122 research outputs found
Size effect in ion transport through angstrom-scale slits
It has been an ultimate but seemingly distant goal of nanofluidics to
controllably fabricate capillaries with dimensions approaching the size of
small ions and water molecules. We report ion transport through ultimately
narrow slits that are fabricated by effectively removing a single atomic plane
from a bulk crystal. The atomically flat angstrom-scale slits exhibit little
surface charge, allowing elucidation of the role of steric effects. We find
that ions with hydrated diameters larger than the slit size can still permeate
through, albeit with reduced mobility. The confinement also leads to a notable
asymmetry between anions and cations of the same diameter. Our results provide
a platform for studying effects of angstrom-scale confinement, which is
important for development of nanofluidics, molecular separation and other
nanoscale technologies
Voids as a Precision Probe of Dark Energy
A signature of the dark energy equation of state may be observed in the shape
of voids. We estimate the constraints on cosmological parameters that would be
determined from the ellipticity distribution of voids from future spectroscopic
surveys already planned for the study of large scale structure.
The constraints stem from the sensitivity of the distribution of ellipticity
to the cosmological parameters through the variance of fluctuations of the
density field smoothed at some length scale. This length scale can be chosen to
be of the order of the comoving radii of voids at very early times when the
fluctuations are Gaussian distributed. We use Fisher estimates to show that the
constraints from void ellipticities are promising. Combining these constraints
with other traditional methods results in the improvement of the Dark Energy
Task Force Figure of Merit on the dark energy parameters by an order of hundred
for future experiments. The estimates of these future constraints depend on a
number of systematic issues which require further study using simulations. We
outline these issues and study the impact of certain observational and
theoretical systematics on the forecasted constraints on dark energy
parameters.Comment: Submitted to PRD, 22 pages 9 figure
Tunable photocatalytic selectivity of TiO2/SiO2 nanocomposites: effect of silica and isolation approach
Can a Reduced Dimension Interface Model be More Computationally Effective than a Docking Algorithm?
2D/3D Visual Tracker for Rover Mast
A visual-tracker computer program controls an articulated mast on a Mars rover to keep a designated feature (a target) in view while the rover drives toward the target, avoiding obstacles. Several prior visual-tracker programs have been tested on rover platforms; most require very small and well-estimated motion between consecutive image frames a requirement that is not realistic for a rover on rough terrain. The present visual-tracker program is designed to handle large image motions that lead to significant changes in feature geometry and photometry between frames. When a point is selected in one of the images acquired from stereoscopic cameras on the mast, a stereo triangulation algorithm computes a three-dimensional (3D) location for the target. As the rover moves, its body-mounted cameras feed images to a visual-odometry algorithm, which tracks two-dimensional (2D) corner features and computes their old and new 3D locations. The algorithm rejects points, the 3D motions of which are inconsistent with a rigid-world constraint, and then computes the apparent change in the rover pose (i.e., translation and rotation). The mast pan and tilt angles needed to keep the target centered in the field-of-view of the cameras (thereby minimizing the area over which the 2D-tracking algorithm must operate) are computed from the estimated change in the rover pose, the 3D position of the target feature, and a model of kinematics of the mast. If the motion between the consecutive frames is still large (i.e., 3D tracking was unsuccessful), an adaptive view-based matching technique is applied to the new image. This technique uses correlation-based template matching, in which a feature template is scaled by the ratio between the depth in the original template and the depth of pixels in the new image. This is repeated over the entire search window and the best correlation results indicate the appropriate match. The program could be a core for building application programs for systems that require coordination of vision and robotic motion
How to detect gravitational waves through the cross-correlation of the galaxy distribution with the CMB polarization
Thompson scattering of cosmic microwave background (CMB) photons off of free
electrons during the reionization epoch induces a correlation between the
distribution of galaxies and the polarization pattern of the CMB, the magnitude
of which is proportional to the quadrupole moment of radiation at the time of
scattering. Since the quadrupole moment generated by gravitational waves (GWs)
gives rise to a different polarization pattern than that produced by scalar
modes, one can put interesting constraints on the strength of GWs on large
scales by cross-correlating the small scale galaxy distribution and CMB
polarization. We use this method together with Fisher analysis to predict how
well future surveys can measure the tensor-to-scalar ratio . We find that
with a future CMB experiment with detector noise Delta_P = 2 mu K-arcmin and a
beam width theta_FWHM = 2' and a future galaxy survey with limiting magnitude
I<25.6 one can measure the tensor-to-scalar ratio with an error sigma_r \simeq
0.09. To measure r \approx 0.01, however, one needs Delta_P \simeq 0.5 mu
K-radian and theta_FWHM \simeq 1'. We also investigate a few systematic
effects, none of which turn out to add any biases to our estimators, but they
increase the error bars by adding to the cosmic variance. The incomplete sky
coverage has the most dramatic effect on our constraints on r for large sky
cuts, with a reduction in signal-to-noise smaller than one would expect from
the naive estimate (S/N)^2 \propto f_sky. Specifically, we find a degradation
factor of f_deg=0.32 \pm 0.01 for a sky cut of |b|>10^\circ (f_sky=0.83) and
f_deg=0.056 \pm 0.004 for a sky cut of |b|>20^\circ (f_sky=0.66). Nonetheless,
given that our method has different systematics than the more conventional
method of observing the large scale B modes directly, it may be used as an
important check in the case of a detection.Comment: 18 pages, 6 figures, to be submitted to PR
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