924 research outputs found
Surface Polymer Network Model and Effective Membrane Curvature Elasticity
A microscopic model of a surface polymer network - membrane system is
introduced, with contact polymer surface interactions that can be either
repulsive or attractive and sliplinks of functionality four randomly
distributed over the supporting membrane surface anchoring the polymers to it.
For the supporting surface perturbed from a planar configuration and a small
relative number of surface sliplinks, we investigate an expansion of the free
energy in terms of the local curvatures of the surface and the surface density
of sliplinks, obtained through the application of the Balian - Bloch -
Duplantier multiple surface scattering method. As a result, the dependence of
the curvature elastic modulus, the Gaussian modulus as well as of the
spontaneous curvature of the "dressed" membrane, ~{\sl i.e.} polymer network
plus membrane matrix, is obtained on the mean polymer bulk end to end
separation and the surface density of sliplinks.Comment: 15 pages with one included compressed uuencoded figure
Fatigue and fracture of a 316 stainless steel metal matrix composite reinforced with 25% titanium diboride
Fatigue and fracture mechanisms have been studied in a steel-based metal matrix composite (MMC), comprising a 316L austenitic matrix reinforced with 25 wt% particulate titanium diboride (TiB2). The fracture toughness was determined in the as-HIPped condition as being slightly below 30 MPaâm. Fatigue crack growth rates have been determined, and corrected for the effects of crack closure. The fracture surfaces have been studied to determine the mechanisms of damage during crack advance, which are determined as matrix fatigue, reinforcement particle fracture, and ductile rupture of the matrix. We show that the occurrence of damage mechanisms during fatigue of the material is linked to Kmax, rather than to âK. This is rationalised in terms of a semi-cohesive process zone within the monotonic plastic zone ahead of the crack tip
First Order Type Models for Multivariable Process Control
It is well known in classical feedback control that many high order linear time-invariant systems can be approximated, for the purpose of feedback design, by a low order state-space model due to the presence of approximately cancelling poles and zeros in the system transfer function. The paper presents an equivalent technique in the case of a multi-variable system described by a strictly proper mxm, minimum-phase and invertible transfer function matrix G (s) by the application of the contraction mapping theorem. It is shown that, in many cases of practical interest, a multi-variable first-order type model is adequate for the purposes of control system design and that such a model can be determined directly from transient response data or, equivalently, by the analysis of the high and low frequency characteristics of the system. The application of the technique is illustrated by the analysis of a high order binary distillation column model and the dynamics of a counter-flow heat exchanger
A cluster randomised controlled trial to investigate the effectiveness and cost effectiveness of the 'Girls Active' intervention: a study protocol
Background: Despite the health benefits of physical activity, data from the UK suggest that a large proportion of adolescents do not meet the recommended levels of moderate-to-vigorous physical activity (MVPA). This is particularly evident in girls, who are less active than boys across all ages and may display a faster rate of decline in physical activity throughout adolescence. The ‘Girls Active’ intervention has been designed by the Youth Sport Trust to target the lower participation rates observed in adolescent girls. ‘Girls Active’ uses peer leadership and marketing to empower girls to influence decision making in their school, develop as role models and promote physical activity to other girls. Schools are provided with training and resources to review their physical activity, sport and PE provision, culture and practices to ensure they are relevant and attractive to adolescent girls. Methods/Design: This study is a two-arm cluster randomised controlled trial (RCT) aiming to recruit 20 secondary schools. Clusters will be randomised at the school level (stratified by school size and proportion of Black and Minority Ethnic (BME) pupils) to receive either the ‘Girls Active’ intervention or carry on with usual practice (1:1). The 20 secondary schools will be recruited from state secondary schools within the Midlands area. We aim to recruit 80 girls aged 11–14 years in each school. Data will be collected at three time points; baseline and seven and 14months after baseline. Our primary aim is to investigate whether ‘Girls Active’ leads to higher objectively measured (GENEActiv) moderate-to-vigorous physical activity in adolescent girls at 14months after baseline assessment compared to the control group. Secondary outcomes include other objectively measured physical activity variables, adiposity, physical activity-related psychological factors and the cost-effectiveness of the ‘Girls Active’ intervention. A thorough process evaluation will be conducted during the course of the intervention delivery. Discussion: The findings of this study will provide valuable information on whether this type of school-based approach to increasing physical activity in adolescent girls is both effective and cost-effective in the UK
Hybrid ablation for atrial fibrillation: A systematic review and meta-analysis
