2,486 research outputs found
Changes in trabecular bone, hematopoiesis and bone marrow vessels in aplastic anemia, primary osteoporosis, and old age
Retrospective histologic analyses of bone biopsies and of post mortem samples from normal persons of different age groups, and of bone biopsies of age- and sex-matched groups of patients with primary osteoporosis and aplastic anemia show characteristic age dependent as well as pathologic changes including atrophy of osseous trabeculae and of hematopoiesis, and changes in the sinusoidal and arterial capillary compartments. These results indicate the possible role of a microvascular defect in the pathogenesis of osteoporosis and aplastic anemia
Cable compliance
The object of the investigation was to solve mechanical problems using cable-in-bending and cable-in-torsion. These problems included robotic contacts, targets, and controls using cable compliance. Studies continued in the use of cable compliance for the handicapped and the elderly. These included work stations, walkers, prosthetic knee joints, elbow joints, and wrist joints. More than half of these objects were met, and models were made and studies completed on most of the others. It was concluded that the many different and versatile solutions obtained only opened the door to many future challenges
Fluctuations of a long, semiflexible polymer in a narrow channel
We consider an inextensible, semiflexible polymer or worm-like chain, with
persistence length and contour length , fluctuating in a cylindrical
channel of diameter . In the regime , corresponding to a long,
tightly confined polymer, the average length of the channel
occupied by the polymer and the mean square deviation from the average vary as
and , respectively, where
and are dimensionless amplitudes. In earlier work
we determined and the analogous amplitude for a
channel with a rectangular cross section from simulations of very long chains.
In this paper we estimate and from the simulations.
The estimates are compared with exact analytical results for a semiflexible
polymer confined in the transverse direction by a parabolic potential instead
of a channel and with a recent experiment. For the parabolic confining
potential we also obtain a simple analytic result for the distribution of
or radial distribution function, which is asymptotically exact
for large and has the skewed shape seen experimentally.Comment: 21 pages, including 4 figure
Surface Critical Behavior of Binary Alloys and Antiferromagnets: Dependence of the Universality Class on Surface Orientation
The surface critical behavior of semi-infinite
(a) binary alloys with a continuous order-disorder transition and
(b) Ising antiferromagnets in the presence of a magnetic field is considered.
In contrast to ferromagnets, the surface universality class of these systems
depends on the orientation of the surface with respect to the crystal axes.
There is ordinary and extraordinary surface critical behavior for orientations
that preserve and break the two-sublattice symmetry, respectively. This is
confirmed by transfer-matrix calculations for the two-dimensional
antiferromagnet and other evidence.Comment: Final version that appeared in PRL, some minor stylistic changes and
one corrected formula; 4 pp., twocolumn, REVTeX, 3 eps fig
Simulation of a semiflexible polymer in a narrow cylindrical pore
The probability that a randomly accelerated particle in two dimensions has
not yet left a simply connected domain after a time decays as
for long times. The same quantity also determines the
confinement free energy per unit length of a
semiflexible polymer in a narrow cylindrical pore with cross section . From simulations of a randomly accelerated particle we estimate the
universal amplitude of for both circular and rectangular cross
sections.Comment: 10 pages, 2 eps figure
Conformal off-diagonal boundary density profiles on a semi-infinite strip
The off-diagonal profile phi(v) associated with a local operator (order
parameter or energy density) close to the boundary of a semi-infinite strip
with width L is obtained at criticality using conformal methods. It involves
the surface exponent x_phi^s and displays a simple universal behaviour which
crosses over from surface finite-size scaling when v/L is held constant to
corner finite-size scaling when v/L -> 0.Comment: 5 pages, 1 figure, IOP macros and eps
Casimir Forces between Spherical Particles in a Critical Fluid and Conformal Invariance
Mesoscopic particles immersed in a critical fluid experience long-range
Casimir forces due to critical fluctuations. Using field theoretical methods,
we investigate the Casimir interaction between two spherical particles and
between a single particle and a planar boundary of the fluid. We exploit the
conformal symmetry at the critical point to map both cases onto a highly
symmetric geometry where the fluid is bounded by two concentric spheres with
radii R_- and R_+. In this geometry the singular part of the free energy F only
depends upon the ratio R_-/R_+, and the stress tensor, which we use to
calculate F, has a particularly simple form. Different boundary conditions
(surface universality classes) are considered, which either break or preserve
the order-parameter symmetry. We also consider profiles of thermodynamic
densities in the presence of two spheres. Explicit results are presented for an
ordinary critical point to leading order in epsilon=4-d and, in the case of
preserved symmetry, for the Gaussian model in arbitrary spatial dimension d.
Fundamental short-distance properties, such as profile behavior near a surface
or the behavior if a sphere has a `small' radius, are discussed and verified.
The relevance for colloidal solutions is pointed out.Comment: 37 pages, 2 postscript figures, REVTEX 3.0, published in Phys. Rev. B
51, 13717 (1995
Local functional models of critical correlations in thin-films
Recent work on local functional theories of critical inhomogeneous fluids and
Ising-like magnets has shown them to be a potentially exact, or near exact,
description of universal finite-size effects associated with the excess
free-energy and scaling of one-point functions in critical thin films. This
approach is extended to predict the two-point correlation function G in
critical thin-films with symmetric surface fields in arbitrary dimension d. In
d=2 we show there is exact agreement with the predictions of conformal
invariance for the complete spectrum of correlation lengths as well as the
detailed position dependence of the asymptotic decay of G. In d=3 and d>=4 we
present new numerical predictions for the universal finite-size correlation
length and scaling functions determining the structure of G across the
thin-film. Highly accurate analytical closed form expressions for these
universal properties are derived in arbitrary dimension.Comment: 4 pages, 1 postscript figure. Submitted to Phys Rev Let
Critical behaviour near multiple junctions and dirty surfaces in the two-dimensional Ising model
We consider m two-dimensional semi-infinite planes of Ising spins joined
together through surface spins and study the critical behaviour near to the
junction. The m=0 limit of the model - according to the replica trick -
corresponds to the semi-infinite Ising model in the presence of a random
surface field (RSFI). Using conformal mapping, second-order perturbation
expansion around the weakly- and strongly-coupled planes limits and
differential renormalization group, we show that the surface critical behaviour
of the RSFI model is described by Ising critical exponents with logarithmic
corrections to scaling, while at multiple junctions (m>2) the transition is
first order. There is a spontaneous junction magnetization at the bulk critical
point.Comment: Old paper, for archiving. 6 pages, 1 figure, IOP macro, eps
Non-Universal Critical Behaviour of Two-Dimensional Ising Systems
Two conditions are derived for Ising models to show non-universal critical
behaviour, namely conditions concerning 1) logarithmic singularity of the
specific heat and 2) degeneracy of the ground state. These conditions are
satisfied with the eight-vertex model, the Ashkin-Teller model, some Ising
models with short- or long-range interactions and even Ising systems without
the translational or the rotational invariance.Comment: 17 page
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