16,974 research outputs found
Minimum permissible leakage resistance established for instrumentation systems
Mathematical formulas are used to determine if, and to what extent, an instrumentation system that has been exposed to the elements should be dried out to restore minimum permissible leakage resistance to ground. Formulas are also derived and used for an intermediate number of systems that are exposed to moisture penetration
Improved version of the eikonal model for absorbing spherical particles
We present a new expression of the scattering amplitude, valid for spherical
absorbing objects, which leads to an improved version of the eikonal method
outside the diffraction region. Limitations of this method are discussed and
numerical results are presented and compared successfully with the Mie theory.Comment: 7 pages, postscript figures available on cpt.univ-mrs.fr, to appear
in J. Mod. Optic
Aerospace lubrication technology transfer to industrial applications
Difficulties encountered in entering industrial markets with an aerospace lubrication and coating technology are discussed along with the technical, financial, and managerial solutions that evolved
Nonequilibrium dynamics and fluctuation-dissipation relation in a sheared fluid
The nonequilibrium dynamics of a binary Lennard-Jones mixture in a simple
shear flow is investigated by means of molecular dynamics simulations. The
range of temperature investigated covers both the liquid, supercooled and
glassy states, while the shear rate covers both the linear and nonlinear
regimes of rheology. The results can be interpreted in the context of a
nonequilibrium, schematic mode-coupling theory developed recently, which makes
the theory applicable to a wide range of soft glassy materials. The behavior of
the viscosity is first investigated. In the nonlinear regime, strong
shear-thinning is obtained. Scaling properties of the intermediate scattering
functions are studied. Standard `mode-coupling properties' of factorization and
time-superposition hold in this nonequilibrium situation. The
fluctuation-dissipation relation is violated in the shear flow in a way very
similar to that predicted theoretically, allowing for the definition of an
effective temperature Teff for the slow modes of the fluid. Temperature and
shear rate dependencies of Teff are studied using density fluctuations as an
observable. The observable dependence of Teff is also investigated. Many
different observables are found to lead to the same value of Teff, suggesting
several experimental procedures to access Teff. It is proposed that tracer
particle of large mass may play the role of an `effective thermometer'. When
the Einstein frequency of the tracers becomes smaller than the inverse
relaxation time of the fluid, a nonequilibrium equipartition theorem holds.
This last result gives strong support to the thermodynamic interpretation of
Teff and makes it experimentally accessible in a very direct way.Comment: Version accepted for publication in Journal of Chemical Physic
Optimal estimates of the diffusion coefficient of a single Brownian trajectory
Modern developments in microscopy and image processing are revolutionizing
areas of physics, chemistry and biology as nanoscale objects can be tracked
with unprecedented accuracy. The goal of single particle tracking is to
determine the interaction between the particle and its environment. The price
paid for having a direct visualization of a single particle is a consequent
lack of statistics. Here we address the optimal way of extracting diffusion
constants from single trajectories for pure Brownian motion. It is shown that
the maximum likelihood estimator is much more efficient than the commonly used
least squares estimate. Furthermore we investigate the effect of disorder on
the distribution of estimated diffusion constants and show that it increases
the probability of observing estimates much smaller than the true (average)
value.Comment: 8 pages, 5 figure
Light scattering of large rough particles application to cometary grains
While the electromagnetic field scattered by a spherical particle is classically obtained by the Helmholtz equation, the general case of an arbitrary particle may be investigated in the general framework of the interaction of a wave with a scattering potential. A wave function then satisfies the Schroedinger equation. The general solution of the Schroedinger equation is given. The main disadvantage of this approach are its restriction to large particles and its scalar nature preventing the calculation of the polarization. However, Perrin and Lamy have shown how to avoid the second limitation and retrieve a vectorial description. They proved that in the case of large spheres when the ad hoc assumptions are satisfied, the expression of the scattering amplitude may be approximated by an expansion series in partial waves, i.e., on a discrete basis. The analogy may be generalized, and the ratio of the two components for a rough particle obtained by taking the ratio of the reflectivities for the two directions of polarization. These reflectivities involve the simple and double reflections calculated following the method developed by Wolff for rough surfaces. The theory is further detailed
Sub-Terahertz Monochromatic Transduction with Semiconductor Acoustic Nanodevices
We demonstrate semiconductor superlattices or nanocavities as narrow band
acoustic transducers in the sub-terahertz range. Using picosecond ultrasonics
experiments in the transmission geometry with pump and probe incident on
opposite sides of the thick substrate, phonon generation and detection
processes are fully decoupled. Generating with the semiconductor device and
probing on the metal, we show that both superlattices and nanocavities generate
spectrally narrow wavepackets of coherent phonons with frequencies in the
vicinity of the zone center and time durations in the nanosecond range,
qualitatively different from picosecond broadband pulses usually involved in
picosecond acoustics with metal generators. Generating in the metal and probing
on the nanoacoustic device, we furthermore evidence that both nanostructured
semiconductor devices may be used as very sensitive and spectrally selective
detectors
Optical properties of irregular interstellar grains
In order to study the interaction of light with interstellar grains, the authors represent an irregular particle by a network of interacting dipoles whose polarizability is determined in a first approach by the Clausius-Mossoti relationship. Typically, 10,000 dipoles are considered. In the case of spherical particles, the results from Mie theory are fully recovered. The main interest of this method is to study with good accuracy the implications of surface roughness and/or inhomogeneities on optical properties in the infrared spectral range, particularly of the silicate emission features
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