1,148 research outputs found
Flow around a sphere in a circular tube
The vector potential for the flow of an ideal fluid through a tube containing a concentric spherical obstacle is found for ratios of sphere radius to tube radius of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95. The flow is confined to the space between sphere and cylinder by thin vortex sheets of variable strength and a table of their circulation intensity on the spherical surface is given. Accuracies vary from about one part in 108 for small spheres to one part in 107 for large ones. The increase in the scalar velocity potential between the ends of the tube caused by the insertion of the sphere is expressed in terms of the effective increase in tube length. This also gives the increase in resistance of a solid conducting cylinder due to the presence of a concentric spherical bubble
No anomalous scaling in electrostatic calibrations for Casimir force measurements
In a recent paper (Phys.Rev.A78, 020101(R) (2008)), Kim at al. have reported
a large anomaly in the scaling law of the electrostatic interaction between a
sphere and a plate, which was observed during the calibration of their Casimir
force set-up. Here we experimentally demonstrate that in proper electrostatic
calibrations the scaling law follows the behavior expected from elementary
electrostatic arguments, even when the electrostatic voltage that one must
apply to minimize the force (typically ascribed to contact potentials) depends
on the separation between the surfaces.Comment: Final versio
Improved perfluoroalkylether fluid development
The feasibility of transforming a commercial linear perfluoroalkylether fluid into a material stable in the presence of metals and metal alloys in oxidizing atmospheres at 300 C without the loss of the desirable viscosity temperature characteristics was determined. The approach consisted of thermal oxidative treatment in the presence of catalyst to remove weak links, followed by transformation of the created functional groups into phospha-s-triazine linkages. It it found that the experimental material obtained in 66% yield from the commercial fluid exhibits, over an 8 hr period at 300 C in the presence of Ti(4Al, 4Mn) alloy, thermal oxidative stability better by a factor of 2.6x1000 based on volatiles evolved than the commercial product. The viscosity and molecular weight of the developed fluid are unchanged and are essentially identical with the commercial material. No metal corrosion occurs with the experimental fluid at 300 C
Melting temperature of screened Wigner crystal on helium films by molecular dynamics
Using molecular dynamics (MD) simulation, we have calculated the melting
temperature of two-dimensional electron systems on \AA-\AA helium
films supported by substrates of dielectric constants
at areal densities varying from cm to cm. Our results are in good agreement with the available
theoretical and experimental results.Comment: 4 pages and 4 figure
Numerical modelling of liquid droplet dynamics in microgravity
Microgravity provides ideal experimental conditions for studying highly reactive and under-cooled materials where there is no contact between the sample and the other experimental apparatus. The non-contact conditions allow material properties to be measured from the oscillating liquid droplet response to perturbations. This work investigates the impact of a strong magnetic field on these measurement processes for weakly viscous, electrically conducting droplets. We present numerical results using an axisymmetric model that employs the pseudo-spectral collocation method and a recently developed 3D model. Both numerical models have been developed to solve the equations describing the coupled electromagnetic and fluid flow processes. The models represent the changing surface shape that results from the interaction between forces inside the droplet and the surface tension imposed boundary conditions. The models are used to examine the liquid droplet dynamics in a strong DC magnetic field. In each case the surface shape is decomposed into a superposition of spherical harmonic modes. The oscillation of the individual mode coefficients is then analysed to determine the oscillation frequencies and damping rates that are then compared to the low amplitude solutions predicted by the published analytical asymptotic theory
Casimir-Polder interaction between an atom and a dielectric slab
We present an explicit analytic calculation of the energy-level shift of an
atom in front of a non-dispersive and non-dissipative dielectric slab. We work
with the fully quantized electromagnetic field, taking retardation into
account. We give the shift as a two-dimensional integral and use asymptotic
analysis to find expressions for it in various retarded and non-retarded
limiting cases. The results can be used to estimate the energy shift of an atom
close to layered microstructures.Comment: 10 pages, incl 7 figure
Fiber-diffraction Interferometer using Coherent Fiber Optic Taper
We present a fiber-diffraction interferometer using a coherent fiber optic
taper for optical testing in an uncontrolled environment. We use a coherent
fiber optic taper and a single-mode fiber having thermally-expanded core. Part
of the measurement wave coming from a test target is condensed through a fiber
optic taper and spatially filtered from a single-mode fiber to be reference
wave. Vibration of the cavity between the target and the interferometer probe
is common to both reference and measurement waves, thus the interference fringe
is stabilized in an optical way. Generation of the reference wave is stable
even with the target movement. Focus shift of the input measurement wave is
desensitized by a coherent fiber optic taper
Long range polarization attraction between two different likely charged macroions
It is known that in a water solution with multivalent counterions (Z-ions),
two likely charged macroions can attract each other due to correlations of
Z-ions adsorbed on their surfaces. This "correlation" attraction is
short-ranged and decays exponentially with increasing distance between
macroions at characteristic distance A/2\pi, where A is the average distance
between Z-ions on the surfaces of macroions. In this work, we show that an
additional long range "polarization" attraction exists when the bare surface
charge densities of the two macroions have the same sign, but are different in
absolute values. The key idea is that with adsorbed Z-ions, two insulating
macroions can be considered as conductors with fixed but different electric
potentials. Each potential is determined by the difference between the entropic
bulk chemical potential of a Z-ion and its correlation chemical potential at
the surface of the macroion determined by its bare surface charge density. When
the two macroions are close enough, they get polarized in such a way that their
adjacent spots form a charged capacitor, which leads to attraction. In a salt
free solution this polarization attractive force is long ranged: it decays as a
power of the distance between the surfaces of two macroions, d. The
polarization force decays slower than the van der Waals attraction and
therefore is much larger than it in a large range of distances. In the presence
of large amount of monovalent salt, when A/2\pi<< d<< r_s (r_s is the
Debye-H\"{u}ckel screening radius), this force is still much stronger than the
van der Waals attraction and the correlation attraction mentioned above.Comment: 12 pages, 7 figures. Small change in the text, no change in result
Counterion-Mediated Weak and Strong Coupling Electrostatic Interaction between Like-Charged Cylindrical Dielectrics
We examine the effective counterion-mediated electrostatic interaction
between two like-charged dielectric cylinders immersed in a continuous
dielectric medium containing neutralizing mobile counterions. We focus on the
effects of image charges induced as a result of the dielectric mismatch between
the cylindrical cores and the surrounding dielectric medium and investigate the
counterion-mediated electrostatic interaction between the cylinders in both
limits of weak and strong electrostatic couplings (corresponding, e.g., to
systems with monovalent and multivalent counterions, respectively). The results
are compared with extensive Monte-Carlo simulations exhibiting good agreement
with the limiting weak and strong coupling results in their respective regime
of validity.Comment: 19 pages, 10 figure
Ion exchange phase transitions in "doped" water--filled channels
Ion transport through narrow water--filled channels is impeded by a high
electrostatic barrier. The latter originates from the large ratio of the
dielectric constants of the water and a surrounding media. We show that
``doping'', i.e. immobile charges attached to the walls of the channel,
substantially reduces the barrier. This explains why most of the biological ion
channels are ``doped''. We show that at rather generic conditions the channels
may undergo ion exchange phase transitions (typically of the first order). Upon
such a transition a finite latent concentration of ions may either enter or
leave the channel, or be exchanged between the ions of different valences. We
discuss possible implications of these transitions for the Ca-vs.-Na
selectivity of biological Ca channels. We also show that transport of divalent
Ca ions is assisted by their fractionalization into two separate excitations.Comment: 16 pages, 27 figure
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