1,690 research outputs found
Stationary and non-stationary fluid flow of a Bose-Einstein condensate through a penetrable barrier
We experimentally study the fluid flow induced by a broad, penetrable barrier
moving through an elongated dilute gaseous Bose-Einstein condensate. The
barrier is created by a laser beam swept through the condensate, and the
resulting dipole potential can be either attractive or repulsive. We examine
both cases and find regimes of stable and unstable fluid flow: At slow speeds
of the barrier, the fluid flow is stationary due to the superfluidity of the
condensate. At intermediate speeds, we observe a non-stationary regime in which
the condensate gets filled with dark solitons. At faster speeds, soliton
formation completely ceases and a remarkable absence of excitation in the
condensate is seen again.Comment: 4 pages, 4 figure
An Energy-Minimization Finite-Element Approach for the Frank-Oseen Model of Nematic Liquid Crystals: Continuum and Discrete Analysis
This paper outlines an energy-minimization finite-element approach to the
computational modeling of equilibrium configurations for nematic liquid
crystals under free elastic effects. The method targets minimization of the
system free energy based on the Frank-Oseen free-energy model. Solutions to the
intermediate discretized free elastic linearizations are shown to exist
generally and are unique under certain assumptions. This requires proving
continuity, coercivity, and weak coercivity for the accompanying appropriate
bilinear forms within a mixed finite-element framework. Error analysis
demonstrates that the method constitutes a convergent scheme. Numerical
experiments are performed for problems with a range of physical parameters as
well as simple and patterned boundary conditions. The resulting algorithm
accurately handles heterogeneous constant coefficients and effectively resolves
configurations resulting from complicated boundary conditions relevant in
ongoing research.Comment: 31 pages, 3 figures, 3 table
Community-based trial of screening for Chlamydia trachomatis to prevent pelvic inflammatory disease: the POPI (prevention of pelvic infection) trial.
BACKGROUND: Pelvic inflammatory disease (PID) is common and can lead to tubal factor infertility, ectopic pregnancy or chronic pelvic pain. Despite major UK government investment in the National Chlamydia Screening Programme, evidence of benefit remains controversial. The main aim of this trial was to investigate whether screening and treatment of chlamydial infection reduced the incidence of PID over 12 months. Secondary aims were to conduct exploratory studies of the role of bacterial vaginosis (BV) in the development of PID and of the natural history of chlamydial infection.
DESIGN: Randomised controlled trial with follow up after 12 months.
SETTING NON-HEALTHCARE: Common rooms and lecture theatres at 20 universities and further education colleges in Greater London.
PARTICIPANTS: 2500 sexually active female students were asked to complete a questionnaire on sexual health and provide self-administered vaginal swabs and smears.
INTERVENTION: Vaginal swabs from intervention women were tested for chlamydia by polymerase chain reaction (PCR) and those infected referred for treatment. Vaginal swabs from control women were stored and analysed after a year. Vaginal smears were Gram stained and analysed for BV.
MAIN OUTCOME MEASURE: Incidence of clinical PID over 12 months in intervention and control groups. Possible cases of PID will be identified from questionnaires and record searches. Confirmation of the diagnosis will be done by detailed review of medical records by three independent researchers blind to whether the woman is in intervention or control group. TRIAL REGISTRATION: Clinical Trials NCT 00115388
A Tunable Echelle Imager
We describe and evaluate a new instrument design called a Tunable Echelle
Imager (TEI). In this instrument, the output from an imaging Fabry-Perot
interferometer is cross-dispersed by a grism in one direction and dispersed by
an echelle grating in the perpendicular direction. This forms a mosaic of
different narrow-band images of the same field on a detector. It offers a
distinct wavelength multiplex advantage over a traditional imaging Fabry-Perot
device.
Potential applications of the TEI include spectrophotometric imaging and
OH-suppressed imaging by rejection.Comment: 11 pages, 12 figures, accepted by PAS
A finite element based formulation for sensitivity studies of piezoelectric systems
Sensitivity Analysis is a branch of numerical analysis which aims to quantify the affects that variability in the parameters of a numerical model have on the model output. A finite element based sensitivity analysis formulation for piezoelectric media is developed here and implemented to simulate the operational and sensitivity characteristics of a piezoelectric based distributed mode actuator (DMA). The work acts as a starting point for robustness analysis in the DMA technology
A contribution to the selection of emission-line galaxies using narrow-band filters in the optical airglow windows
Emission line galaxies are an invaluable tool for our understanding of the
evolution of galaxies in the Universe. Imaging of deep fields with narrow-band
filters allows not only the selection of these objects, but also to infer the
line flux and the equivalent width of the emission line with some assumptions.
The narrow-band filter technique provides homogeneous samples of galaxies in
small comoving volumes in the sky. We present an analysis of the selection of
emission-line galaxies using narrow-band filters. Different methods of
observation are considered: broad-band -- narrow-band filters and two
broad-band and one narrow-band filters.
We study also the effect of several lines entering simultaneously inside the
filters (this is the case of Halpha). In each case the equations to obtain the
equivalent width and line flux from the photometry are obtained. Candidates to
emission-line objects are selected by their color excess in a magnitude-color
diagram. For different narrow-band filters, we compute the mean colors of stars
and galaxies, showing that, apart from galaxies, some types of stars could be
selected with certain filter sets. We show how to compute the standard
deviation of the colors of the objects even in the usual case when there are
not enough objects to determine the standard deviation from the data. We
present also helpful equations to compute the narrow-band and the broad-band
exposure times in order to obtain minimum dispersion in the ratio of fluxes of
both bands with minimum total exposure time.Comment: Accepted for publication in PASP 48 pages, 10 figures Corrected
typos, fixed references. Updated reference to T
Independent control of polar and azimuthal anchoring
Monte Carlo simulation, experiment and continuum theory are used to examine
the anchoring exhibited by a nematic liquid crystal at a patterned substrate
comprising a periodic array of rectangles that, respectively, promote vertical
and planar alignment. It is shown that the easy axis and effective anchoring
energy promoted by such surfaces can be readily controlled by adjusting the
design of the pattern. The calculations reveal rich behavior: for strong
anchoring, as exhibited by the simulated system, for rectangle ratios
the nematic aligns in the direction of the long edge of the rectangles, the
azimuthal anchoring coefficient changing with pattern shape. In weak anchoring
scenarios, however, including our experimental systems, preferential anchoring
is degenerate between the two rectangle diagonals. Bistability between
diagonally-aligned and edge-aligned arrangement is predicted for intermediate
combinations of anchoring coefficient and system length-scale.Comment: 12 pages, 12 figure
A Role for the Vacuolating Cytotoxin, VacA, in Colonization and Helicobacter pylori-Induced Metaplasia in the Stomach
Carriage of Helicobacter pylori strains producing more active (s1/i1) forms of VacA is strongly associated with gas-tric adenocarcinoma. To our knowledge, we are the
first to determine effects of different polymorphic forms of VacA on inflammation and metaplasia in the mouse stomach. Bacteria producing the less active s2/i2 form of VacA colonized mice more efficiently than mutants null for VacA or producing more active forms of it, providing the
first evidence of a positive role for the minimally active s2/i2 toxin. Strains producing more active toxin forms induced more severe and extensive metaplasia and in flammation in the mouse stomach than strains producing weakly active (s2/i2) toxin. We also examined the association in humans, controlling for cag PAI status. In human gastric biopsy specimens, the vacA i1 allele was strongly associated with precancerous intestinal metaplasia, with almost complete absence of intestinal metaplasia in subjects infected with i2-type strains, even in a vacA s1, cagA+ background
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