227 research outputs found
Dynamics of Strongly Deformed Polymers in Solution
Bead spring models for polymers in solution are nonlinear if either the
finite extensibility of the polymer, excluded volume effects or hydrodynamic
interactions between polymer segments are taken into account. For such models
we use a powerful method for the determination of the complete relaxation
spectrum of fluctuations at {\it steady state}. In general, the spectrum and
modes differ significantly from those of the linear Rouse model. For a tethered
polymer in uniform flow the differences are mainly caused by an inhomogeneous
distribution of tension along the chain and are most pronounced due to the
finite chain extensibility. Beyond the dynamics of steady state fluctuations we
also investigate the nonlinear response of the polymer to a {\em large sudden
change} in the flow. This response exhibits several distinct regimes with
characteristic decay laws and shows features which are beyond the scope of
single mode theories such as the dumbbell model.Comment: 7 pages, 3 figure
Two-Loop Corrections to the Neutral Higgs Boson Masses in the CP-Violating NMSSM
We present our calculation of the two-loop corrections of to the neutral Higgs boson masses of the CP-violating
Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM). The
calculation is performed in the Feynman diagrammatic approach in the gaugeless
limit at vanishing external momentum. We apply a mixed
-on-shell (OS) renormalization scheme for the NMSSM
input parameters. Furthermore, we exploit a as well as
an OS renormalization in the top/stop sector. The corrections are implemented
in the Fortran code NMSSMCALC for the calculation of the Higgs spectrum both in
the CP-conserving and CP-violating NMSSM. The code also provides the Higgs
boson decays including the state-of-the-art higher-order corrections. The
corrections computed in this work improve the already available corrections in
NMSSMCALC which are the full one-loop corrections without any approximation and
the two-loop corrections in the gaugeless limit
and at vanishing external momentum. Depending on the chosen parameter point, we
find that the corrections add about
4-7% to the one-loop mass of the SM-like Higgs boson for
renormalization in the top/stop sector and they reduce
the mass by about 6-9% if OS renormalization is applied. For an estimate of the
theoretical uncertainty we vary the renormalization scale and change the
renormalization scheme and show that care has to be taken in the corresponding
interpretation
Polymer drift in a solvent by force acting on one polymer end
We investigate the effect of hydrodynamic interactions on the non-equilibrium
drift dynamics of an ideal flexible polymer pulled by a constant force applied
at one end of the polymer using the perturbation theory and the renormalization
group method. For moderate force, if the polymer elongation is small, the
hydrodynamic interactions are not screened and the velocity and the
longitudinal elongation of the polymer are computed using the renormalization
group method. Both the velocity and elongation are nonlinear functions of the
driving force in this regime. For large elongation we found two regimes. For
large force but finite chain length the hydrodynamic interactions are
screened. For large chain lengths and a finite force the hydrodynamic
interactions are only partially screened, which in three dimensions results in
unusual logarithmic corrections to the velocity and the longitudinal
elongation.Comment: 6 page
Direct measurement of shear-induced cross-correlations of Brownian motion
Shear-induced cross-correlations of particle fluctuations perpendicular and
along stream-lines are investigated experimentally and theoretically. Direct
measurements of the Brownian motion of micron-sized beads, held by optical
tweezers in a shear-flow cell, show a strong time-asymmetry in the
cross-correlation, which is caused by the non-normal amplification of
fluctuations. Complementary measurements on the single particle probability
distribution substantiate this behavior and both results are consistent with a
Langevin model. In addition, a shear-induced anti-correlation between
orthogonal random-displacements of two trapped and hydrodynamically interacting
particles is detected, having one or two extrema in time, depending on the
positions of the particles.Comment: 4 pages, 4 figure
Benchmarking of CFD Modelling Closures for Two-Phase Turbulent Bubbly Flows
Eulerian-Eulerian computational fluid dynamic models are used in the prediction of multiphase gas-liquid flows in nuclear reactor thermal hydraulics and in many other chemical and process engineering applications. The modelling approach, based on the concept of interpenetrating continua, allows the calculation of complex and large-scale industrial flows with a relatively limited computational load. However, interfacial transfer processes need to be entirely modelled through numerous closure relations. A large number of different optimized closure sets are available, each often showing remarkable accuracy, but generally only over a few experimental data sets. This specificity makes it difficult to compare the overall accuracy of the models and obstructs the development of more general and robust approaches. In this paper, the bubbly flow models developed at the University of Leeds and the Helmholtz-Zentrum Dresden - Rossendorf are benchmarked against relevant experiments. These two research groups follow a similar modelling approach, aimed at identifying a single universal set of widely applicable closures. The models, implemented respectively in Star-CCM+ and CFX, are applied to a large selection of bubbly flows in different geometries. The main focus is on the momentum transfer, mainly responsible for the lateral bubble distribution in any flow, and on turbulence closures. Therefore, monodispersed bubbly flows that can be effectively characterized with a single average bubble diameter are selected. Overall, the models are found to be generally reliable and robust, and additional developments towards further improved accuracy, increased generality and the definition of a common unified set of model
closures are identified. In future, additional benchmark exercises of this kind will be performed, and potentially the definition of proven sets of reference experiments will be recommended
Many-body interactions and melting of colloidal crystals
We study the melting behavior of charged colloidal crystals, using a
simulation technique that combines a continuous mean-field Poisson-Boltzmann
description for the microscopic electrolyte ions with a Brownian-dynamics
simulation for the mesoscopic colloids. This technique ensures that many-body
interactions between the colloids are fully taken into account, and thus allows
us to investigate how many-body interactions affect the solid-liquid phase
behavior of charged colloids. Using the Lindemann criterion, we determine the
melting line in a phase-diagram spanned by the colloidal charge and the salt
concentration. We compare our results to predictions based on the established
description of colloidal suspensions in terms of pairwise additive Yukawa
potentials, and find good agreement at high-salt, but not at low-salt
concentration. Analyzing the effective pair-interaction between two colloids in
a crystalline environment, we demonstrate that the difference in the melting
behavior observed at low salt is due to many-body interactions
Heavy Scalar Top Quark Decays in the Complex MSSM: A Full One-Loop Analysis
We evaluate all two-body decay modes of the heavy scalar top quark in the
Minimal Supersymmetric Standard Model with complex parameters (cMSSM) and no
generation mixing. The evaluation is based on a full one-loop calculation of
all decay channels, also including hard QED and QCD radiation. The
renormalization of the complex parameters is described in detail. The
dependence of the heavy scalar top quark decay on the relevant cMSSM parameters
is analyzed numerically, including also the decay to Higgs bosons and another
scalar quark or to a top quark and the lightest neutralino. We find sizable
contributions to many partial decay widths and branching ratios. They are
roughly of O(10%) of the tree-level results, but can go up to 30% or higher.
