel phase-transition-templating process for the generation of micron-scale, vesicle-like nanoparticle shells stabilized by mesogenic ligand-ligand interactions. The mesogenic ligand’s flexible arm structure enhances ligand alignment with the local LC director, providing control over the dispersion and stabilization of nanoparticles in liquid crystal phases. In this paper we explore the capsule formation process in detail, generating QD-based capsules over a surprisingly wide range of radii. We demonstrate that the initial nanoparticle concentration and cooling rate are important parameters influencing capsule size. By increasing particle concentration of nanoparticles and reducing the cooling rate we developed large shells up to 96±19 μm in diameter whereas decreasing concentration and increasing the cooling rate produces shells as small as 4±1 μm.
KEYWORDS: Switching, Molecules, Liquid crystals, Polarization, Temperature metrology, Plasmonics, Optical switching, Near field optics, Near field, Nanoparticles
We have developed an all-optical method to control the in- and out-of-plane spatial orientation of nematic liquid crystal
(NLC) molecules by leveraging the highly localized electric fields produced in the near-field regime of gold nanoparticle
(AuNP) layers. A 1-2 micron thick NLC film is deposited on a close-packed drop-cast AuNP layer, excited with tunable
optical sources and the transmission of white light through it analyzed using polarization optics as a function of incident
light wavelength, excitation power and sample temperature. Our findings, supported by simulations using discrete-dipole
approximations, establish the optical switching effect to be repeatable, reversible, spectrally-selective, operational over a
broad temperature range, including room temperature, and requiring very small on-resonance excitation intensity (0.3
W/cm2). For the case of the in-plane switching we have additionally demonstrated that controlling the incident excitation
polarization can continuously vary the alignment of the NLC molecules, allowing for grayscale transmission.
The ability to control and direct self-assembly of nanostructures into specific geometries with new functionalities, while
preserving their original optical and electronic properties, is an attractive research endeavor. We have fabricated liquid
crystal (LC) based matrices into which chemically synthesized nanostructures of varied morphologies and compositions
are uniformly dispersed. Using high resolution spatially- and time-resolved scanning photoluminescence (PL)
measurements, we have demonstrated directed nanoparticle assembly and manipulation in situ. This includes (a)
directional assembly and electric field modulated re-orientation of disk-shaped gallium selenide nanoparticles using a
nematic LC matrix, and (b) spectral modulation of chemically synthesized core shell CdSe/ZnS quantum dots (QDs)
embedded in a cholesteric liquid crystal (CLC) matrix. Our work opens up the possibility of designing new QD based
optical devices where spatial control of orientation, wavelength and polarization of the embedded QDs would allow
great flexibility and added functionalities.
Organic luminescent solar concentrators (LSCs) have been widely investigated due to their potential in dramatically
decreasing the cost of collecting solar energy. We designed, fabricated organic LSCs at different sizes and characterized
their optical and electrical properties. The output efficiency enhancement methods for LSCs photovoltaics (PVs) are
explored including attaching white diffusers on the bottom surfaces of LSCs, and adding a refractive index matched
optical gel between the LSC edge surfaces and the attached PV cells. To further improve the output power conversion
efficiency, multi-layered LSCs are studied and compared with single layered LSCs. The distribution of the output current
from the LSC edges varies slightly, which is beneficial to collection of the concentrated light by attached PV cells. Also,
in comparison with applying a wavelength selective film, the alignment of dye molecules using polymerized liquid
crystal is discussed as a promising optical design and efficiency improvement method.
In recent years the dispersion and directed assembly of nano-particles in liquid crystal media has proved an interesting
field for investigation and one that may yield new hybrid materials for optical applications and fundamental research. In
this paper, we investigate the dispersion of quantum dots in different liquid crystal phases, looking at aggregation and
pattern formation. Quantum dot self-assembly in liquid crystals is dependent on particle surface properties and
concentration in the liquid crystal medium. By varying these parameters we observe some fascinating structures and
phase behavior using polarized optical microscopy and fluorescence microscopy.
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