Background: Both catheter and surgical ablation strategies offer effective treatments of atrial fibrillation (AF). The hybrid (joint surgical and catheter) ablation for AF is an emerging rhythm control strategy. We sought to determine the efficacy and safety of hybrid ablation of AF. Methods: Systematic review and meta-analysis interrogating PubMed, EMBASE, and Cochrane databases from January 1, 1991, to November 30, 2017, using the following search terms: âCox-maze,â âmini-maze,â âablation methods (including radiofrequency, cryoablation, cryomaze),â and âsurgery.â Included studies required ablation procedures to be hybrid and report rhythm follow-up. Results: We included 925 patients with AF (38% persistent, 51% longstanding persistent) from 22 single-center studies (mean follow-up of 19 months). The surgical lesion set consisted of pulmonary vein isolation (n = 11) or box lesion (n = 11) with variable additional linear ablation. This was followed by sequential (n = 9), staged (n = 9), or combination (n = 4) catheter-based ablation to ensure isolation of pulmonary veins and to facilitate additional ablation or consolidation of surgically ablated lines. Overall, sinus rhythm maintenance was 79.4% (95% confidence interval [CI], 72.4-85.7] and 70.7% (95% CI, 62.2-78.7) with and without antiarrhythmic drugs, respectively at 19 ± 25 (range, 6-128) months. The use of the bipolar AtriCure Synergy system and left atrial appendage exclusion conferred superior rhythm outcome without antiarrhythmic drugs (P †.01). The overall complication rate was 6.5% (95% CI, 3.4-10.2): mortality 0.2% (95% CI, 0-0.9); stroke 0.3% (95% CI, 0-1.1); reoperation for bleeding 1.6% (95% CI, 0.6-3.0); permanent pacing ~0% (95% CI, 0-0.5); conversion to sternotomy 0.3% (95% CI, 0-1.1); atrioesophageal fistula ~0% (95% CI, 0-0.5); and phrenic nerve injury 0.3% (95% CI, 0-1.1). Conclusions: Hybrid ablation therapy for AF demonstrates favorable rhythm outcome with acceptable complication rates.Jason A. Varzaly, Dennis H. Lau, Darius Chapman, James Edwards, Michael Worthington, and Prashanthan Sander
Friedel oscillations in a two-band Hubbard model for CuO chains
Friedel oscillations induced by open boundary conditions in a two-band
Hubbard model for CuO chains are numerically studied. We find that for
physically realistic parameters and close to quarter filling, these
oscillations have a 2k_F modulation according with experimental results on
YBa_2Cu_3O_{7-delta}. In addition, we predict that, for the same parameters, as
hole doping is reduced from quarter filling to half filling, Friedel
oscillations would acquire a 4k_F modulation, typical of a strongly correlated
electrons regime. The 4k_F modulation dominates also in the electron doped
region. The range of parameters varied is very broad, and hence the results
reported could apply to other cuprates and other strongly correlated compounds
with quasi-one dimensional structures. On a more theoretical side, we stress
the fact that the copper and oxygen subsystems should be described by two
different Luttinger liquid exponents.Comment: 7 pages, 7 eps figure
Effects of inhomogeneities on apparent cosmological observables: "fake" evolving dark energy
Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing
cosmological constant , we investigate how the presence of a local
spherically-symmetric inhomogeneity can affect apparent cosmological
observables, such as the deceleration parameter or the effective equation of
state of dark energy (DE), derived from the luminosity distance under the
assumption that the real space-time is exactly homogeneous and isotropic. The
presence of a local underdensity is found to produce apparent phantom behavior
of DE, while a locally overdense region leads to apparent quintessence
behavior. We consider relatively small large scale inhomogeneities which today
are not linear and could be seeded by primordial curvature perturbations
compatible with CMB bounds. Our study shows how observations in an
inhomogeneous CDM universe with initial conditions compatible with the
inflationary beginning, if interpreted under the wrong assumption of
homogeneity, can lead to the wrong conclusion about the presence of "fake"
evolving dark energy instead of .Comment: 22 pages, 19 figures,Final version to appear in European Physical
Journal
Lagrangian Becchi-Rouet-Stora-Tyutin treatment of collective coordinates
The Becchi-Rouet-Stora-Tyutin (BRST) treatment for the quantization of
collective coordinates is considered in the Lagrangian formalism. The motion of
a particle in a Riemannian manifold is studied in the case when the classical
solutions break a non-abelian global invariance of the action. Collective
coordinates are introduced, and the resulting gauge theory is quantized in the
BRST antifield formalism. The partition function is computed perturbatively to
two-loops, and it is shown that the results are independent of gauge-fixing
parameters.Comment: LaTeX file, 26 pages, PostScript figures at end of fil
A self-consistent treatment of non-equilibrium spin torques in magnetic multilayers
It is known that the transfer of spin angular momenta between current
carriers and local moments occurs near the interface of magnetic layers when
their moments are non-collinear. However, to determine the magnitude of the
transfer, one should calculate the spin transport properties far beyond the
interface regions. Based on the spin diffusion equation, we present a
self-consistent approach to evaluate the spin torque for a number of layered
structures. One of the salient features is that the longitudinal and transverse
components of spin accumulations are inter-twined from one layer to the next,
and thus, the spin torque could be significantly amplified with respect to
treatments which concentrate solely on the transport at the interface due to
the presence of the much longer longitudinal spin diffusion length. We conclude
that bare spin currents do not properly estimate the spin angular momentum
transferred between to the magnetic background; the spin transfer that occurs
at interfaces should be self-consistently determined by embedding it in our
globally diffuse transport calculations.Comment: 21 pages, 6 figure
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