822 research outputs found
Modeling and Testing of the Viscoelastic Properties of a Graphite Nanoplatelet/Epoxy Composite
In order to facilitate the interpretation of experimental data, a micromechanical modeling procedure is developed to predict the viscoelastic properties of a graphite nanoplatelet/epoxy composite as a function of volume fraction and nanoplatelet diameter. The predicted storage and loss moduli for the composite are compared to measured values from the same material using three test methods; Dynamical Mechanical Analysis, nanoindentation, and quasi-static tensile tests. In most cases, the model and experiments indicate that for increasing volume fractions of nanoplatelets, both the storage and loss moduli increase. Also, the results indicate that for nanoplatelet sizes above 15 microns, nanoindentation is capable of measuring properties of individual constituents of a composite system. Comparison of the predicted values to the measured data helps illustrate the relative similarities and differences between the bulk and local measurement techniques
Comparison of Some Exact and Perturbative Results for a Supersymmetric SU() Gauge Theory
We consider vectorial, asymptotically free supersymmetric
SU() gauge theories with copies of massless chiral super fields in
various representations and study how perturbative predictions for the lower
boundary of the infrared conformal phase, as a function of , compare with
exact results. We make use of two-loop and three-loop calculations of the beta
function and anomalous dimension of the quadratic chiral super field operator
product for this purpose. The specific chiral superfield contents that we
consider are copies of (i) , (ii) , (iii) ,
and (iv) , where , , , and denote,
respectively, the fundamental, adjoint, and symmetric and antisymmetric rank-2
tensor representations. We find that perturbative results slightly overestimate
the value of relative to the respective exact results for these
representations, i.e., slightly underestimate the interval in for which
the theory has infrared conformal behavior. Our results provide a measure of
how closely perturbative calculations reproduce exact results for these
theories.Comment: 16 pages, 3 figure
On the Inconsistency of Fayet-Iliopoulos Terms in Supergravity Theories
Motivated by recent discussions, we revisit the issue of whether globally
supersymmetric theories with non-zero Fayet-Iliopoulos terms may be
consistently coupled to supergravity. In particular, we examine claims that a
fundamental inconsistency arises due to the conflicting requirements which are
imposed on the -symmetry properties of the theory by the supergravity
framework. We also prove that certain kinds of Fayet-Iliopoulos contributions
to the supercurrent supermultiplets of theories with non-zero Fayet-Iliopoulos
terms fail to exist. A key feature of our discussion is an explicit comparison
between results from the chiral (or ``old minimal'') and linear (or ``new
minimal'') formulations of supergravity, and the effects within each of these
formalisms that are induced by the presence of non-zero Fayet-Iliopoulos terms.Comment: Comments: 69 pages, LaTeX, 2 figures, 7 tables. Significant new
material on conformal-compensator formalisms added, previous results
clarified and extended, references adde
Effects of Hikers and Boats on Tule Elk Behavior in a National Park Wilderness Area
Human disturbance of wildlife may cause disruption of normal feeding, resting, reproduction, or care for juveniles. Such disturbance may be particularly undesirable in federally managed wilderness areas designed to minimize human influences on natural resources. We recorded tule elk (Cervus elephus nannodes) responses (standing, walking away, running) to off-trail hikers, off-shore boats, and other natural and anthropogenic factors in Point Reyes National Seashore in northern California during 2002 to 2008. Most disturbance behaviors were related to other elk exhibiting rutting behaviors, but off-trail hikers still explained a 100% increase and off-shore boats a 15% increase in baseline disturbance behaviors by elk. However, off-trail hikers and boats did not cause elk to enter or leave the study area during the sample periods. Elk were more prone to human disturbance when herd sizes wereand, to a lesser extent, offshore boats appear to disturb natural tule elk behavior, but the physiological or population-level effects of this disturbance are unknown. Our quantitative results may help park managers minimize or mitigate human–elk interactions in wilderness areas
Short-Term Thermal Acclimation Modifies the Metabolic Condition of the Coral Holobiont
The nutritional symbiosis between coral hosts and photosynthetic dinoflagellates is fundamental to the functioning of coral reefs. Rising seawater temperatures destabilize this relationship, resulting in drastic declines in world-wide coral cover. Thermal history is thought to play an important role in shaping a coral\u27s response to subsequent thermal stress. Here, we exposed Pocillopora damicornis to two thermal acclimation regimes (ambient vs. warm) and compared the effect that acclimation had on the coral holobiont\u27s response to a subsequent seven day heat stress event. We conducted daily physiological measurements at the holobiont level (gross photosynthesis, respiration, host protein content, symbiont density and chlorophyll content) throughout the heating event, as well as cellular-level imaging of 13C-bicarbonate and 15N-nitrate assimilation (using NanoSIMS) at the end of the heat stress event. Thermal acclimation history had a negligible effect on the measurements conducted at the holobiont level during the heat stress event. No differences were observed in the O2-budget between ambient and warm-acclimated corals and only small fluctuations in host protein, symbiont density and chlorophyll content were detected. In contrast, this lack of differential response, was not mirrored at the cellular level. Warm-acclimated corals had substantially higher 13C enrichment in the host gastrodermis and lipid bodies, but significantly lower 15N-nitrate assimilation in the symbionts and the host tissue layers, relative to the ambient-acclimated corals. We discuss potential reasons for the disconnect that occurred between symbiont bicarbonate and nitrate assimilation (in the absence of photosynthetic breakdown) in the warm-acclimated corals. We suggest this represents either a shift in nitrogen utilization, or supply limitation by the host. Our findings raise several interesting hypotheses regarding the role that nitrogen metabolism plays in thermal stress, which will warrant further investigation if we are to understand the acclimatization capacity of the coral holobiont
Organization theory and military metaphor: time for a reappraisal?
A ‘conventional’ use of military metaphor would use it to convey attributes such as hierarchical organization, vertical communication and limited autonomy. This is often used in contrast to a looser form of organization based on the metaphor of the network. However, this article argues that military practice is more complex, with examples of considerable autonomy within the constraints of central direction. It is suggested that not only might this be a more useful metaphor for many contemporary organizations, but also that simplistic uses of military metaphor divert our attention away from the functions that management hierarchies play. The discussion is embedded within a critical realist account of metaphor, arguing for both its value and the need for its further development
Stable bundles on hypercomplex surfaces
A hypercomplex manifold is a manifold equipped with three complex structures
I, J, K satisfying the quaternionic relations. Let M be a 4-dimensional compact
smooth manifold equipped with a hypercomplex structure, and E be a vector
bundle on M. We show that the moduli space of anti-self-dual connections on E
is also hypercomplex, and admits a strong HKT metric. We also study manifolds
with (4,4)-supersymmetry, that is, Riemannian manifolds equipped with a pair of
strong HKT-structures that have opposite torsion. In the language of Hitchin's
and Gualtieri's generalized complex geometry, (4,4)-manifolds are called
``generalized hyperkaehler manifolds''. We show that the moduli space of
anti-self-dual connections on M is a (4,4)-manifold if M is equipped with a
(4,4)-structure.Comment: 17 pages. Version 3.0: reference adde
Characterization of an electron conduit between bacteria and the extracellular environment
A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning ß-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment
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