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Temperature driven anchoring
transitions in Polymer Dispersed Liquid Crystals
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We study temperature driven anchoring transition in
nematic/polymer composites as well as chiral nematic/polymer composites.
Polymer dispersed liquid crystalline films are formed by photo-polymerizing
the mixture of nematic fluid, monomer and crosslinking agent. As the
photo-polymerization proceeds, the polymer polygonal cells that encapsulate
the nematic liquid crystal. Since the anchoring behavior of liquid crystal
is controlled by surface characteristics of the polymer, we modify and
study the anchoring behavior by changing the length and branching of side
groups
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Ordered Arrays of
Holes in Polymer Films
by
Breath Figure
Templated Assembly
This is a simple and elegant way of producing macroporous
polymer films with ordered arrays of bubbles. Flow of moist air over a
dilute polymer solution with volatile solvent causes water drops to
nucleate and form over the film with solvent evaporating. These breath
figures then template a three dimensional ordered array of bubbles in
polymer films. We have shown that we can create such structures in
conjugated polymers, and since the bubbles are of dimensions of the order
of wavelength of light, these materials are of interest in say photonic
band gap materials.
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Nano-optics in Biological World
In a world that is as colorful as ours, it is not
surprising to find that colors produced in nature by a variety of creatures
have attracted the attention of scientific giants such as Newton,
Michelson, and Lord Rayleigh, just to name a few. It is perhaps somewhat
surprising to find that there are still many questions remaining to be
answered with regard to color produced by animals, like butterflies and
beetles, and what that color means to them. This gets us into the realm of
color vision and color perception in the animal world. We primarily focus
on color production in nature purely by physical means such as diffraction,
interference, and scattering, and attempt to learn from nature better ways
of producing colors and optical effects purely by physical means.
Understanding fiber coating process: Instability
development and drop shape analysis
Fibers are often coated by drawing them through a complex
fluid to provide them with a thin layer of coating/finish chosen say to
provide chemical and mechanical protection to fiber or an anti-static layer
or as in composites to alter surface characteristics of the fiber or simply
to impart color. A fluid film on a fiber is generally unstable, and it
spontaneously undulates and breaks into a periodic array of drops. This
instability simply results due to surface tension of the fluid and is known
as Rayleigh instability for pioneering work done by Lord Rayleigh in 1882
and can be easily observed by noticing how water beads up on a spider web.
We are investigating the the instability development by analyzing droplet
formation and droplet shapes for a variety of complex fluids.
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Rotaxanated Anisotropic
Structures: Organic solar cells
Rotaxanes consist of
macrocyclic rings trapped onto linear molecules by end capping the
threading molecule with two bulky substituents. We synthesize and
characterize rotaxanes based on cyclodextrins (cyclic sugars with 6, 7 or 8
glucose units) and are studying their optical and electro-optical
properties with aim to make supramolecular devices based on them. The
cyclodextrins have a hydrophobic interior and a hydrophilic exterior and
thus improve the water solubility of several 'threading' molecules, say
conjugated structures. In addition to improving water solubility which
makes them candidates for say improving solubility of pharmaceutical
compounds, these systems are ideal host-guest compounds forming model
systems to receptor-substrate systems.. The architecture and specific
molecular interactions provide ways of controlling luminescence, charge
transport as well as chemical and mechanical stability in these molecular
materials. We study the interactions and the photo-physical properties of
our model compounds both to understand the underlying science and to
provide material for making organic solar cells.
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Dye diffusion in fibers: 3D
reconstruction of trilobal nylon fiber dyed with fluorescein studied with
laser confocal microscopy
We study of dye
diffusion in fibers using confocal microscopy, where the diffusion
coefficient of the dye is determined by imaging the fluorescence intensity
profile across the fiber cross-section as a function of time.
LSCM-FRAP (Laser Scanning Confocal Microscopy-Florescence Recovery After
Photo-bleaching) is a non-invasive technique based on irreversibly
bleaching a fluorescent probe in a well defined volume element with a high
intensity laser. Since LSCM employs spot illumination and strongly
eliminates out of the focus light, the internal structure of the fiber can
be visualized without physical sectioning of the fiber. This research is
motivated both by desire to better understand diffusion of dyes into fibers
and by desire to provide solution to the problem of dye streaking in
fabrics observed in industry.
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Stereocomplex fibers and self reinforced
composites (funded by NSF)
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(Details will be
added subsequently, meanwhile see also abstracts of selected publications here)
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maintained by Vivek Sharma.
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