6,301 research outputs found
The mechanical response of semiflexible networks to localized perturbations
Previous research on semiflexible polymers including cytoskeletal networks in
cells has suggested the existence of distinct regimes of elastic response, in
which the strain field is either uniform (affine) or non-uniform (non-affine)
under external stress. Associated with these regimes, it has been further
suggested that a new fundamental length scale emerges, which characterizes the
scale for the crossover from non-affine to affine deformations. Here, we extend
these studies by probing the response to localized forces and force dipoles. We
show that the previously identified nonaffinity length [D.A. Head et al. PRE
68, 061907 (2003).] controls the mesoscopic response to point forces and the
crossover to continuum elastic behavior at large distances.Comment: 16 pages, 18 figures; substantial changes to text and figures to
clarify the crossover to continuum elasticity and the role of finite-size
effect
Transcriptomes of parents identify parenting strategies and sexual conflict in a subsocial beetle
This work was funded by UK NERC grants to M.G.R. and A.J.M. an NERC studentship to D.J.P. the University of Georgia and a US NSF grant to A.J.M. and M.G.R.Parenting in the burying beetle Nicrophorus vespilloides is complex and, unusually, the sex and number of parents that can be present is flexible. Such flexibility is expected to involve specialized behaviour by the two sexes under biparental conditions. Here, we show that offspring fare equally well regardless of the sex or number of parents present. Comparing transcriptomes, we find a largely overlapping set of differentially expressed genes in both uniparental and biparental females and in uniparental males including vitellogenin, associated with reproduction, and takeout, influencing sex-specific mating and feeding behaviour. Gene expression in biparental males is similar to that in non-caring states. Thus, being ‘biparental’ in N. vespilloides describes the family social organization rather than the number of directly parenting individuals. There was no specialization; instead, in biparental families, direct male parental care appears to be limited with female behaviour unchanged. This should lead to strong sexual conflict.Publisher PDFPeer reviewe
Glassy dynamics in granular compaction
Two models are presented to study the influence of slow dynamics on granular
compaction. It is found in both cases that high values of packing fraction are
achieved only by the slow relaxation of cooperative structures. Ongoing work to
study the full implications of these results is discussed.Comment: 12 pages, 9 figures; accepted in J. Phys: Condensed Matter,
proceedings of the Trieste workshop on 'Unifying concepts in glass physics
Lunar International Science Coordination/Calibration Targets
A new era of international lunar exploration has begun and will expand over the next four years with data acquired from at least four sophisticated remote sensing missions: KAGUYA (SELENE) [Japan], Chang'E [China], Chandrayaan-l [India], and LRO [United States]. It is recognized that this combined activity at the Moon with modern sophisticated sensors wi II provide unprecedented new information about the Moon and will dramatically improve our understanding of Earth's nearest neighbor. It is anticipated that the blooming of scientific exploration of the Moon by nations involved in space activities will seed and foster peaceful international coordination and cooperation that will benefit all. Summarized here are eight Lunar International Science Coordination/Calibration Targets (L-ISCT) that are intended to a) allow cross-calibration of diverse multi-national instruments and b) provide a focus for training young scientists about a range of lunar science issues. The targets, discussed at several scientific forums, were selected for coordinated science and instrument calibration of orbital data. All instrument teams are encouraged to participate in a coordinated activity of early-release data that will improve calibration and validation of data across independent and diverse instruments
Ab Initio Study of Screw Dislocations in Mo and Ta: A new picture of plasticity in bcc transition metals
We report the first ab initio density-functional study of screw
dislocations cores in the bcc transition metals Mo and Ta. Our results suggest
a new picture of bcc plasticity with symmetric and compact dislocation cores,
contrary to the presently accepted picture based on continuum and interatomic
potentials. Core energy scales in this new picture are in much better agreement
with the Peierls energy barriers to dislocation motion suggested by
experiments.Comment: 3 figures, 3 table
Force-extension relation of cross-linked anisotropic polymer networks
Cross-linked polymer networks with orientational order constitute a wide
class of soft materials and are relevant to biological systems (e.g., F-actin
bundles). We analytically study the nonlinear force-extension relation of an
array of parallel-aligned, strongly stretched semiflexible polymers with random
cross-links. In the strong stretching limit, the effect of the cross-links is
purely entropic, independent of the bending rigidity of the chains. Cross-links
enhance the differential stretching stiffness of the bundle. For hard
cross-links, the cross-link contribution to the force-extension relation scales
