1,199 research outputs found
The Conditions for Initiating "All-or-Nothing" Repolarization in Cardiac Muscle
Solutions have been computed for the point polarization of an infinite cable-like membrane obeying the equations used to reproduce the Purkinje fiber action potential (Noble, 1960, 1962a) in order to determine the conditions for initiating all-or-nothing repolarization during the action potential plateau. It was found that all-or-nothing repolarization would not be obtainable during the first half of the action potential in spite of the fact that the membrane current-voltage relations contain regions of negative conductance. At the point at which the all-or-nothing response is first obtained, the computed threshold is large and repolarization almost back to the resting potential would be required in order to initiate the response. The results are discussed in relation to the experimental evidence at present available on repolarization in heart muscle
Dissolution experiments in halite cores: comparisons in cavity shape and controls between brine and seawater experiments
There is an increasing need for underground storage of
natural gas (and potentially hydrogen) to meet the UK’s
energy demands and ensure its energy security. In addition,
the growth of renewable energy technologies, such as wind
power, will be facilitated by the development of grid-scale
energy storage facilities to balance grid demand. One solution
lies in creating large-scale compressed-air energy storage
(CAES) facilities underground. Whilst a number of
lithologies offer storage potential, only three operational
CAES facilities exist in the UK. They are constructed in
specifically designed solution-mined salt (halite) caverns,
similar to those currently used for natural gas storage. The
influences exerted on salt dissolution by petrology, structure
and fabric during cavern construction are not fully
understood, with some occurences of caverns with noncircular
cross-sections being less than optimum for gas
storage and especially CAES
Radiocarbon Date List X: Baffin Bay, Baffin Island, Iceland, Labrador Sea, and the Northern North Atlantic
Date List X contains an annotated listing of 213 radiocarbon dates determined on samples from marine and terrestrial environments. The marine samples were collected from the East Greenland, Iceland, Spitzbergen, and Norwegian margins, Baffin Bay, and Labrador Sea. The terrestrial samples were collected from Vestfirdir, Iceland and Baffin Island. The samples were submitted by INSTAAR and researchers affiliated with INSTAAR\u27s Micropaleontology Laboratory under the direction of Dr.’s John T. Andrews and Anne E. Jennings. All of the dates from marine sediment cores were determined from either shells or foraminifera (both benthic and planktic). All dates were obtained by the Accelerator Mass Spectrometry (AMS) method. Regions of concentrated marine research include: Baffin Bay, Baffin Island, Labrador Sea, East Greenland fjords, shelf and slope, Denmark Strait, the southwestern and northwestern Iceland shelves, and Vestfirdir, Iceland. The non-marine radiocarbon dates are from peat, wood, plant microfossils, and mollusc. The radiocarbon dates have been used to address a variety of research objectives such as: 1. determining the timing of northern hemisphere high latitude environmental changes including glacier advance and retreat, and 2. assessing the accuracy of a fluctuating reservoir correction. Thus, most of the dates constrain the timing, rate, and interaction of late Quaternary paleoenvironmental fluctuations in sea level, glacier extent, sediment input, and changes in ocean circulation patterns. Where significant, stratigraphic and sample contexts are presented for each core to document the basis for interpretations
Unusual morphologies and the occurrence of pseudomorphs after ikaite (CaCO3•6H2O) in fast growing, hyperalkaline speleothem
Unusual speleothem, associated with hyperalkaline (pH>12) groundwaters have formed within a shallow, abandoned railway tunnel at Peak Dale, Derbyshire, UK. The hyperalkaline groundwaters are produced by the leaching of a thin layer (<2 m) of old lime kiln waste above the soil-bedrock surface above the tunnel by rainwater. This results in a different reaction and chemical process to that more commonly associated with the formation of calcium carbonate speleothems from Ca-HCO3-type groundwaters and degassing of CO2. Stalagmites within the Peak Dale tunnel have grown rapidly (averaging 33 mm y-1), following the closure of the tunnel 70 years ago. They have an unusual morphology comprising a central sub-horizontally-laminated column of micro- to nano-crystalline calcium carbonate encompassed by an outer sub-vertical assymetric ripple laminated layer. The stalagmites are largely composed of secondary calcite forming pseudomorphs (<1 mm) which we believe to be predominantly after the ‘cold climate’ calcium carbonate polymorph, ikaite (calcium carbonate hexahydrate: CaCO3•6H2O), with minor volumes of small (<5 μm) pseudomorphs after vaterite. The tunnel has a near constant temperature of 8-9°C which is slightly above the previously published crystallisation temperatures for ikaite (<6°C). Analysis of a stalagmite actively growing at the time of sampling, and preserved immediately within a dry nitrogen cryogenic vessel, indicates that following crystallisation of ikaite, decomposition to calcite occurs rapidly, if not instantaneously. We believe this is the first occurrence of this calcium carbonate polymorph observed within speleothem
Vale of York 3-D borehole interpretation and cross-sections study
The Vale of York between Doncaster and Scunthorpe in the south and York and Bugthorpe in
the north is largely underlain by bedrock of the Sherwood Sandstone Group – one of the regions
principal aquifers. Significant superficial deposits of Quaternary age overlie the Sherwood
Sandstone. This study aims to investigate the nature of these superficial deposits with respect to
their relationship with the underlying aquifer.
