73 research outputs found
Phase field model of premelting of grain boundaries
We present a phase field model of solidification which includes the effects
of the crystalline orientation in the solid phase. This model describes grain
boundaries as well as solid-liquid boundaries within a unified framework. With
an appropriate choice of coupling of the phase field variable to the gradient
of the crystalline orientation variable in the free energy, we find that high
angle boundaries undergo a premelting transition. As the melting temperature is
approached from below, low angle grain boundaries remain narrow. The width of
the liquid layer at high angle grain boundaries diverges logarithmically. In
addition, for some choices of model coupling, there may be a discontinuous jump
in the width of the fluid layer as function of temperature.Comment: 6 pages, 9 figures, RevTeX
Dynamics of Shear-Transformation Zones in Amorphous Plasticity: Energetic Constraints in a Minimal Theory
We use energetic considerations to deduce the form of a previously uncertain
coupling term in the shear-transformation-zone (STZ) theory of plastic
deformation in amorphous solids. As in the earlier versions of the STZ theory,
the onset of steady deformation at a yield stress appears here as an exchange
of dynamic stability between jammed and plastically deforming states. We show
how an especially simple ``quasilinear'' version of this theory accounts
qualitatively for many features of plasticity such as yielding, strain
softening, and strain recovery. We also show that this minimal version of the
theory fails to describe certain other phenomena, and argue that these
limitations indicate needs for additional internal degrees of freedom beyond
those included here.Comment: 19 pages, 6 figure
Frequency-dependent selection in vaccine-associated pneumococcal population dynamics
Many bacterial species are composed of multiple lineages distinguished by extensive variation in gene content. These often cocirculate in the same habitat, but the evolutionary and ecological processes that shape these complex populations are poorly understood. Addressing these questions is particularly important for Streptococcus pneumoniae, a nasopharyngeal commensal and respiratory pathogen, because the changes in population structure associated with the recent introduction of partial-coverage vaccines have substantially reduced pneumococcal disease. Here we show that pneumococcal lineages from multiple populations each have a distinct combination of intermediate-frequency genes. Functional analysis suggested that these loci may be subject to negative frequency-dependent selection (NFDS) through interactions with other bacteria, hosts or mobile elements. Correspondingly, these genes had similar frequencies in four populations with dissimilar lineage compositions. These frequencies were maintained following substantial alterations in lineage prevalences once vaccination programmes began. Fitting a multilocus NFDS model of post-vaccine population dynamics to three genomic datasets using Approximate Bayesian Computation generated reproducible estimates of the influence of NFDS on pneumococcal evolution, the strength of which varied between loci. Simulations replicated the stable frequency of lineages unperturbed by vaccination, patterns of serotype switching and clonal replacement. This framework highlights how bacterial ecology affects the impact of clinical interventions.Accessory loci are shown to have similar frequencies in diverse Streptococcus pneumoniae populations, suggesting negative frequency-dependent selection drives post-vaccination population restructuring
Singular Shape of a Fluid Drop in an Electric or Magnetic Field
Beyond a threshold, electric or magnetic fields cause a dielectric or
ferromagnetic fluid drop respectively to develop conical tips. We analyze the
appearance of the conical tips and the associated shape transition of the drop
using a local force balance as well as a global energy argument. We find that a
conical interface is possible only when the dielectric constant (or
permeability) of the fluid exceeds a critical value . For a
fluid with , a conical interface is possible at two
angles, one stable and one unstable. We calculate the critical field required
to sustain a drop with stable conical tips. Such a drop is energetically
favored at sufficiently high field. Our results also apply to the formation of
conical dimples when a pool of fluid is placed in a normal field.Comment: 10 pages, Plain TeX, 3 postscript figures available on request via
emai
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