inversely proportional to the force. Its dependence on the cross-link density,
close to the gelation transition, is the same as that of the shear modulus. The
qualitative behavior is captured by a toy model of two chains with a single
cross-link in the middle.Comment: 7 pages, 4 figure
Fluctuation-dissipation relations in trap models
Trap models are intuitively appealing and often solvable models of glassy
dynamics. In particular, they have been used to study aging and the resulting
out-of-equilibrium fluctuation-dissipation relations between correlations and
response functions. In this note I show briefly that one such relation, first
given by Bouchaud and Dean, is valid for a general class of mean-field trap
models: it relies only on the way a perturbation affects the transition rates,
but is independent of the distribution of trap depths and the form of the
unperturbed transition rates, and holds for all observables that are
uncorrelated with the energy. The model with Glauber dynamics and an
exponential distribution of trap depths, as considered by Barrat and Mezard,
does not fall into this class if the perturbation is introduced in the standard
way by shifting all trap energies. I show that a similar relation between
response and correlation nevertheless holds for the out-of-equilibrium dynamics
at low temperatures. The results point to intriguing parallels between trap
models with energetic and entropic barriers.Comment: Extended introduction and discussion of relation to results of
cond-mat/0303445. 13 pages, 2 figures, IOP styl
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Investments in respiratory infectious disease research 1997–2010: a systematic analysis of UK funding
Objectives: Respiratory infections are responsible for a large global burden of disease. We assessed the public and philanthropic investments awarded to UK institutions for respiratory infectious disease research to identify areas of underinvestment. We aimed to identify projects and categorise them by pathogen, disease and position along the research and development value chain. Setting: The UK. Participants: Institutions that host and carry out infectious disease research. Primary and secondary outcome measures The total amount spent and number of studies with a focus on several different respiratory pathogens or diseases, and to correlate these against the global burden of disease; also the total amount spent and number of studies relating to the type of science, the predominant funder in each category and the mean and median award size. Results: We identified 6165 infectious disease studies with a total investment of £2·6 billion. Respiratory research received £419 million (16.1%) across 1192 (19.3%) studies. The Wellcome Trust provided greatest investment (£135.2 million; 32.3%). Tuberculosis received £155 million (37.1%), influenza £80 million (19.1%) and pneumonia £27.8 million (6.6%). Despite high burden, there was relatively little investment in vaccine-preventable diseases including diphtheria (£0.1 million, 0.03%), measles (£5.0 million, 1.2%) and drug-resistant tuberculosis. There were 802 preclinical studies (67.3%) receiving £273 million (65.2%), while implementation research received £81 million (19.3%) across 274 studies (23%). There were comparatively few phase I–IV trials or product development studies. Global health research received £68.3 million (16.3%). Relative investment was strongly correlated with 2010 disease burden. Conclusions: The UK predominantly funds preclinical science. Tuberculosis is the most studied respiratory disease. The high global burden of pneumonia-related disease warrants greater investment than it has historically received. Other priority areas include antimicrobial resistance (particularly within tuberculosis), economics and proactive investments for emerging infectious threats
Introduction to protein folding for physicists
The prediction of the three-dimensional native structure of proteins from the
knowledge of their amino acid sequence, known as the protein folding problem,
is one of the most important yet unsolved issues of modern science. Since the
conformational behaviour of flexible molecules is nothing more than a complex
physical problem, increasingly more physicists are moving into the study of
protein systems, bringing with them powerful mathematical and computational
tools, as well as the sharp intuition and deep images inherent to the physics
discipline. This work attempts to facilitate the first steps of such a
transition. In order to achieve this goal, we provide an exhaustive account of
the reasons underlying the protein folding problem enormous relevance and
summarize the present-day status of the methods aimed to solving it. We also
provide an introduction to the particular structure of these biological
heteropolymers, and we physically define the problem stating the assumptions
behind this (commonly implicit) definition. Finally, we review the 'special
flavor' of statistical mechanics that is typically used to study the
astronomically large phase spaces of macromolecules. Throughout the whole work,
much material that is found scattered in the literature has been put together
here to improve comprehension and to serve as a handy reference.Comment: 53 pages, 18 figures, the figures are at a low resolution due to
arXiv restrictions, for high-res figures, go to http://www.pabloechenique.co
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