792 research outputs found
Shape and symmetry determine two-dimensional melting transitions of hard regular polygons
The melting transition of two-dimensional (2D) systems is a fundamental
problem in condensed matter and statistical physics that has advanced
significantly through the application of computational resources and
algorithms. 2D systems present the opportunity for novel phases and phase
transition scenarios not observed in 3D systems, but these phases depend
sensitively on the system and thus predicting how any given 2D system will
behave remains a challenge. Here we report a comprehensive simulation study of
the phase behavior near the melting transition of all hard regular polygons
with vertices using massively parallel Monte Carlo simulations
of up to one million particles. By investigating this family of shapes, we show
that the melting transition depends upon both particle shape and symmetry
considerations, which together can predict which of three different melting
scenarios will occur for a given . We show that systems of polygons with as
few as seven edges behave like hard disks; they melt continuously from a solid
to a hexatic fluid and then undergo a first-order transition from the hexatic
phase to the fluid phase. We show that this behavior, which holds for all
, arises from weak entropic forces among the particles. Strong
directional entropic forces align polygons with fewer than seven edges and
impose local order in the fluid. These forces can enhance or suppress the
discontinuous character of the transition depending on whether the local order
in the fluid is compatible with the local order in the solid. As a result,
systems of triangles, squares, and hexagons exhibit a KTHNY-type continuous
transition between fluid and hexatic, tetratic, and hexatic phases,
respectively, and a continuous transition from the appropriate "x"-atic to the
solid. [abstract truncated due to arxiv length limitations]
Ecosystem-based biomimicry as a potential informer of regenerative architecture: a proposed regenerative research centre on the docks of the Durban Bay.
Masters Degree. University of KwaZulu-Natal, Durban.The context of the 21st century is the epoch of the ‘Anthropocene’ which characterizes the
contemporary geological age we have entered – one defined by causative human activity and its
decisive influence on planetary systems. This stems from the conceptualisation of human systems to
be separate from natural systems leading to a problematic relationship between man and nature. This
is manifested through our built environments where processes of urbanization and industrialization
continue to pollute and degrade ecosystems both in & beyond their hinterlands.
The built environment has been identified as a potential medium for mitigating the causes for
ecosystem degradation and the loss of biodiversity – because of its role as a driver of ecosystem
degradation, because it is the primary habitat for humans, and because the built environment
presents potential opportunities for change.
The purpose of this research was to explore regenerative design as a means for the built environment
to restore the capacity of ecosystems. The research focussed on ecosystem-based biomimicry as a
potential informer for regenerative architecture, so that it may motivate architecture as a vital
component in the regeneration of local biodiversity and the wider ecological habitat. The research
was primarily focused on the city of Durban and in particular the Durban Bay.
The study is of an interdisciplinary nature and gathers insight from experts in their respective fields.
The disciplines explored were Architecture, Urban Design and Town planning, Ecology, and
Biomimicry.
The research findings indicate that there is a need for socio-ecological change in the way we conceive
of our built environments, to re-think the problematic relationship between man, the built
environment, and the ecologies that they are an intrinsic part of. Regenerative Design begins to alter
this relationship, and promotes the positive integration with ecological systems, regenerating
ecosystems particularly in human dominated urban contexts. Mimicking ecosystems goes beyond a
basic understanding of local ecological systems, but rather critically changes how buildings should
function in relation to nature, shifting from a consumer of ecosystems towards a producer of natural
resources
Deficiency of the bone mineralization inhibitor NPP1 protects against obesity and diabetes
The emergence of bone as an endocrine regulator has prompted a re-evaluation of the role of bone mineralization factors in the development of metabolic disease. Ectonucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) controls bone mineralization through the generation of pyrophosphate, and levels of NPP1 are elevated both in dermal fibroblast cultures and muscle of individuals with insulin resistance. We investigated the metabolic phenotype associated with impaired bone metabolism in mice lacking the gene that encodes NPP1 (Enpp1−/− mice). Enpp1−/− mice exhibited mildly improved glucose homeostasis on a normal diet but showed a pronounced resistance to obesity and insulin resistance in response to chronic high-fat feeding. Enpp1−/− mice had increased levels of the insulin-sensitizing bone-derived hormone osteocalcin but unchanged insulin signalling within osteoblasts. A fuller understanding of the pathways of NPP1 could inform the development of novel therapeutic strategies for treating insulin resistance
Lifelong CRF overproduction is associated with altered gene expression and sensitivity of discrete GABAA and mGlu receptor subtypes
RATIONALE: Repeated activation of corticotropin-releasing factor (CRF) receptors is associated with increased anxiety and enhanced stress responsivity, which may be mediated via limbic GABAergic and glutamatergic transmission. OBJECTIVE: The present study investigated molecular and functional alterations in GABA(A) receptor (GABA(A)R) and metabotropic glutamate receptor (mGluR) responsivity in transgenic mice that chronically overexpress CRF. METHODS: CRF(1) receptor, GABA(A)R, and mGluR sensitivity were determined in CRF-overexpressing mice using the stress-induced hyperthermia (SIH) test. In addition, we measured mRNA expression levels of GABA(A)R α subunits and mGluRs in the amygdala and hypothalamus. RESULTS: CRF-overexpressing mice were less sensitive to the anxiolytic effects of the CRF(1) receptor antagonists CP154,526 and DMP695, the GABA(A)R α(3)-selective agonist TP003 (0–3 mg/kg) and the mGluR(2/3) agonist LY379268 (0–10 mg/kg) in the SIH test. The hypothermic effect of the non-selective GABA(A)R agonist diazepam (0–4 mg/kg) and the α(1)-subunit-selective GABA(A)R agonist zolpidem (0–10 mg/kg) was reduced in CRF-overexpressing mice. No genotype differences were found using the GABA(A)R α(5)-subunit preferential compound SH-053-2′F-R-CH(3) and mGluR(5) antagonists MPEP and MTEP. CRF-overexpressing mice showed decreased expression levels of GABA(A)R α(2) subunit and mGluR(3) mRNA levels in the amygdala, whereas these expression levels were increased in the hypothalamus. CRF-overexpressing mice also showed increased hypothalamic mRNA levels of α(1) and α(5) GABA(A)R subunits. CONCLUSIONS: We found that lifelong CRF overproduction is associated with altered gene expression and reduced functional sensitivity of discrete GABA(A) and mGluR receptor subtypes. These findings suggest that sustained over-activation of cerebral CRF receptors may contribute to the development of altered stress-related behavior via modulation of GABAergic and glutamatergic transmission
