1,537 research outputs found
Duality in Shearing Rheology Near the Athermal Jamming Transition
We consider the rheology of soft-core frictionless disks in two dimensions in
the neighborhood of the athermal jamming transition. From numerical simulations
of bidisperse, overdamped, particles, we argue that the divergence of the
viscosity below jamming is characteristic of the hard-core limit, independent
of the particular soft-core interaction. We develop a mapping from soft-core to
hard-core particles that recovers all the critical behavior found in earlier
scaling analyses. Using this mapping we derive a duality relation that gives
the exponent of the non-linear Herschel-Bulkley rheology above jamming in terms
of the exponent of the diverging viscosity below jamming.Comment: 5 pages, 4 figures. Manuscript revisions: new title, additional text
concerning connections to experiment, revised Fig. 4, other minor changes and
clarifications in text. Conclusions remain essentially unchanged. Accepted
for publication in Phys. Rev. Let
Phase behaviour and particle-size cutoff effects in polydisperse fluids
We report a joint simulation and theoretical study of the liquid-vapor phase
behaviour of a fluid in which polydispersity in the particle size couples to
the strength of the interparticle interactions. Attention is focussed on the
case in which the particles diameters are distributed according to a fixed
Schulz form with degree of polydispersity . The coexistence
properties of this model are studied using grand canonical ensemble Monte Carlo
simulations and moment free energy calculations. We obtain the cloud and shadow
curves as well as the daughter phase density distributions and fractional
volumes along selected isothermal dilution lines. In contrast to the case of
size-{\em independent} interaction strengths (N.B. Wilding, M. Fasolo and P.
Sollich, J. Chem. Phys. {\bf 121}, 6887 (2004)), the cloud and shadow curves
are found to be well separated, with the critical point lying significantly
below the cloud curve maximum. For densities below the critical value, we
observe that the phase behaviour is highly sensitive to the choice of upper
cutoff on the particle size distribution. We elucidate the origins of this
effect in terms of extremely pronounced fractionation effects and discuss the
likely appearance of new phases in the limit of very large values of the
cutoff.Comment: 12 pages, 15 figure
EFFECTS OF HERBICIDES AND GRAZING ON FLORISTIC QUALITY OF NATIVE TALLGRASS PASTURES IN EASTERN SOUTH DAKOTA AND SOUTHWESTERN MINNESOTA
Historic herbicide use and grazing have influenced natural diversity and quality of native pasturelands in the Great Plains. Floristic quality assessments are useful to assist agencies in prioritizing conservation practices to enhance native grasslands. The objective of this study was to determine the effects of past land-use practices on the floristic quality of remnant native pastures in eastern South Dakota and southwestern Minnesota. Floristic quality assessments were conducted on 30 native pastures and categorized by past management practices (herbicide application and grazing intensity). Mean coefficient of conservatism (C) and floristic quality index (FQI) were calculated for each site~Results showed that increased herbicide use and grazing intensity resulted in a lower species richness, forb C ,and FQI. However, grass and grasslike plants were minimally affected. Pastures that were infrequently sprayed with herbicides and lightly grazed consistently had the highest species richness, C ,and FQI. Pastures with no grazing produced similar values to those with moderate grazing. Pastures managed as preserves or wildlife habitat areas had higher FQI than those managed for livestock grazing. The implications of this study should further help ecologists and managers understand the positive and negative effects of grazing practices and herbicide application on tallgrass prairie remnants
EFFECTS OF HERBICIDES AND GRAZING ON FLORISTIC QUALITY OF NATIVE TALLGRASS PASTURES IN EASTERN SOUTH DAKOTA AND SOUTHWESTERN MINNESOTA
Historic herbicide use and grazing have influenced natural diversity and quality of native pasturelands in the Great Plains. Floristic quality assessments are useful to assist agencies in prioritizing conservation practices to enhance native grasslands. The objective of this study was to determine the effects of past land-use practices on the floristic quality of remnant native pastures in eastern South Dakota and southwestern Minnesota. Floristic quality assessments were conducted on 30 native pastures and categorized by past management practices (herbicide application and grazing intensity). Mean coefficient of conservatism (C) and floristic quality index (FQI) were calculated for each site~Results showed that increased herbicide use and grazing intensity resulted in a lower species richness, forb C ,and FQI. However, grass and grasslike plants were minimally affected. Pastures that were infrequently sprayed with herbicides and lightly grazed consistently had the highest species richness, C ,and FQI. Pastures with no grazing produced similar values to those with moderate grazing. Pastures managed as preserves or wildlife habitat areas had higher FQI than those managed for livestock grazing. The implications of this study should further help ecologists and managers understand the positive and negative effects of grazing practices and herbicide application on tallgrass prairie remnants
Rheological State Diagrams for Rough Colloids in Shear Flow
To assess the role of particle roughness in the rheological phenomena of concentrated colloidal suspensions, we develop model colloids with varying surface roughness length scales up to 10% of the particle radius. Increasing surface roughness shifts the onset of both shear thickening and dilatancy towards lower volume fractions and critical stresses. Experimental data are supported by computer simulations of spherical colloids with adjustable friction coefficients, demonstrating that a reduction in the onset stress of thickening and a sign change in the first normal stresses occur when friction competes with lubrication. In the quasi-Newtonian flow regime, roughness increases the effective packing fraction of colloids. As the shear stress increases and suspensions of rough colloids approach jamming, the first normal stresses switch signs and the critical force required to generate contacts is drastically reduced. This is likely a signature of the lubrication films giving way to roughness-induced tangential interactions that bring about load-bearing contacts in the compression axis of flow.National Science Foundation (U.S.) (Award 1232937)National Science Foundation (U.S.) (Award 1602183)United States. Army Research Office (Grant W911NF10-1-0518
