1,093 research outputs found
Stable suspension and dispersion-induced transitions from repulsive Casimir forces between fluid-separated eccentric cylinders
Using an exact numerical method for finite nonplanar objects, we demonstrate
a stable mechanical suspension of a silica cylinder within a metallic cylinder
separated by ethanol, via a repulsive Casimir force between the silica and the
metal. We investigate cylinders with both circular and square cross sections,
and show that the latter exhibit a stable orientation as well as a stable
position, via a method to compute Casimir torques for finite objects.
Furthermore, the stable orientation of the square cylinder is shown to undergo
an unusual 45 degrees transition as a function of the separation lengthscale,
which is explained as a consequence of material dispersion.Comment: Published in Physical Review Letters. Vol. 101, page, 190404 (2008
Measurements of the Casimir-Lifshitz force in fluids: the effect of electrostatic forces and Debye screening
In this work, we present detailed measurements of the Casimir-Lifshitz force
between two gold surfaces (a sphere and a plate) immersed in ethanol and study
the effect of residual electrostatic forces, which are dominated by static
fields within the apparatus and can be reduced with proper shielding.
Electrostatic forces are further reduced by Debye screening through the
addition of salt ions to the liquid. Additionally, the salt leads to a
reduction of the Casimir-Lifshitz force by screening the zero-frequency
contribution to the force; however, the effect is small between gold surfaces
at the measured separations and within experimental error. An improved
calibration procedure is described and compared to previous methods. Finally,
the experimental results are compared to Lifshitz's theory and found to be
consistent for the materials used in the experiment.Comment: 11 figures. PRA in pres
On the torque on birefringent plates induced by quantum fluctuations
We present detailed numerical calculations of the mechanical torque induced
by quantum fluctuations on two parallel birefringent plates with in plane
optical anisotropy, separated by either vacuum or a liquid (ethanol). The
torque is found to vary as , where represents the angle
between the two optical axes, and its magnitude rapidly increases with
decreasing plate separation . For a 40 m diameter disk, made out of
either quartz or calcite, kept parallel to a Barium Titanate plate at nm, the maximum torque (at ) is of the order of
Nm. We propose an experiment to observe this torque
when the Barium Titanate plate is immersed in ethanol and the other
birefringent disk is placed on top of it. In this case the retarded van der
Waals (or Casimir-Lifshitz) force between the two birefringent slabs is
repulsive. The disk would float parallel to the plate at a distance where its
net weight is counterbalanced by the retarded van der Waals repulsion, free to
rotate in response to very small driving torques.Comment: 7 figures, submitted to Phys. Rev.
Remote stereocontrol in [3,3]-sigmatropic rearrangements: Application to the total synthesis of the immunosuppressant mycestericin G
On electrostatic and Casimir force measurements between conducting surfaces in a sphere-plane configuration
We report on measurements of forces acting between two conducting surfaces in
a spherical-plane configuration in the 35 nm-1 micrometer separation range. The
measurements are obtained by performing electrostatic calibrations followed by
a residual analysis after subtracting the electrostatic-dependent component. We
find in all runs optimal fitting of the calibrations for exponents smaller than
the one predicted by electrostatics for an ideal sphere-plane geometry. We also
find that the external bias potential necessary to minimize the electrostatic
contribution depends on the sphere-plane distance. In spite of these anomalies,
by implementing a parametrixation-dependent subtraction of the electrostatic
contribution we have found evidence for short-distance attractive forces of
magnitude comparable to the expected Casimir-Lifshitz force. We finally discuss
the relevance of our findings in the more general context of Casimir-Lifshitz
force measurements, with particular regard to the critical issues of the
electrical and geometrical characterization of the involved surfaces.Comment: 22 pages, 15 figure
Geometry and material effects in Casimir physics - Scattering theory
We give a comprehensive presentation of methods for calculating the Casimir
force to arbitrary accuracy, for any number of objects, arbitrary shapes,
susceptibility functions, and separations. The technique is applicable to
objects immersed in media other than vacuum, to nonzero temperatures, and to
spatial arrangements in which one object is enclosed in another. Our method
combines each object's classical electromagnetic scattering amplitude with
universal translation matrices, which convert between the bases used to
calculate scattering for each object, but are otherwise independent of the
