48,759 research outputs found
Testing and evaluation of solid lubricants for gas bearings
The testing and results of testing solid film lubricants for gas lubricated bearing applications are reported. The tests simulated operational hazards of tilting pad gas bearings. The presence of a low coefficient of friction and the endurance of the solid film lubricant were the criteria for judging superior performance. All solid lubricants tested were applied to a plasma sprayed chrome oxide surface. Molybdenum disulfide and graphite fluoride were the solid lubricants tested; other test parameters included the method of application of the solid lubricant and the surface finish of the plasma sprayed coating. In general, the application of a solid film lubricant was found to significantly improve the coefficient of friction of the rubbing surfaces
Enhanced tracer transport by the spiral defect chaos state of a convecting fluid
To understand how spatiotemporal chaos may modify material transport, we use
direct numerical simulations of the three-dimensional Boussinesq equations and
of an advection-diffusion equation to study the transport of a passive tracer
by the spiral defect chaos state of a convecting fluid. The simulations show
that the transport is diffusive and is enhanced by the spatiotemporal chaos.
The enhancement in tracer diffusivity follows two regimes. For large Peclet
numbers (that is, small molecular diffusivities of the tracer), we find that
the enhancement is proportional to the Peclet number. For small Peclet numbers,
the enhancement is proportional to the square root of the Peclet number. We
explain the presence of these two regimes in terms of how the local transport
depends on the local wave numbers of the convection rolls. For large Peclet
numbers, we further find that defects cause the tracer diffusivity to be
enhanced locally in the direction orthogonal to the local wave vector but
suppressed in the direction of the local wave vector.Comment: 11 pages, 12 figure
Creating better superconductors by periodic nanopatterning
The quest to create superconductors with higher transition temperatures is as
old as superconductivity itself. One strategy, popular after the realization
that (conventional) superconductivity is mediated by phonons, is to chemically
combine different elements within the crystalline unit cell to maximize the
electron-phonon coupling. This led to the discovery of NbTi and Nb3Sn, to name
just the most technologically relevant examples. Here, we propose a radically
different approach to transform a `pristine' material into a better (meta-)
superconductor by making use of modern fabrication techniques: designing and
engineering the electronic properties of thin films via periodic patterning on
the nanoscale. We present a model calculation to explore the key effects of
different supercells that could be fabricated using nanofabrication or
deliberate lattice mismatch, and demonstrate that specific pattern will enhance
the coupling and the transition temperature. We also discuss how numerical
methods could predict the correct design parameters to improve
superconductivity in materials including Al, NbTi, and MgB
Glycerol confined in zeolitic imidazolate frameworks: The temperature-dependent cooperativity length scale of glassy freezing
In the present work, we employ broadband dielectric spectroscopy to study the
molecular dynamics of the prototypical glass former glycerol confined in two
microporous zeolitic imidazolate frameworks (ZIF-8 and ZIF-11) with
well-defined pore diameters of 1.16 and 1.46 nm, respectively. The spectra
reveal information on the modified alpha relaxation of the confined supercooled
liquid, whose temperature dependence exhibits clear deviations from the typical
super-Arrhenius temperature dependence of the bulk material, depending on
temperature and pore size. This allows assigning well-defined cooperativity
length scales of molecular motion to certain temperatures above the glass
transition. We relate these and previous results on glycerol confined in other
host systems to the temperature-dependent length scale deduced from nonlinear
dielectric measurements. The combined experimental data can be consistently
described by a critical divergence of this correlation length as expected
within theoretical approaches assuming that the glass transition is due to an
underlying phase transition.Comment: 14 pages, 5 figures + Supplemental Material (4 pages, 6 figures).
Final version as accepted for publicatio
Space/time noncommutative field theories and causality
As argued previously, amplitudes of quantum field theories on noncommutative
space and time cannot be computed using naive path integral Feynman rules. One
of the proposals is to use the Gell-Mann--Low formula with time-ordering
applied before performing the integrations. We point out that the previously
given prescription should rather be regarded as an interaction point
time-ordering. Causality is explicitly violated inside the region of
interaction. It is nevertheless a consistent procedure, which seems to be
related to the interaction picture of quantum mechanics. In this framework we
compute the one-loop self-energy for a space/time noncommutative \phi^4 theory.
Although in all intermediate steps only three-momenta play a role, the final
result is manifestly Lorentz covariant and agrees with the naive calculation.
Deriving the Feynman rules for general graphs, we show, however, that such a
picture holds for tadpole lines only.Comment: 16 pages, LaTeX, uses feynmf macros, one reference added; ooops,
version 2 was an older one
Characterization of the domain chaos convection state by the largest Lyapunov exponent
Using numerical integrations of the Boussinesq equations in rotating cylindrical domains with realistic boundary conditions, we have computed the value of the largest Lyapunov exponent lambda1 for a variety of aspect ratios and driving strengths. We study in particular the domain chaos state, which bifurcates supercritically from the conducting fluid state and involves extended propagating fronts as well as point defects. We compare our results with those from Egolf et al., [Nature 404, 733 (2000)], who suggested that the value of lambda1 for the spiral defect chaos state of a convecting fluid was determined primarily by bursts of instability arising from short-lived, spatially localized dislocation nucleation events. We also show that the quantity lambda1 is not intensive for aspect ratios Gamma over the range 20<Gamma<40 and that the scaling exponent of lambda1 near onset is consistent with the value predicted by the amplitude equation formalism
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