780 research outputs found
Towards Measures for Translation : An Exploration of Neurophysiological Biomarkers of Disability in Multiple Sclerosis
Preserving surface area and porosity during fabrication of silicon aerocrystal particles from anodized wafers
Qualification and Issues with Space Flight Laser Systems and Components
The art of flight quality solid-state laser development is still relatively young, and much is still unknown regarding the best procedures, components, and packaging required for achieving the maximum possible lifetime and reliability when deployed in the harsh space environment. One of the most important issues is the limited and unstable supply of quality, high power diode arrays with significant technological heritage and market lifetime. Since Spectra Diode Labs Inc. ended their involvement in the pulsed array business in the late 199O's, there has been a flurry of activity from other manufacturers, but little effort focused on flight quality production. This forces NASA, inevitably, to examine the use of commercial parts to enable space flight laser designs. System-level issues such as power cycling, operational derating, duty cycle, and contamination risks to other laser components are some of the more significant unknown, if unquantifiable, parameters that directly effect transmitter reliability. Designs and processes can be formulated for the system and the components (including thorough modeling) to mitigate risk based on the known failures modes as well as lessons learned that GSFC has collected over the past ten years of space flight operation of lasers. In addition, knowledge of the potential failure modes related to the system and the components themselves can allow the qualification testing to be done in an efficient yet, effective manner. Careful test plan development coupled with physics of failure knowledge will enable cost effect qualification of commercial technology. Presented here will be lessons learned from space flight experience, brief synopsis of known potential failure modes, mitigation techniques, and options for testing from the system level to the component level
Range corrections for two-neutron halo nuclei in effective theory
The range corrections to the universal properties and structure of
two-neutron halo nuclei are investigated within an effective quantum mechanics
framework. Treating the nucleus as an effective three-body system, we make a
systematic improvement upon previous calculations by calculating the linear
range corrections at next-to-leading order. Since the effective ranges for the
neutron-core interactions are not known, we estimate the effective range to be
set by the inverse of the pion mass. We investigate the possibility of excited
Efimov states in two-neutron halo nuclei and calculate their mean square radii
to next-to-leading order. We find that the effective range corrections are
generally small and the leading order predictions are very robust.Comment: 19 pages, 4 eps figures, revtex4, final version to appear in Nucl.
Phys.
Stresses in lipid membranes
The stresses in a closed lipid membrane described by the Helfrich
hamiltonian, quadratic in the extrinsic curvature, are identified using
Noether's theorem. Three equations describe the conservation of the stress
tensor: the normal projection is identified as the shape equation describing
equilibrium configurations; the tangential projections are consistency
conditions on the stresses which capture the fluid character of such membranes.
The corresponding torque tensor is also identified. The use of the stress
tensor as a basis for perturbation theory is discussed. The conservation laws
are cast in terms of the forces and torques on closed curves. As an
application, the first integral of the shape equation for axially symmetric
configurations is derived by examining the forces which are balanced along
circles of constant latitude.Comment: 16 pages, introduction rewritten, other minor changes, new references
added, version to appear in Journal of Physics
Investigation of a Mesoporous Silicon Based Ferromagnetic Nanocomposite
A semiconductor/metal nanocomposite is composed of a porosified silicon wafer and embedded ferromagnetic nanostructures. The obtained hybrid system possesses the electronic properties of silicon together with the magnetic properties of the incorporated ferromagnetic metal. On the one hand, a transition metal is electrochemically deposited from a metal salt solution into the nanostructured silicon skeleton, on the other hand magnetic particles of a few nanometres in size, fabricated in solution, are incorporated by immersion. The electrochemically deposited nanostructures can be tuned in size, shape and their spatial distribution by the process parameters, and thus specimens with desired ferromagnetic properties can be fabricated. Using magnetite nanoparticles for infiltration into porous silicon is of interest not only because of the magnetic properties of the composite material due to the possible modification of the ferromagnetic/superparamagnetic transition but also because of the biocompatibility of the system caused by the low toxicity of both materials. Thus, it is a promising candidate for biomedical applications as drug delivery or biomedical targeting
Effective Area-Elasticity and Tension of Micro-manipulated Membranes
We evaluate the effective Hamiltonian governing, at the optically resolved
scale, the elastic properties of micro-manipulated membranes. We identify
floppy, entropic-tense and stretched-tense regimes, representing different
behaviors of the effective area-elasticity of the membrane. The corresponding
effective tension depends on the microscopic parameters (total area, bending
rigidity) and on the optically visible area, which is controlled by the imposed
external constraints. We successfully compare our predictions with recent data
on micropipette experiments.Comment: To be published in Phys. Rev. Let
Fission of a multiphase membrane tube
A common mechanism for intracellular transport is the use of controlled
deformations of the membrane to create spherical or tubular buds. While the
basic physical properties of homogeneous membranes are relatively well-known,
the effects of inhomogeneities within membranes are very much an active field
of study. Membrane domains enriched in certain lipids in particular are
attracting much attention, and in this Letter we investigate the effect of such
domains on the shape and fate of membrane tubes. Recent experiments have
demonstrated that forced lipid phase separation can trigger tube fission, and
we demonstrate how this can be understood purely from the difference in elastic
constants between the domains. Moreover, the proposed model predicts timescales
for fission that agree well with experimental findings
Helfrich-Canham bending energy as a constrained non-linear sigma model
The Helfrich-Canham bending energy is identified with a non-linear sigma
model for a unit vector. The identification, however, is dependent on one
additional constraint: that the unit vector be constrained to lie orthogonal to
the surface. The presence of this constraint adds a source to the divergence of
the stress tensor for this vector so that it is not conserved. The stress
tensor which is conserved is identified and its conservation shown to reproduce
the correct shape equation.Comment: 5 page
Hamilton's equations for a fluid membrane: axial symmetry
Consider a homogenous fluid membrane, or vesicle, described by the
Helfrich-Canham energy, quadratic in the mean curvature. When the membrane is
axially symmetric, this energy can be viewed as an `action' describing the
motion of a particle; the contours of equilibrium geometries are identified
with particle trajectories. A novel Hamiltonian formulation of the problem is
presented which exhibits the following two features: {\it (i)} the second
derivatives appearing in the action through the mean curvature are accommodated
in a natural phase space; {\it (ii)} the intrinsic freedom associated with the
choice of evolution parameter along the contour is preserved. As a result, the
phase space involves momenta conjugate not only to the particle position but
also to its velocity, and there are constraints on the phase space variables.
This formulation provides the groundwork for a field theoretical generalization
to arbitrary configurations, with the particle replaced by a loop in space.Comment: 11 page
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