1,594 research outputs found
Weak Long-Ranged Casimir Attraction in Colloidal Crystals
We investigate the influence of geometric confinement on the free energy of
an idealized model for charge-stabilized colloidal suspensions. The mean-field
Poisson-Boltzmann formulation for this system predicts pure repulsion among
macroionic colloidal spheres. Fluctuations in the simple ions' distribution
provide a mechanism for the macroions to attract each other at large
separations. Although this Casimir interaction is long-ranged, it is too weak
to influence colloidal crystals' dynamics.Comment: 5 pages 2 figures ReVTe
Colloidal transport through optical tweezer arrays
Viscously damped particles driven past an evenly spaced array of potential
energy wells or barriers may become kinetically locked in to the array, or else
may escape from the array. The transition between locked-in and free-running
states has been predicted to depend sensitively on the ratio between the
particles' size and the separation between wells. This prediction is confirmed
by measurements on monodisperse colloidal spheres driven through arrays of
holographic optical traps.Comment: 4 pages, 4 figure
Observation of Flux Reversal in a Symmetric Optical Thermal Ratchet
We demonstrate that a cycle of three holographic optical trapping patterns
can implement a thermal ratchet for diffusing colloidal spheres, and that the
ratchet-driven transport displays flux reversal as a function of the cycle
frequency and the inter-trap separation. Unlike previously described ratchet
models, the approach we describe involves three equivalent states, each of
which is locally and globally spatially symmetric, with spatiotemporal symmetry
being broken by the sequence of states.Comment: 4 pages, 2 figures, submitted for publication in Physical Review
Letter
Correlated particle dynamics in concentrated quasi-two-dimensional suspensions
We investigate theoretically and experimentally how the hydrodynamically
correlated lateral motion of particles in a suspension confined between two
surfaces is affected by the suspension concentration. Despite the long range of
the correlations (decaying as 1/r^2 with the inter-particle distance r), the
concentration effect is present only at short inter-particle distances for
which the static pair correlation is nonuniform. This is in sharp contrast with
the effect of hydrodynamic screening present in unconfined suspensions, where
increasing the concentration changes the prefactor of the large-distance
correlation.Comment: 13 page
Melting of Quasi-Two-Dimensional Charge Stripes in La5/3Sr1/3NiO4
Commensurability effects for nickelates have been studied by the first
neutron experiments on La5/3Sr1/3NiO4. Upon cooling, this system undergoes
three successive phase transitions associated with quasi-two-dimensional (2D)
commensurate charge and spin stripe ordering in the NiO planes. The two
lower temperature phases (denoted as phase II and III) are stripe lattice
states with quasi-long-range in-plane charge correlation. When the lattice of
2D charge stripes melts, it goes through an intermediate glass state (phase I)
before becoming a disordered liquid state. This glass state shows short-range
charge order without spin order, and may be called a "stripe glass" which
resembles the hexatic/nematic state in 2D melting.Comment: 10 pages, RevTex, 4 figures available on request to
[email protected]
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
Elastic Theory of pinned flux lattices
The pinning of flux lattices by weak impurity disorder is studied in the
absence of free dislocations using both the gaussian variational method and, to
, the functional renormalization group. We find universal
logarithmic growth of displacements for : and persistence of algebraic quasi-long range
translational order. When the two methods can be compared they agree within
on the value of . We compute the function describing the crossover
between the ``random manifold'' regime and the logarithmic regime. This
crossover should be observable in present decoration experiments.Comment: 12 pages, Revtex 3.
On the geometry of four qubit invariants
The geometry of four-qubit entanglement is investigated. We replace some of
the polynomial invariants for four-qubits introduced recently by new ones of
direct geometrical meaning. It is shown that these invariants describe four
points, six lines and four planes in complex projective space . For
the generic entanglement class of stochastic local operations and classical
communication they take a very simple form related to the elementary symmetric
polynomials in four complex variables. Moreover, their magnitudes are
entanglement monotones that fit nicely into the geometric set of -qubit ones
related to Grassmannians of -planes found recently. We also show that in
terms of these invariants the hyperdeterminant of order 24 in the four-qubit
amplitudes takes a more instructive form than the previously published
expressions available in the literature. Finally in order to understand two,
three and four-qubit entanglement in geometric terms we propose a unified
setting based on furnished with a fixed quadric.Comment: 19 page
Retrospective examination of injuries and physical fitness during Federal Bureau of Investigation new agent training
<p>Abstract</p> <p>Background</p> <p>A retrospective examination was conducted of injuries, physical fitness, and their association among Federal Bureau of Investigation (FBI) new agent trainees.</p> <p>Methods</p> <p>Injuries and activities associated with injuries were obtained from a review of medical records in the medical clinic that served the new agents. A physical fitness test (PFT) was administered at Weeks 1, 7 and 14 of the 17-week new agent training course. The PFT consisted of push-ups, sit-ups, pull-ups, a 300-meter sprint, and a 1.5-mile run. Injury data were available from 2000 to 2008 and fitness data were available from 2004 to early 2009.</p> <p>Results</p> <p>During the survey period, 37% of men and 44% of women experienced one or more injuries during the new agent training course (risk ratio (women/men) = 1.18, 95% confidence interval = 1.07-1.31). The most common injury diagnoses were musculoskeletal pain (not otherwise specified) (27%), strains (11%), sprains (10%), contusions (9%), and abrasions/lacerations (9%). Activities associated with injury included defensive tactics training (48%), physical fitness training (26%), physical fitness testing (6%), and firearms training (6%). Over a 6-year period, there was little difference in performance of push-ups, sit-ups, pull-ups, or the 300-meter sprint; 1.5-mile run performance was higher in recent years. Among both men and women, higher injury incidence was associated with lower performance on any of the physical fitness measures.</p> <p>Conclusion</p> <p>This investigation documented injury diagnoses, activities associated with injury, and changes in physical fitness, and demonstrated that higher levels of physical fitness were associated with lower injury risk.</p
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