4,386 research outputs found
Measurement of the linear viscoelastic behavior of antimisting kerosene
Measurements of dynamic viscoelastic properties in very small oscillating shear deformations was made on solutions of a jet fuel, Jet A, containing an antimisting polymeric additive, FM-9. A few measurements were also made on solutions of FM-9 in a mixed solvent of mineral oil, Tetralin, and 0-terphenyl. Two samples of FM-9 had approximate number-average molecular weights of 12,000,000 and 8,100,000 as deduced from analysis of the measurements. The ranges of variables were 2.42 to 4.03 g/1 in concentration (0.3 to 0.5% by weight), 1 to 35 in temperature, 1.3 to 9.4 cp in solvent viscosity, and 103 to 6100 Hz in frequency. Measurements in the Jet A solvent were made both with and without a modifying carrier. The results were compared with the Zimm theory and the viscoelastic behavior was found to resemble rather closely that of ordinary non-polar polymers in theta solvents. The relation of the results to the antithixotropic behavior of such solutions a high shear rates is discussed in terms of intramolecular and intermolecular interactions
On the mechanism of the highly viscous flow
The asymmetry model for the highly viscous flow postulates thermally
activated jumps from a practically undistorted ground state to strongly
distorted, but stable structures, with a pronounced Eshelby backstress from the
distorted surroundings. The viscosity is ascribed to those stable distorted
structures which do not jump back, but relax by the relaxation of the
surrounding viscoelastic matrix. It is shown that this mechanism implies a
description in terms of the shear compliance, with a viscosity which can be
calculated from the cutoff of the retardation spectrum. Consistency requires
that this cutoff lies close to the Maxwell time. The improved asymmetry model
compares well with experiment.Comment: 8 pages, 3 figures, 49 references; revised version accepted in
Journal of Chemical Physic
Scattering of Dirac electrons by circular mass barriers: valley filter and resonant scattering
The scattering of two-dimensional (2D) massless Dirac electrons is
investigated in the presence of a random array of circular mass barriers. The
inverse momentum relaxation time and the Hall factor are calculated and used to
obtain parallel and perpendicular resistivity components within linear
transport theory. We found a non zero perpendicular resistivity component which
has opposite sign for electrons in the different K and K' valleys. This
property can be used for valley filter purposes. The total cross-section for
scattering on penetrable barriers exhibit resonances due to the presence of
quasi-bound states in the barriers that show up as sharp gaps in the
cross-section while for Schr\"{o}dinger electrons they appear as peaks.Comment: 10 pages, 11 figure
Low-Energy Conductivity of Single- and Double-Layer Graphene from the Uncertainty Principle
The minimum conductivity value as well as the linear dependence of
conductivity on the charge density near the Dirac point in single and
doublelayer graphene is derived from the energy-time uncertainty principle
applied to ballistic charge carriers
Nonlinear Transport of Bose-Einstein Condensates Through Waveguides with Disorder
We study the coherent flow of a guided Bose-Einstein condensate incident over
a disordered region of length L. We introduce a model of disordered potential
that originates from magnetic fluctuations inherent to microfabricated guides.
This model allows for analytical and numerical studies of realistic transport
experiments. The repulsive interaction among the condensate atoms in the beam
induces different transport regimes. Below some critical interaction (or for
sufficiently small L) a stationary flow is observed. In this regime, the
transmission decreases exponentially with L. For strong interaction (or large
L), the system displays a transition towards a time dependent flow with an
algebraic decay of the time averaged transmission.Comment: 15 pages, 9 figure
Energy Requirement of Control: Comments on Szilard's Engine and Maxwell's Demon
In mathematical physical analyses of Szilard's engine and Maxwell's demon, a
general assumption (explicit or implicit) is that one can neglect the energy
needed for relocating the piston in Szilard's engine and for driving the trap
door in Maxwell's demon. If this basic assumption is wrong, then the
conclusions of a vast literature on the implications of the Second Law of
Thermodynamics and of Landauer's erasure theorem are incorrect too. Our
analyses of the fundamental information physical aspects of various type of
control within Szilard's engine and Maxwell's demon indicate that the entropy
production due to the necessary generation of information yield much greater
energy dissipation than the energy Szilard's engine is able to produce even if
all sources of dissipation in the rest of these demons (due to measurement,
decision, memory, etc) are neglected.Comment: New, simpler and more fundamental approach utilizing the physical
meaning of control-information and the related entropy production. Criticism
of recent experiments adde
Coherent transport through graphene nanoribbons in the presence of edge disorder
We simulate electron transport through graphene nanoribbons of experimentally
realizable size (length L up to 2 micrometer, width W approximately 40 nm) in
the presence of scattering at rough edges. Our numerical approach is based on a
modular recursive Green's function technique that features sub-linear scaling
with L of the computational effort. We identify the influence of the broken A-B
sublattice (or chiral) symmetry and of K-K' scattering by Fourier spectroscopy
of individual scattering states. For long ribbons we find Anderson-localized
scattering states with a well-defined exponential decay over 10 orders of
magnitude in amplitude.Comment: 8 pages, 6 Figure
Investigation of DC-8 nacelle modifications to reduce fan-compressor noise in airport communities. Part 5 - Economic implications of retrofit Technical report, May 1967 - Oct. 1969
Economic impact of modifications to DC-8 aircraft nacelles to reduce fan-compressor noise - Part
Underscreened Kondo effect in S=1 magnetic quantum dots: Exchange, anisotropy and temperature effects
We present a theoretical analysis of the effects of uniaxial magnetic
anisotropy and contact-induced exchange field on the underscreened Kondo effect
in S=1 magnetic quantum dots coupled to ferromagnetic leads. First, by using
the second-order perturbation theory we show that the coupling to
spin-polarized electrode results in an effective exchange field
and an effective magnetic anisotropy . Second, we confirm these
findings by using the numerical renormalization group method, which is employed
to study the dependence of the quantum dot spectral functions, as well as
quantum dot spin, on various parameters of the system. We show that the
underscreened Kondo effect is generally suppressed due to the presence of
effective exchange field and can be restored by tuning the anisotropy constant,
when . The Kondo effect can also be restored by
sweeping an external magnetic field, and the restoration occurs twice in a
single sweep. From the distance between the restored Kondo resonances one can
extract the information about both the exchange field and the effective
anisotropy. Finally, we calculate the temperature dependence of linear
conductance for the parameters where the Kondo effect is restored and show that
the restored Kondo resonances display a universal scaling of Kondo
effect.Comment: 13 pages, 9 figures (version as accepted for publication in Physical
Review B
Viscoelasticity and metastability limit in supercooled liquids
A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp
exists below which the liquid relaxation time exceeds the crystal nucleation
time. We revisit classical nucleation theory taking into account the
viscoelastic response of the liquid to the formation of crystal nuclei and find
that the kinetic spinodal is strongly influenced by elastic effects. We
introduce a dimensionless parameter \lambda, which is essentially the ratio
between the infinite frequency shear modulus and the enthalpy of fusion of the
crystal. In systems where \lambda is larger than a critical value \lambda_c the
metastability limit is totally suppressed, independently of the surface
tension. On the other hand, if \lambda < \lambda_c a kinetic spinodal is
present and the time needed to experimentally observe it scales as
exp[\omega/(\lambda_c-\lambda)^2], where \omega is roughly the ratio between
surface tension and enthalpy of fusion
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