3,692 research outputs found
A mosaic infrared sensor for space astronomy, phase 3
Short wavelength (1 to 3 micron) HgCdTe mosaic detector arrays for space astronomy purposes were fabricated and studied. Honeywell will test and analyze these arrays at moderate temperatures (300-130K). Low temperature testing will be performed at the University of Hawaii. Short wavelength mosaic arrays were fabricated on three wafers and one array from each wafer was tested and analyzed. The p-type base carrier concentration on these wafers was an order of magnitude lower than typically used so far on this program (10 to the 14/cc as compared to 10 to the 15/cc). Tunneling currents are expected to decrease with this decrease in carrier concentration, resulting in improved performance at very low temperatures. The risk with such a low carrier concentration is that fixed charge in the surface passivating layer must be carefully controlled to prevent surface inversion layers
Effect of silica colloids on the rheology of viscoelastic gels formed by the surfactant cetyl trimethylammonium tosylate
The effects of the addition of sub-micrometer sized colloidal silica spheres
on the linear and nonlinear rheology of semi-dilute solutions of a viscoelastic
gel are studied. For a 1.4 wt.% solution of the surfactant CTAT, a peak in the
zero shear rate viscosity is observed at approximately equal
weight percents of silica and CTAT. This peak shifts to lower silica
concentrations on increasing either the CTAT concentration or the surface
charge on silica and disappears when the CTAT concentration is increased to
2.6wt%. The increases in and the high frequency plateau modulus
G on the introduction of SiO are explained by considering the
increasingly entangled wormlike micelles that are formed due to the enhanced
screening of the electrostatic interactions. The observed decrease in the
values of G and at higher concentrations of silica
particles is explained in terms of the formation of surfactant bilayers due to
the adsorption of the positively charged cetyl trimethylammonium to the
negatively charged silica.Comment: 28 pages, includes 8 eps and 2 png figures; accepted for publication
in Jl. Colloid Interface Sc
Nonequilibrium Fluctuation Relation for Sheared Micellar Gel in a Jammed State
We show that the shear rate at a fixed shear stress in a micellar gel in a
jammed state exhibits large fluctuations, showing positive and negative values,
with the mean shear rate being positive. The resulting probability distribution
functions (PDF's) of the global power flux to the system vary from Gaussian to
non-Gaussian, depending on the driving stress and in all cases show similar
symmetry properties as predicted by Gallavotti-Cohen steady state fluctuation
relation. The fluctuation relation allows us to determine an effective
temperature related to the structural constraints of the jammed state. We have
measured the stress dependence of the effective temperature. Further,
experiments reveal that the effective temperature and the standard deviation of
the shear rate fluctuations increase with the decrease of the system size.Comment: 5 pages 3 figure
Site-Specific Colloidal Crystal Nucleation by Template-enhanced Particle Transport
The monomer surface mobility is the single most important parameter that
decides the nucleation density and morphology of islands during thin film
growth. During template-assisted surface growth in particular, low surface
mobilities can prevent monomers from reaching target sites and this results in
a partial to complete loss of nucleation control. While in atomic systems a
broad range of surface mobilities can be readily accessed, for colloids, owing
to their large size, this window is substantially narrow and therefore imposes
severe restrictions in extending template-assisted growth techniques to steer
their self-assembly. Here, we circumvented this fundamental limitation by
designing templates with spatially varying feature sizes, in this case moire
patterns, which in the presence of short-range depletion attraction presented
surface energy gradients for the diffusing colloids. The templates serve a dual
purpose, first, directing the particles to target sites by enhancing their
surface mean free paths and second, dictating the size and symmetry of the
growing crystallites. Using optical microscopy, we directly followed the
nucleation and growth kinetics of colloidal islands on these surfaces at the
single-particle level. We demonstrate nucleation control, with high fidelity,
in a regime that has remained unaccessed in theoretical, numerical and
experimental studies on atoms and molecules as well. Our findings pave the way
for fabricating non-trivial surface architectures composed of complex colloids
and nanoparticles.Comment: 12 pages, 3 figure
Origin of the unusual dependence of Raman D band on excitation wavelength in graphite-like materials
We have revisited the still unresolved puzzle of the dispersion of the Raman
disordered-induced D band as a function of laser excitation photon energy E
in graphite-like materials. We propose that the D-mode is a combination of an
optic phonon at the K-point in the Brillioun zone and an acoustic phonon whose
momentum is determined uniquely by the double resonance condition. The fit of
the experimental data with the double-resonance model yields the reduced
effective mass of 0.025m for the electron-hole pairs corresponding to the
A transition, in agreement with other experiments. The model can also
explain the difference between and for D and
D modes, and predicts its dependence on the Raman excitation
frequency.Comment: 4 figures in eps forma
Banded spatiotemporal chaos in sheared nematogenic fluids
We present the results of a numerical study of a model of the hydrodynamics
of a sheared nematogenic fluid, taking into account the effects of order
parameter stresses on the velocity profile, but allowing spatial variations
only in the gradient direction. When parameter values are such that the stress
from orientational distortions is comparable to the bare viscous stress, the
system exhibits steady states with the characteristics of shear banding. In
addition, nonlinearity in the coupling of extensional flow to orientation leads
to the appearance of a new steady state in which the features of both
spatiotemporal chaos and shear banding are present.Comment: 4 pages, 6 figures.(Some of the figures have low resolution so as to
fit the requirements of arxiv.
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