8,296 research outputs found
Micromachined membrane particle filters
We report here several particle membrane filters (8 x 8 mm^2) with circular, hexagonal and rectangular through holes. By varying hole dimensions from 6 to 12 pm, opening factors from 4 to 45 % are achieved. In order to improve the filter robustness, a composite silicon nitride/Parylene membrane technology is developed. More importantly, fluid dynamic performance of the filters is also studied by both experiments and numerical simulations. It is found that the gaseous flow through the filters depends strongly on opening factors, and the measured pressure drops are much lower than that from numerical simulation using the Navier-Stokes equation. Interestingly, surface velocity slip can only account for a minor part of the discrepancy. This suggests that a very interesting topic for micro fluid mechanics research is identified
Lower bounds on the blow-up rate of the axisymmetric Navier-Stokes equations II
Consider axisymmetric strong solutions of the incompressible Navier-Stokes
equations in with non-trivial swirl. Let denote the axis of symmetry
and measure the distance to the z-axis. Suppose the solution satisfies
either or, for some \e > 0, for and
allowed to be large. We prove that is regular at time zero.Comment: More explanations and a new appendi
Minimal ureagenesis is necessary for survival in the murine model of hyperargininemia treated by AAV-based gene therapy.
Hyperammonemia is less severe in arginase 1 deficiency compared with other urea cycle defects. Affected patients manifest hyperargininemia and infrequent episodes of hyperammonemia. Patients typically suffer from neurological impairment with cortical and pyramidal tract deterioration, spasticity, loss of ambulation, seizures and intellectual disability; death is less common than with other urea cycle disorders. In a mouse model of arginase I deficiency, the onset of symptoms begins with weight loss and gait instability, which progresses toward development of tail tremor with seizure-like activity; death typically occurs at about 2 weeks of life. Adeno-associated viral vector gene replacement strategies result in long-term survival of mice with this disorder. With neonatal administration of vector, the viral copy number in the liver greatly declines with hepatocyte proliferation in the first 5 weeks of life. Although the animals do survive, it is not known from a functional standpoint how well the urea cycle is functioning in the adult animals that receive adeno-associated virus. In these studies, we administered [1-13C] acetate to both littermate controls and adeno-associated virus-treated arginase 1 knockout animals and examined flux through the urea cycle. Circulating ammonia levels were mildly elevated in treated animals. Arginine and glutamine also had perturbations. Assessment 30âmin after acetate administration demonstrated that ureagenesis was present in the treated knockout liver at levels as low at 3.3% of control animals. These studies demonstrate that only minimal levels of hepatic arginase activity are necessary for survival and ureagenesis in arginase-deficient mice and that this level of activity results in control of circulating ammonia. These results may have implications for potential therapy in humans with arginase deficiency
Fermat's principle of least time in the presence of uniformly moving boundaries and media
The refraction of a light ray by a homogeneous, isotropic and non-dispersive
transparent material half-space in uniform rectilinear motion is investigated
theoretically. The approach is an amalgamation of the original Fermat's
principle and the fact that an isotropic optical medium at rest becomes
optically anisotropic in a frame where the medium is moving at a constant
velocity. Two cases of motion are considered: a) the material half-space is
moving parallel to the interface; b) the material half-space is moving
perpendicular to the interface. In each case, a detailed analysis of the
obtained refraction formula is provided, and in the latter case, an intriguing
backward refraction of light is noticed and thoroughly discussed. The results
confirm the validity of Fermat's principle when the optical media and the
boundaries between them are moving at relativistic speeds.Comment: 11 pages, 6 figures, RevTeX 4, comments welcome; V2: revised, Fig. 7
added; V3: several typos corrected, accepted for publication in European
Journal of Physics (online at: http://stacks.iop.org/EJP/28/933
Examining summer laboratory research apprenticeships for high school students as a factor in entry to MD/PhD programs at matriculation
Vector modulation instability induced by vacuum fluctuations in highly birefringent fibers in the anomalous dispersion regime
We report a detailed experimental study of vector modulation instability in
highly birefringent optical fibers in the anomalous dispersion regime. We prove
that the observed instability is mainly induced by vacuum fluctuations. The
detuning of the spectral peaks agrees with linear perturbation analysis. The
exact shape of the spectrum is well reproduced by numerical integration of
stochastic nonlinear Schrodinger equations describing quantum propagation.Comment: 11 pages, 4 figures, to be published in Optics Letter
Personalised nutrition and health
This article is one of a series commissioned by The BMJ. Open access fees for the series were funded by SwissRe, which had no input into the commissioning or peer review of the articles.S
Recording advances for neural prosthetics
An important challenge for neural prosthetics research is to record from populations of neurons over long periods of time, ideally for the lifetime of the patient. Two new advances toward this goal are described, the use of local field potentials (LFPs) and autonomously positioned recording electrodes. LFPs are the composite extracellular potential field from several hundreds of neurons around the electrode tip. LFP recordings can be maintained for longer periods of time than single cell recordings. We find that similar information can be decoded from LFP and spike recordings, with better performance for state decodes with LFPs and, depending on the area, equivalent or slightly less than equivalent performance for signaling the direction of planned movements. Movable electrodes in microdrives can be adjusted in the tissue to optimize recordings, but their movements must be automated to be a practical benefit to patients. We have developed automation algorithms and a meso-scale autonomous electrode testbed, and demonstrated that this system can autonomously isolate and maintain the recorded signal quality of single cells in the cortex of awake, behaving monkeys. These two advances show promise for developing very long term recording for neural prosthetic applications
D-instantons and Closed String Tachyons in Misner Space
We investigate closed string tachyon condensation in Misner space, a toy
model for big bang universe. In Misner space, we are able to condense tachyonic
modes of closed strings in the twisted sectors, which is supposed to remove the
big bang singularity. In order to examine this, we utilize D-instanton as a
probe. First, we study general properties of D-instanton by constructing
boundary state and effective action. Then, resorting to these, we are able to
show that tachyon condensation actually deforms the geometry such that the
singularity becomes milder.Comment: 24 pages, 1 figure, minor change
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