details of the individual objects. This approach, which combines methods of
statistical physics and scattering theory, is well suited to analyze many
diverse phenomena. We illustrate its power and versatility by a number of
examples, which show how the interplay of geometry and material properties
helps to understand and control Casimir forces. We also examine whether
electrodynamic Casimir forces can lead to stable levitation. Neglecting
permeabilities, we prove that any equilibrium position of objects subject to
such forces is unstable if the permittivities of all objects are higher or
lower than that of the enveloping medium; the former being the generic case for
ordinary materials in vacuum.Comment: 44 pages, 11 figures, to appear in upcoming Lecture Notes in Physics
volume in Casimir physic
Informational needs of general practitioners regarding discharge medication: content, timing and pharmacotherapeutic advice
textabstractObjective: To investigate the needs of Dutch general practitioners on discharge medication, both regarding content, timing and the appreciation of pharma-cotherapeutic advices from clinical pharmacists. Setting: A general teaching hospital in Amsterdam, the Netherlands. Method: A prospective observational study was performed. A questionnaire with regard to the content, optimal timing (including way of information transfer) and appreciation of pharmacotherapeutic advices was posted to 464 general practitioners. One reminder was sent. Main outcome measure: Description of the needs of general practitioners was assessed. For each question and categories of comments frequency tables were made. The Fisher-exact test was used to study associations between the answers to the questions. Results: In total, 149 general practitioners (32%) responded. Most general practitioners (75%) experienced a delay in receiving discharge medication information and preferred to receive this on the day of discharge. GPs wished to receive this information mainly through e-mail (44%). There was a significant correlation (P = 0.002) between general practitioners who wanted to know whether and why medication had been stopped (87%) and changed (88%) during hospital admission. The general practitioners (88%) appreciated pharmacotherapeutic advices from clinical pharmacists. Conclusion: This study indicates how information transfer on discharge medication to GPs can be optimised in the Netherlands. The information arrives late and GPs want to be informed on the day of discharge mainly by e-mail. GPs wish to know why medication is changed or discontinued and appreciate pharmacotherapeutic advices from clinical pharmacists
Tunable bipolar optical interactions between guided lightwaves
The optical binding forces between guided lightwaves in dielectric waveguides
can be either repulsive or attractive. So far only attractive force has been
observed. Here we experimentally demonstrate a bipolar optical force between
coupled nanomechanical waveguides. Both attractive and repulsive optical forces
are obtained. The sign of the force can be switched reversibly by tuning the
relative phase of the interacting lightwaves. This tunable, bipolar interaction
forms the foundation for the operation of a new class of light force devices
and circuits.Comment: 4 figure
Measurement of the branching ratio of the decay
From the 2002 data taking with a neutral kaon beam extracted from the
CERN-SPS, the NA48/1 experiment observed 97 candidates with a background contamination of events.
From this sample, the BR() is measured to be
Cytokinesis in bloodstream stage Trypanosoma brucei requires a family of katanins and spastin
Microtubule severing enzymes regulate microtubule dynamics in a wide range of organisms and are implicated in important cell cycle processes such as mitotic spindle assembly and disassembly, chromosome movement and cytokinesis. Here we explore the function of several microtubule severing enzyme homologues, the katanins (KAT80, KAT60a, KAT60b and KAT60c), spastin (SPA) and fidgetin (FID) in the bloodstream stage of the African trypanosome parasite, Trypanosoma brucei. The trypanosome cytoskeleton is microtubule based and remains assembled throughout the cell cycle, necessitating its remodelling during cytokinesis. Using RNA interference to deplete individual proteins, we show that the trypanosome katanin and spastin homologues are non-redundant and essential for bloodstream form proliferation. Further, cell cycle analysis revealed that these proteins play essential but discrete roles in cytokinesis. The KAT60 proteins each appear to be important during the early stages of cytokinesis, while downregulation of KAT80 specifically inhibited furrow ingression and SPA depletion prevented completion of abscission. In contrast, RNA interference of FID did not result in any discernible effects. We propose that the stable microtubule cytoskeleton of T. brucei necessitates the coordinated action of a family of katanins and spastin to bring about the cytoskeletal remodelling necessary to complete cell divisio
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