1,118 research outputs found
Characterization of soil and postlaunch pad debris from Cape Canaveral launch complex and analysis of soil interaction with aqueous HCl
Soil samples were fractionated and analyzed in order to assess the physical and chemical interactions of entrained soil with solid-rocket exhaust clouds. The sandy soil consisted primarily of quartz (silica) particles, 30 to 500 microns in diameter, and also contained seashell fragments. Differential and cumulative soil-mass size distributions are presented along with mineralogy, elemental compositions, and solution pH histories. About 90 percent of the soil mass consisted of particles 165 microns in diameter. Characteristic reaction times in aqueous HC1 slurries varied from a few minutes to several days, and capacities for reaction under acidic conditions varied from 10 to 40 g HCl/kg soil, depending on particle size. Airborne lifetimes of particles 165 microns are conservatively 30 min, and this major grouping is predicted to represent a small short-term chemical sink for up to 5% of the total HC1. The smaller and more minor fractions, below a 165 micron diameter, may act as giant cloud condensation nuclei over much longer airborne lifetimes. Finally, the demonstrated time dependency of neutralization is a complicating factor; it can influence the ability to deduce in-cloud HCl scavenging with reaction and can affect the accuracy of measured chemical compositions of near-field wet deposition
Computer Simulation of Cytoskeleton-Induced Blebbing in Lipid Membranes
Blebs are balloon-shaped membrane protrusions that form during many
physiological processes. Using computer simulation of a particle-based model
for self-assembled lipid bilayers coupled to an elastic meshwork, we
investigated the phase behavior and kinetics of blebbing. We found that blebs
form for large values of the ratio between the areas of the bilayer and the
cytoskeleton. We also found that blebbing can be induced when the cytoskeleton
is subject to a localized ablation or a uniform compression. The results
obtained are qualitatively in agreement with the experimental evidence and the
model opens up the possibility to study the kinetics of bleb formation in
detail.Comment: To appear in Physical Review
Interstellar Scintillation Observations of 146 Extragalactic Radio Sources
From 1979--1996 the Green Bank Interferometer was used by the Naval Research
Laboratory to monitor the flux density from 146 compact radio sources at
frequencies near 2 and 8 GHz. We filter the ``light curves'' to separate
intrinsic variations on times of a year or more from more rapid interstellar
scintilation (ISS) on times of 5--50 d. Whereas the intrinsic variation at 2
GHz is similar to that at 8 GHz (though diminished in amplitude), the ISS
variation is much stronger at 2 than at 8 GHz. We characterize the ISS
variation by an rms amplitude and a timescale and examine the statistics of
these parameters for the 121 sources with significant ISS at 2 GHz. We model
the scintillations using the NE2001 Galactic electron model assuming the
sources are brightness-limited.
We find the observed rms amplitude to be in general agreement with the model,
provided that the compact components of the sources have about 50% of their
flux density in a component with maximum brightness temperatures
--K. Thus our results are consistent with cm-wavelength VLBI
studies of compact AGNs, in that the maximum brightness temperatures found are
consistent with the inverse synchrotron limit at K, boosted
in jet configurations by Doppler factors up to about 20. The average of the
observed 2 GHz ISS timescales is in reasonable agreement with the model at
Galactic latitudes above about 10\de. At lower latitudes the observed
timescales are too fast, suggesting that the transverse plasma velocity
increases more than expected beyond about 1 kpc.Comment: 32 pages, 16 figures. Submitted to Ap
Biomineralization of PbS and PbS-CdS core-shell nanocrystals and their application in quantum dot sensitized solar cells
Biomineralization utilizes biological systems to synthesize functional inorganic materials for application in diverse fields. In the current work, we enable biomineralization of quantum confined PbS and PbSâCdS coreâshell nanocrystals and demonstrate their application in quantum dot sensitized solar cells (QDSSCs). An engineered strain of Stenotrophomonas maltophilia is utilized to generate a cystathionine Îł-lyase that is active for the biomineralization of metal sulfide nanocrystals from a buffered aqueous solution of metal salts and L-cysteine. In the presence of lead acetate, this enzymatic route generates rock salt structured PbS nanocrystals. Controlling the growth conditions yields âŒ4 nm PbS crystals with absorption and photoluminescence peaks at 910 nm and 1080 nm, respectively, consistent with the expected