1,140 research outputs found
Atomistic Hydrodynamics and the Dynamical Hydrophobic Effect in Porous Graphene
Mirroring their role in electrical and optical physics, two-dimensional
crystals are emerging as novel platforms for fluid separations and water
desalination, which are hydrodynamic processes that occur in nanoscale
environments. For numerical simulation to play a predictive and descriptive
role, one must have theoretically sound methods that span orders of magnitude
in physical scales, from the atomistic motions of particles inside the channels
to the large-scale hydrodynamic gradients that drive transport. Here, we use
constraint dynamics to derive a nonequilibrium molecular dynamics method for
simulating steady-state mass flow of a fluid moving through the nanoscopic
spaces of a porous solid. After validating our method on a model system, we use
it to study the hydrophobic effect of water moving through pores of
electrically doped single-layer graphene. The trend in permeability that we
calculate does not follow the hydrophobicity of the membrane, but is instead
governed by a crossover between two competing molecular transport mechanisms.Comment: 6 pages, 3 figure
On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion
Recent transient absorption experiments on CdS nanorods suggest that
photoexcited holes rapidly trap to the surface of these particles and then
undergo diffusion along the rod surface. In this paper, we present a
semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment
on the nature of both the hole-trap states and the mechanism by which the holes
diffuse. Hole states near the top of the valence band form an energetic near
continuum with the bulk, and localize to the non-bonding sp orbitals on
surface sulfur atoms. After localization, the holes form nonadiabatic small
polarons that move between the sulfur orbitals on the surface of the particle
in a series of uncorrelated, incoherent, thermally-activated hops at room
temperature. The surface-trapped holes are deeply in the weak-electronic
coupling limit and, as a result, undergo slow diffusion.Comment: 4 figure
Diverse Demographic Factors Influence Apparent Juvenile Survival In A Migratory Songbird
To better understand the dynamics of avian populations and their role in population trends, we require an in-depth understanding of the factors influencing the survival of adults and juveniles. How-ever, assessing survival in juveniles is often challenging, especially in small, migratory species where individuals typically disperse from the study area and are not available for recapture in subsequent years. Bobolinks (Dolichonyx oryzivorus) are a long-distance migrant that exhibits natal philopatry in at least one population, allowing for more comprehensive juvenile survival analyses than in many other long-distance avian migrants. Using a 17-yr dataset from two sites representing a Vermont population of Bobolinks, we used Program MARK to assess factors influencing apparent juvenile survival, including factors related to nesting timing, nest attempt number, the philopatric behaviors of relatives, body mass, brood size, and agricultural management scheme. Our top models indicated that nest attempt number and whether or not a nest mate also survived and returned to breed locally were important factors explaining variation in apparent survival in juvenile Bobolinks. Specifically, juveniles from first nest attempts that fledged earlier in the season, with siblings that did not survive and return to breed locally, showed higher apparent survival. Factors such as site and the philopatric behavior of females associated with nests also appeared in top-ranking models, while factors such as body mass and brood size did not. These results indicate the importance of providing high-quality breeding habitat to birds early in the season when juvenile survival is greatest and indicate that individuals may be utilizing inbreeding avoidance strategies. These results provide new insight into the ecological and agricultural management factors influencing survival in migratory species that use managed habitats and underscore the importance of integrating juvenile survival data into current management schemes to better support this and other declining species
Nestling Sex Ratios Do Not Support Long-Term Parity In Two Species With Different Life-History Strategies
To maximize fitness, breeding adults may respond to environmental processes by adjusting their progeny’s sex ratios. R. A. Fisher in 1930 hypothesized that frequency-dependent selection would result in equal investment in sons and daughters over the long term, yielding a balanced sex ratio if the costs of raising a son and daughter are equal. Diverse hypotheses have tried to explain population and brood-by-brood deviations from this mean as well as annual variation by focusing on adult sex ratios, resources, abiotic conditions, and female and male quality. We collected data in 2002-2010 to explore population-level variation in nestling sex ratios in 2 migratory grassland songbird species: the Bobolink (Dolichonyx oryzivorus) and Savannah Sparrow (Passerculus sandwichensis). These species differ in migratory strategy (long-distance vs. short-distance), and morphological dimorphism. Fisher’s hypothesis was rejected for Savannah Sparrows (n = 684 nestlings; 39% male) but not rejected for Bobolinks (n = 390 nestlings; 53.8% male). No relationship was found between nestling and adult sex ratios measured in the same year. In descriptive analyses at the brood level, male and female body size and age, and ecological conditions (temperature and precipitation) failed to predict nestling sex ratios. Although male nestlings were heavier than female nestlings and resource availability changed through the season, these factors did not influence sex ratios relative to female body size or seasonality. For Savannah Sparrows, larger broods tended to be male-biased. While we were otherwise not able to explain deviation in offspring sex ratio for Savannah Sparrows, our results suggest that the ecological and evolutionary pressures that affect sex ratios may be both species- and population-specific
Factors Associated with Marburg Hemorrhagic Fever:
Background. Reliable on-site polymerase chain reaction (PCR) testing for Marburg hemorrhagic fever (MHF)
is not always available. Therefore, clinicians triage patients on the basis of presenting symptoms and contact history.
