46,616 research outputs found
Non-equilibrium dynamics of an active colloidal "chucker"
We report Monte Carlo simulations of the dynamics of a "chucker": a colloidal
particle which emits smaller solute particles from its surface, isotropically
and at a constant rate k_c. We find that the diffusion constant of the chucker
increases for small k_c, as recently predicted theoretically. At large k_c the
chucker diffuses more slowly due to crowding effects. We compare our simulation
results to those of a "point particle" Langevin dynamics scheme in which the
solute concentration field is calculated analytically, and in which
hydrodynamic effects can be included albeit in an approximate way. By
simulating the dragging of a chucker, we obtain an estimate of its apparent
mobility coefficient which violates the fluctuation-dissipation theorem. We
also characterise the probability density profile for a chucker which sediments
onto a surface which either repels or absorbs the solute particles, and find
that the steady state distributions are very different in the two cases. Our
simulations are inspired by the biological example of
exopolysaccharide-producing bacteria, as well as by recent experimental,
simulation and theoretical work on phoretic colloidal "swimmers".Comment: re-submission after referee's comment
A theory of human error
Human errors tend to be treated in terms of clinical and anecdotal descriptions, from which remedial measures are difficult to derive. Correction of the sources of human error requires an attempt to reconstruct underlying and contributing causes of error from the circumstantial causes cited in official investigative reports. A comprehensive analytical theory of the cause-effect relationships governing propagation of human error is indispensable to a reconstruction of the underlying and contributing causes. A validated analytical theory of the input-output behavior of human operators involving manual control, communication, supervisory, and monitoring tasks which are relevant to aviation, maritime, automotive, and process control operations is highlighted. This theory of behavior, both appropriate and inappropriate, provides an insightful basis for investigating, classifying, and quantifying the needed cause-effect relationships governing propagation of human error
Evaluation and study of advanced optical contamination, deposition, measurement, and removal techniques
A program is described to design, fabricate and install an experimental work chamber assembly (WCA) to provide a wide range of experimental capability. The WCA incorporates several techniques for studying the kinetics of contaminant films and their effect on optical surfaces. It incorporates the capability for depositing both optical and contaminant films on temperature-controlled samples, and for in-situ measurements of the vacuum ultraviolet reflectance. Ellipsometer optics are mounted on the chamber for film thickness determinations, and other features include access ports for radiation sources and instrumentation. Several supporting studies were conducted to define specific chamber requirements, to determine the sensitivity of the measurement techniques to be incorporated in the chamber, and to establish procedures for handling samples prior to their installation in the chamber. A bibliography and literature survey of contamination-related articles is included
Quantitative multielement analysis using high energy particle bombardment
Charged particles ranging in energy from 0.8 to 4.0 MeV are used to induce resonant nuclear reactions, Coulomb excitation (gamma X-rays), and X-ray emission in both thick and thin targets. Quantitative analysis is possible for elements from Li to Pb in complex environmental samples, although the matrix can severely reduce the sensitivity. It is necessary to use a comparator technique for the gamma-rays, while for X-rays an internal standard can be used. A USGS standard rock is analyzed for a total of 28 elements. Water samples can be analyzed either by nebulizing the sample doped with Cs or Y onto a thin formvar film or by extracting the sample (with or without an internal standard) onto ion exchange resin which is pressed into a pellet
Effect of vertical active vibration isolation on tracking performance and on ride qualities
An investigation to determine the effect on pilot performance and comfort of an active vibration isolation system for a commercial transport pilot seat is reported. The test setup consisted of: a hydraulic shaker which produced random vertical vibration inputs; the active vibration isolation system; the pilot seat; the pilot control wheel and column; the side-arm controller; and a two-axis compensatory tracking task. The effects of various degrees of pilot isolation on short-term (two-minute) tracking performance and comfort were determined
Chemical Segregation in Hot Cores With Disk Candidates: An investigation with ALMA
In the study of high-mass star formation, hot cores are empirically defined
stages where chemically rich emission is detected toward a massive YSO. It is
unknown whether the physical origin of this emission is a disk, inner envelope,
or outflow cavity wall and whether the hot core stage is common to all massive
stars. We investigate the chemical make up of several hot molecular cores to
determine physical and chemical structure. We use high spectral and spatial
resolution Cycle 0 ALMA observations to determine how this stage fits into the
formation sequence of a high mass star. We observed the G35.20-0.74N and
G35.03+0.35 hot cores at 350 GHz. We analyzed spectra and maps from four
continuum peaks (A, B1, B2 and B3) in G35.20, separated by 1000-2000 AU, and
one continuum peak in G35.03. We made all possible line identifications across
8 GHz of spectral windows of molecular emission lines and determined column
densities and temperatures for as many as 35 species assuming local
thermodynamic equilibrium. In comparing the spectra of the four peaks, we find
each has a distinct chemical composition expressed in over 400 different
transitions. In G35.20, B1 and B2 contain oxygen- and sulfur-bearing organic
and inorganic species but few nitrogen-bearing species whereas A and B3 are
strong sources of O, S, and N-bearing species (especially those with the
CN-bond). CHDCN is clearly detected in A and B3 with D/H ratios of 8 and
13, respectively, but is much weaker at B1 and undetected at B2. No
deuterated species are detected in G35.03, but similar molecular abundances to
G35.20 were found in other species. We also find co-spatial emission of HNCO
and NHCHO in both sources indicating a strong chemical link between the two
species. The chemical segregation between N-bearing organic species and others
in G35.20 suggests the presence of multiple protostars, surrounded by a disk or
torus.Comment: 14 pages with 13 figures main text, 54 pages appendi
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