Investigating energetic electron precipitation through combining ground-based and balloon observations
A detailed comparison is undertaken of the energetic electron spectra and fluxes of two precipitation events that were observed in 18/19 January 2013. A novel but powerful technique of combining simultaneous ground-based subionospheric radio wave data and riometer absorption measurements with X-ray fluxes from a Balloon Array for Relativistic Radiation-belt Electron Losses (BARREL) balloon is used for the first time as an example of the analysis procedure. The two precipitation events are observed by all three instruments, and the relative timing is used to provide information/insight into the spatial extent and evolution of the precipitation regions. The two regions were found to be moving westward with drift periods of 5–11 h and with longitudinal dimensions of ~20° and ~70° (1.5–3.5 h of magnetic local time). The electron precipitation spectra during the events can be best represented by a peaked energy spectrum, with the peak in flux occurring at ~1–1.2 MeV. This suggests that the radiation belt loss mechanism occurring is an energy-selective process, rather than one that precipitates the ambient trapped population. The motion, size, and energy spectra of the patches are consistent with electromagnetic ion cyclotron-induced electron precipitation driven by injected 10–100 keV protons. Radio wave modeling calculations applying the balloon-based fluxes were used for the first time and successfully reproduced the ground-based subionospheric radio wave and riometer observations, thus finding strong agreement between the observations and the BARREL measurements
A Roadmap For Scientific Ballooning 2020-2030
From 2018 to 2020, the Scientific Balloon Roadmap Program Analysis Group
(Balloon Roadmap PAG) served as an community-based, interdisciplinary forum for
soliciting and coordinating community analysis and input in support of the NASA
Scientific Balloon Program. The Balloon Roadmap PAG was tasked with
articulating and prioritizing the key science drivers and needed capabilities
of the Balloon Program for the next decade. Additionally, the Balloon Roadmap
PAG was asked to evaluate the potential for achieving science goals and
maturing technologies of the Science Mission Directorate, evaluate the Balloon
Program goals towards community outreach, and asses commercial balloon launch
opportunities. The culmination of this work has been a written report submitted
to the NASA Astrophysics Division Director.Comment: 95 pages, 69 figures, prepared by the NASA Balloon Program Analysis
Group for the NASA Astrophysics Division Director and the 2020 Astrophysics
Decadal Surve
Progress on agriculture in the UN climate talks: How COP21 can ensure a food-secure future
Agriculture, and consequently food security and livelihoods, is already being affected by climate change, according to latest science from the IPCC (Porter et al. 2014). The IPCC agrees that the world needs to produce at least 50% more food than we do today in order to meet the goal of feeding a projected 9 billion people by 2050. This must be achieved in the face of climatic variability and change, growing constraints on water and land for crops and livestock, and declining wild capture fishery stocks.
Although the protection of food security lies within the core objective of the United Nations Framework Convention on Climate Change (UNFCCC) (Article 2), formal arrangements for addressing agriculture within COP21 are unlikely. CGIAR would welcome the strengthening of aspirations for food security through action on mitigation and adaptation within a new agreement. We recognise that the new climate agreement is unlikely to be prescriptive about how adaptation in agriculture is supported and how agriculture might contribute to emission cuts. These issues are addressed within countries’ INDCs and determined at national level
Astrometry with the Keck-Interferometer: the ASTRA project and its science
The sensitivity and astrometry upgrade ASTRA of the Keck Interferometer is
introduced. After a brief overview of the underlying interferometric
principles, the technology and concepts of the upgrade are presented. The
interferometric dual-field technology of ASTRA will provide the KI with the
means to observe two objects simultaneously, and measure the distance between
them with a precision eventually better than 100 uas. This astrometric
functionality of ASTRA will add a unique observing tool to fields of
astrophysical research as diverse as exo-planetary kinematics, binary
astrometry, and the investigation of stars accelerated by the massive black
hole in the center of the Milky Way as discussed in this contribution.Comment: 22 pages, 10 figures (low resolution), contribution to the
summerschool "Astrometry and Imaging with the Very Large Telescope
Interferometer", 2 - 13 June, 2008, Keszthely, Hungary, corrected authorlis
Ecological and Evolutionary Benefits of Temperate Phage: What Does or Doesn't Kill You Makes You Stronger
Infection by a temperate phage can lead to death of the bacterial cell, but sometimes these phages integrate into the bacterial chromosome, offering the potential for a more long-lasting relationship to be established. Here we define three major ecological and evolutionary benefits of temperate phage for bacteria: as agents of horizontal gene transfer (HGT), as sources of genetic variation for evolutionary innovation, and as weapons of bacterial competition. We suggest that a coevolutionary perspective is required to understand the roles of temperate phages in bacterial populations
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