Using patient data collected in Uige, Angola, in 2005, we assessed the sensitivity and specificity of these factors
to evaluate the validity of World Health Organization (WHO)–recommended case definitions for MHF.
Methods. Multivariable logistic regression was used to identify independent predictors of PCR confirmation
of MHF. A data-derived algorithm was developed to obtain new MHF case definitions with improved sensitivity
and specificity.
Results. A MHF case definition comprising (1) an epidemiological link or (2) the combination of myalgia or
arthralgia and any hemorrhage could potentially serve as an alternative to current case definitions. Our dataderived
case definitions maintained the sensitivity and improved the specificity of current WHO-recommended
case definitions.
Conclusions. Continued efforts to improve clinical documentation during filovirus outbreaks would aid in
the refinement of case definitions and facilitate outbreak control
Dimerization and Incommensurate Spiral Spin Correlations in the Zigzag Spin Chain: Analogies to the Kondo Lattice
Using the density matrix renormalization group and a bosonization approach,
we study a spin-1/2 antiferromagnetic Heisenberg chain with near-neighbor
coupling and frustrating second-neighbor coupling , particularly in
the limit . This system exhibits both dimerization and
incommensurate spiral spin correlations. We argue that this system is closely
related to a doped, spin-gapped phase of the one-dimensional Kondo lattice.Comment: 18 pages, with 13 embedded encapsulated Postscript figures, uses
epsf.sty. Corrects a misstatement about the pitch angle, and contains
additional reference
Ebola viral load at diagnosis associates with patient outcome and outbreak evolution
BACKGROUND. Ebola virus (EBOV) causes periodic outbreaks of life-threatening EBOV disease in Africa. Historically, these outbreaks have been relatively small and geographically contained; however, the magnitude of the EBOV outbreak that began in 2014 in West Africa has been unprecedented. The aim of this study was to describe the viral kinetics of EBOV during this outbreak and identify factors that contribute to outbreak progression.
METHODS. From July to December 2014, one laboratory in Sierra Leone processed over 2,700 patient samples for EBOV detection by quantitative PCR (qPCR). Viremia was measured following patient admission. Age, sex, and approximate time of symptom onset were also recorded for each patient. The data was analyzed using various mathematical models to find trends of potential interest.
RESULTS. The analysis revealed a significant difference (P = 2.7 × 10–77) between the initial viremia of survivors (4.02 log10 genome equivalents [GEQ]/ml) and nonsurvivors (6.18 log10 GEQ/ml). At the population level, patient viral loads were higher on average in July than in November, even when accounting for outcome and time since onset of symptoms. This decrease in viral loads temporally correlated with an increase in circulating EBOV-specific IgG antibodies among individuals who were suspected of being infected but shown to be negative for the virus by PCR.
CONCLUSIONS. Our results indicate that initial viremia is associated with outcome of the individual and outbreak duration; therefore, care must be taken in planning clinical trials and interventions. Additional research in virus adaptation and the impacts of host factors on EBOV transmission and pathogenesis is needed
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