These contributions are important for the correct interpretation of scalar top
quark decays at the LHC and, if kinematically allowed, at the ILC. The
evaluation of the branching ratios of the heavy scalar top quark will be
implemented into the Fortran code FeynHiggs.Comment: 86 pages, 38 figures; minor changes, version published as Phys. Rev.
D86 (2012) 03501
Effect of many-body interactions on the solid-liquid phase-behavior of charge-stabilized colloidal suspensions
The solid-liquid phase-diagram of charge-stabilized colloidal suspensions is
calculated using a technique that combines a continuous Poisson-Boltzmann
description for the microscopic electrolyte ions with a molecular-dynamics
simulation for the macroionic colloidal spheres. While correlations between the
microions are neglected in this approach, many-body interactions between the
colloids are fully included. The solid-liquid transition is determined at a
high colloid volume fraction where many-body interactions are expected to be
strong. With a view to the Derjaguin-Landau-Verwey-Overbeek theory predicting
that colloids interact via Yukawa pair-potentials, we compare our results with
the phase diagram of a simple Yukawa liquid. Good agreement is found at high
salt conditions, while at low ionic strength considerable deviations are
observed. By calculating effective colloid-colloid pair-interactions it is
demonstrated that these differences are due to many-body interactions. We
suggest a density-dependent pair-potential in the form of a truncated Yukawa
potential, and show that it offers a considerably improved description of the
solid-liquid phase-behavior of concentrated colloidal suspensions
Higgs-boson masses and mixing matrices in the NMSSM: analysis of on-shell calculations
We analyze the Higgs-boson masses and mixing matrices in the NMSSM based on an on-shell (OS) renormalization of the gauge-boson and Higgs-boson masses and the parameters of the top/scalar top sector. We compare the implementation of the OS calculations in the codes NMSSMCALC and NMSSM-FeynHiggs up to O(αtαs). We identify the sources of discrepancies at the one- and at the twoloop level. Finally we compare the OS and DR evaluation as implemented in NMSSMCALC. The results are important ingredients for an estimate of the theoretical precision of Higgs-boson mass calculations in the NMSSM
Early infant feeding and adiposity risk: from infancy to adulthood
Introduction: Systematic reviews suggest that a longer duration of breast-feeding is associated with a reduction in the risk of later overweight and obesity. Most studies examining breast-feeding in relation to adiposity have not used longitudinal analysis. In our study, we aimed to examine early infant feeding and adiposity risk in a longitudinal cohort from birth to young adulthood using new as well as published data.
Methods: Data from the Western Australian Pregnancy Cohort (Raine) Study in Perth, W.A., Australia, were used to examine associations between breast-feeding and measures of adiposity at 1, 2, 3, 6, 8, 10, 14, 17, and 20 years.
Results: Breast-feeding was measured in a number of ways. Longer breast-feeding (in months) was associated with reductions in weight z-scores between birth and 1 year (β = -0.027; p \u3c 0.001) in the adjusted analysis. At 3 years, breast-feeding for \u3c4 months increased the odds of infants experiencing early rapid growth (OR 2.05; 95% CI 1.43-2.94; p \u3c 0.001). From 1 to 8 years, children breast-fed for ≤4 months compared to ≥12 months had a significantly greater probability of exceeding the 95th percentile of weight. The age at which breast-feeding was stopped and a milk other than breast milk was introduced (introduction of formula milk) played a significant role in the trajectory of the BMI from birth to 14 years; the 4-month cutoff point was consistently associated with a higher BMI trajectory. Introduction of a milk other than breast milk before 6 months compared to at 6 months or later was a risk factor for being overweight or obese at 20 years of age (OR 1.47; 95% CI 1.12-1.93; p = 0.005).
Discussion: Breast-feeding until 6 months of age and beyond should be encouraged and is recommended for protection against increased adiposity in childhood, adolescence, and young adulthood. Adverse long-term effects of early growth acceleration are fundamental in later overweight and obesity. Formula feeding stimulates a higher postnatal growth velocity, whereas breast-feeding promotes slower growth and a reduced likelihood of overweight and obesity. Biological mechanisms underlying the protective effect of breast-feeding against obesity are based on the unique composition and metabolic and physiological responses to human milk
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