732 research outputs found
Couette Flow of Two-Dimensional Foams
We experimentally investigate flow of quasi two-dimensional disordered foams
in Couette geometries, both for foams squeezed below a top plate and for freely
floating foams. With the top-plate, the flows are strongly localized and rate
dependent. For the freely floating foams the flow profiles become essentially
rate-independent, the local and global rheology do not match, and in particular
the foam flows in regions where the stress is below the global yield stress. We
attribute this to nonlocal effects and show that the "fluidity" model recently
introduced by Goyon {\em et al.} ({\em Nature}, {\bf 454} (2008)) captures the
essential features of flow both with and without a top plate.Comment: 6 pages, 5 figures, revised versio
Dielectric susceptibility of the Coulomb-glass
We derive a microscopic expression for the dielectric susceptibility
of a Coulomb glass, which corresponds to the definition used in classical
electrodynamics, the derivative of the polarization with respect to the
electric field. The fluctuation-dissipation theorem tells us that is a
function of the thermal fluctuations of the dipole moment of the system. We
calculate numerically for three-dimensional Coulomb glasses as a
function of temperature and frequency
Investigation of Sea-surface Microlayer and Phytoplankton Culture Samples by Monolayer Techniques and Brewster Angle Microscopy
Natural samples of sea-surface microlayer and phytoplankton culture samples have been studied by monolayer techniques and by Brewster angle microscopy (BAM). Surface pressure-area (π-A) and surface potential-area (ΔV-A) isotherms have been measured. Simultaneously BAM video images have been recorded.
The π-A isotherms, as well as BAM images of monolayers of dipalmitoyl phosphatidylcholine (DPPC), dimyristoylphosphatidic acid (DMPA) and dioctadecyldimethyl ammonium bromide (DOMA) spread on an aqueous subphase containing a sample of phytoplankton culture show that surface active substances released by phytoplankton influence molecular organization as well as domain morphology of the lipid monolayers.
The sea-surface microlayer sample spread at the air/water interface exhibits the characteristics of a liquid expanded phase without undergoing a phase transition. The BAM images taken from this film depend on surface pressure, showing at low surface pressures liquid condensed domains surrounded by a liquid expanded phase, and at higher surface densities only a liquid condensed phase
Investigation of Sea-surface Microlayer and Phytoplankton Culture Samples by Monolayer Techniques and Brewster Angle Microscopy
Natural samples of sea-surface microlayer and phytoplankton culture samples have been studied by monolayer techniques and by Brewster angle microscopy (BAM). Surface pressure-area (π-A) and surface potential-area (ΔV-A) isotherms have been measured. Simultaneously BAM video images have been recorded.
The π-A isotherms, as well as BAM images of monolayers of dipalmitoyl phosphatidylcholine (DPPC), dimyristoylphosphatidic acid (DMPA) and dioctadecyldimethyl ammonium bromide (DOMA) spread on an aqueous subphase containing a sample of phytoplankton culture show that surface active substances released by phytoplankton influence molecular organization as well as domain morphology of the lipid monolayers.
The sea-surface microlayer sample spread at the air/water interface exhibits the characteristics of a liquid expanded phase without undergoing a phase transition. The BAM images taken from this film depend on surface pressure, showing at low surface pressures liquid condensed domains surrounded by a liquid expanded phase, and at higher surface densities only a liquid condensed phase
Optimization by thermal cycling
Thermal cycling is an heuristic optimization algorithm which consists of
cyclically heating and quenching by Metropolis and local search procedures,
respectively, where the amplitude slowly decreases. In recent years, it has
been successfully applied to two combinatorial optimization tasks, the
traveling salesman problem and the search for low-energy states of the Coulomb
glass. In these cases, the algorithm is far more efficient than usual simulated
annealing. In its original form the algorithm was designed only for the case of
discrete variables. Its basic ideas are applicable also to a problem with
continuous variables, the search for low-energy states of Lennard-Jones
clusters.Comment: Submitted to Proceedings of the Workshop "Complexity, Metastability
and Nonextensivity", held in Erice 20-26 July 2004. Latex, 7 pages, 3 figure
The Effect of Air on Granular Size Separation in a Vibrated Granular Bed
Using high-speed video and magnetic resonance imaging (MRI) we study the
motion of a large sphere in a vertically vibrated bed of smaller grains. As
previously reported we find a non-monotonic density dependence of the rise and
sink time of the large sphere. We find that this density dependence is solely
due to air drag. We investigate in detail how the motion of the intruder sphere
is influenced by size of the background particles, initial vertical position in
the bed, ambient pressure and convection. We explain our results in the
framework of a simple model and find quantitative agreement in key aspects with
numerical simulations to the model equations.Comment: 14 pages, 16 figures, submitted to PRE, corrected typos, slight
change
Three-dimensional shear in granular flow
The evolution of granular shear flow is investigated as a function of height
in a split-bottom Couette cell. Using particle tracking, magnetic-resonance
imaging, and large-scale simulations we find a transition in the nature of the
shear as a characteristic height is exceeded. Below there is a
central stationary core; above we observe the onset of additional axial
shear associated with torsional failure. Radial and axial shear profiles are
qualitatively different: the radial extent is wide and increases with height
while the axial width remains narrow and fixed.Comment: 4 pages, 5 figure
Precision Pointing of IBEX-Lo Observations
Post-launch boresight of the IBEX-Lo instrument onboard the Interstellar
Boundary Explorer (IBEX) is determined based on IBEX-Lo Star Sensor
observations. Accurate information on the boresight of the neutral gas camera
is essential for precise determination of interstellar gas flow parameters.
Utilizing spin-phase information from the spacecraft attitude control system
(ACS), positions of stars observed by the Star Sensor during two years of IBEX
measurements were analyzed and compared with positions obtained from a star
catalog. No statistically significant differences were observed beyond those
expected from the pre-launch uncertainty in the Star Sensor mounting. Based on
the star observations and their positions in the spacecraft reference system,
pointing of the IBEX satellite spin axis was determined and compared with the
pointing obtained from the ACS. Again, no statistically significant deviations
were observed. We conclude that no systematic correction for boresight geometry
is needed in the analysis of IBEX-Lo observations to determine neutral
interstellar gas flow properties. A stack-up of uncertainties in attitude
knowledge shows that the instantaneous IBEX-Lo pointing is determined to within
\sim 0.1\degr in both spin angle and elevation using either the Star Sensor
or the ACS. Further, the Star Sensor can be used to independently determine the
spacecraft spin axis. Thus, Star Sensor data can be used reliably to correct
the spin phase when the Star Tracker (used by the ACS) is disabled by bright
objects in its field-of-view. The Star Sensor can also determine the spin axis
during most orbits and thus provides redundancy for the Star Tracker.Comment: 22 pages, 18 figure
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