Formation of early-type galaxies from cosmological initial conditions
We describe high resolution Smoothed Particle Hydrodynamics (SPH) simulations
of three approximately field galaxies starting from \LCDM initial
conditions. The simulations are made intentionally simple, and include
photoionization, cooling of the intergalactic medium, and star formation but
not feedback from AGN or supernovae. All of the galaxies undergo an initial
burst of star formation at , accompanied by the formation of a
bubble of heated gas. Two out of three galaxies show early-type properties at
present whereas only one of them experienced a major merger. Heating from
shocks and -PdV work dominates over cooling so that for most of the gas the
temperature is an increasing function of time. By a significant
fraction of the final stellar mass is in place and the spectral energy
distribution resembles those of observed massive red galaxies. The galaxies
have grown from on average by 25% in mass and in size by gas poor
(dry) stellar mergers. By the present day, the simulated galaxies are old
(), kinematically hot stellar systems surrounded by hot
gaseous haloes. Stars dominate the mass of the galaxies up to
effective radii ( kpc). Kinematic and most photometric properties
are in good agreement with those of observed elliptical galaxies. The galaxy
with a major merger develops a counter-rotating core. Our simulations show that
realistic intermediate mass giant elliptical galaxies with plausible formation
histories can be formed from \LCDM initial conditions even without requiring
recent major mergers or feedback from supernovae or AGN.Comment: accepted for publication in Ap
The Evolution of Early-Type Galaxies in Distant Clusters
We present results from an optical-IR photometric study of early-type
galaxies in 19 galaxy clusters out to z=0.9. The galaxy sample is selected on
the basis of morphologies determined from HST WFPC2 images, and is
photometrically defined in the K-band to minimize redshift-dependent selection
biases. The optical-IR colors of the early-type cluster galaxies become bluer
with increasing redshift in a manner consistent with the passive evolution of
an old stellar population formed at an early cosmic epoch. The degree of color
evolution is similar for clusters at similar redshift, and does not depend
strongly on the optical richness or X-ray luminosity of the cluster, suggesting
that the history of early-type galaxies is relatively insensitive to
environment. The slope of the color-magnitude relationship shows no significant
change out to z=0.9, providing evidence that it arises from a correlation
between galaxy mass and metallicity, not age. Finally, the intrinsic scatter in
the optical-IR colors is small and nearly constant with redshift, indicating
that the majority of giant, early-type galaxies in clusters share a common star
formation history, with little perturbation due to uncorrelated episodes of
later star formation. Taken together, our results are consistent with models in
which most early-type galaxies in rich clusters are old, formed the majority of
their stars at high redshift in a well-synchronized fashion, and evolved
quiescently thereafter.Comment: 55 pages, 24 figures, uses AASTeX. Accepted for publication in The
Astrophysical Journa
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Inferred summer precipitation for southern Ontario back to AD 610, as reconstructed from ring widths of Thuja occidentalis
We present a network of seven ring-width chronologies of eastern white-cedar (Thuja occidentalis L.) from the Niagara Escarpment in southern Ontario, Canada. Using principal component regression, a 350-year June-July precipitation reconstruction (SOR) is developed for the region. Prior to the 20th century, the SOR series shows reasonable coherence, particularly at the decadal scale, with an independent tree-ring-based reconstruction of the Palmer drought severity index (PDSI) for roughly the same region. A weakening of the tree-growth – climate relationship in recent decades results in a regression model explaining 21% of the variance in the original climate series when the recent data are used for calibration. We therefore compromise with a model, calibrated for the period 1900–1960, which explains 33% of the variance. The model, while not terribly strong, does pass verification tests, indicating some degree of predictive skill. The longest chronology in our network, the 2787-year Flowerpot Island (FLOW) chronology, also exhibits common variability with the PDSI reconstruction, particularly on decadal and longer time scales and was used to infer hydroclimatic conditions back to AD 610. The combined information of the SOR, PDSI, and FLOW series suggests that dry conditions existed for the periods 1700–1725, 1750–1800, and 1840–1900, and wet conditions for the periods 1675–1700, 1730–1750, and 1810–1840. Over longer time scales, the FLOW chronology shows that summer precipitation was particularly variable during the 7th, 9th, 13th, and 16th centuries
Immunization: vital progress, unfinished agenda.
Vaccination against infectious diseases has changed the future of the human species, saving millions of lives every year, both children and adults, and providing major benefits to society as a whole. Here we show, however, that national and sub-national coverage of vaccination varies greatly and major unmet needs persist. Although scientific progress opens exciting perspectives in terms of new vaccines, the pathway from discovery to sustainable implementation can be long and difficult, from the financing, development and licensing to programme implementation and public acceptance. Immunization is one of the best investments in health and should remain a priority for research, industry, public health and society
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