strong quantum confinement of PbS at this size. Quantum yields (QY) of the biomineralized PbS quantum dots, determined after phase transfer to the organic phase, range between 16 and 45%. These are the highest reported QY values for any biomineralized quantum dot materials to date and are comparable with QYs reported for chemically synthesized materials. Subsequent exposure to cadmium acetate results in the biomineralization of a thin CdS shell on the PbS core with a resultant blue-shift in optical properties. The photoluminescence peak shifts to 980 nm, consistent with the expected decrease in band gap energy of a PbSâCdS coreâshell heterostructured quantum dot. HAADF-STEM imaging confirms the crystalline structure and size of the particles with complimentary XEDS analysis confirming the presence of Cd, Pb, and S in individual nanocrystals. Integration of these QDs into QDSSCs yields open circuit potentials of 0.43 V and 0.59 V for PbS and PbSâCdS, respectively, consistent with expectations for these materials and previously reported values for chemically synthesized QDs
Models of OH Maser Variations in U Her
Arecibo spectra of the mainline OH maser emission from U Her over more than a
decade show variations of the OH emission over these time scales. These
observations are combined with high spatial resolution VLBA maps to investigate
the causes of the variations in the velocities of the maser components. Global
properties of the dust shell, such as accelerations, variations in the pump and
shell-wide magnetic field changes are examined as possibilities, and
eliminated. A possible solution to the problem involving plasma turbulence and
the local magnetic field is introduced, and the relevant time scales of the
turbulence are calculated. The turbulent velocity field yields time scales of
the turbulence are calculated. The turbulent velocity field yields time scales
that are too long (of order centuries), while the turbulent magnetic field
produces variations on appropriate time scales of a few years. A line-of-sight
model of the turbulence is developed and investigated. The complete exploration
of this solution requires extensive theoretical and observational work.
Possible avenues of investigation of the plasma turbulence model are presented.Comment: 23 pages, 17 figures, ApJ: accepted Sept, 199
Enzymatic biomineralization of biocompatible CuInS2, (CuInZn)S2 and CuInS2/ZnS core/shell nanocrystals for bioimaging
This work demonstrates a bioenabled, aqueous phase, room temperature route to synthesize CuInS2/ZnS quantum dots conjugated to IgG antibodies for fluorescent tagging of THP-1 leukemia cells.</p
Ursinus College Bulletin Vol. 10, No. 1, October 1893
A digitized copy of the October 1893 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1088/thumbnail.jp
Ursinus College Bulletin Vol. 9, No. 5, February 1893
A digitized copy of the February 1893 Ursinus College Bulletin.https://digitalcommons.ursinus.edu/ucbulletin/1083/thumbnail.jp
A nonextensive entropy approach to solar wind intermittency
The probability distributions (PDFs) of the differences of any physical
variable in the intermittent, turbulent interplanetary medium are scale
dependent. Strong non-Gaussianity of solar wind fluctuations applies for short
time-lag spacecraft observations, corresponding to small-scale spatial
separations, whereas for large scales the differences turn into a Gaussian
normal distribution. These characteristics were hitherto described in the
context of the log-normal, the Castaing distribution or the shell model. On the
other hand, a possible explanation for nonlocality in turbulence is offered
within the context of nonextensive entropy generalization by a recently
introduced bi-kappa distribution, generating through a convolution of a
negative-kappa core and positive-kappa halo pronounced non-Gaussian structures.
The PDFs of solar wind scalar field differences are computed from WIND and ACE
data for different time lags and compared with the characteristics of the
theoretical bi-kappa functional, well representing the overall scale dependence
of the spatial solar wind intermittency. The observed PDF characteristics for
increased spatial scales are manifest in the theoretical distribution
functional by enhancing the only tuning parameter , measuring the
degree of nonextensivity where the large-scale Gaussian is approached for
. The nonextensive approach assures for experimental studies
of solar wind intermittency independence from influence of a priori model
assumptions. It is argued that the intermittency of the turbulent fluctuations
should be related physically to the nonextensive character of the
interplanetary medium counting for nonlocal interactions via the entropy
generalization.Comment: 17 pages, 7 figures, accepted for publication in Astrophys.
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