The Vale of York project area represents a varied glaciated terrain, consisting of pro-glacial finegrained
sediments, coarser glaciofluvilal sediments and extensive glacial tills. These diverse
superficial units vary in thickness throughout the project area. The hydrogeological nature of the
natural superficial sequence is consequently highly variable. Units may be considered as
aquitards, while others may act as aquifers, providing a potential pathway to the underlying
sandstone. The classification of lithologies as aquifer or aquitard is described in detail in this
report.
To investigate the hydrogeological nature of the superficial sequence, six east-west and three
north-south lithostratigraphical cross-sections were constructed. A range of geoscientific
information was considered, including existing geological mapping and over 3000 fully
attributed and coded boreholes. The cross-sections show a subdivision of the superficial
sequence into lithostratigraphical units. Each unit is described in detail in this report.
In addition, a series of thematic maps were generated from the lithological component of the
digital borehole data. Total superficial aquifer and superficial aquitard maps show how the
lithological nature of the superficial sequence varies across the area. Rockhead elevation and
superficial thickness maps indicate where the sandstone aquifer outcrops at the ground surface.
In summary, four main lithostratigraphical units overlie the Sherwood Sandstone Group aquifer
in the project area: a basal sequence of glaciofluvial sand and gravel (interpreted as a superficial
aquifer), glaciolacustrine laminated silt & clay (aquitard), glacial till comprising sandy gravelly
clay (aquitard), and a cover sequence of fluvial and aeolian sand, clay and peat (aquifer /
aquitard). The correlations illustrate that in certain areas, superficial deposits are thin or absent
and that in these areas the Sherwood Sandstone aquifer comes directly to ground surface
Biodeterioration and bioprotection of concrete assets in the coastal environment
The deleterious effects (biodeterioration) and the protective benefits (bioprotection) of biological colonisation on
manmade structures have long been debated. Lichens, biofilms, algae, bivalves and gastropods contribute both
directly and indirectly to damaging substrata in the coastal zone which can enhance abiotic erosive forces that
exploit biologically induced superficial damage. There is mounting evidence that these same species may also
provide protective benefits. This debate often impacts approaches to managing fouling on concrete assets in the
coastal environment. The net benefit or detriment a species or assemblage has on a structure is spatially and
temporally dynamic and subject to the influence of various abiotic and biotic factors at different scales. However,
the net outcome may be more pronounced under different contexts, particularly under warming and ocean
acidifying climate change scenarios which is where further research should focus. Additionally, as bioprotection
represents a potentially valuable ecosystem service, it supports the argument for increasing and improving
habitat availability and biodiversity on artificial coastal structures via ecological enhancement. Quantifying
bioprotection in useful metrics, such as monetary value or time added to serviceable life, would help demonstrate
the benefits of bioprotective species in a meaningful wa
Positive P simulations of spin squeezing in a two-component Bose condensate
The collisional interaction in a Bose condensate represents a non-linearity
which in analogy with non-linear optics gives rise to unique quantum features.
In this paper we apply a Monte Carlo method based on the positive P
pseudo-probability distribution from quantum optics to analyze the efficiency
of spin squeezing by collisions in a two-component condensate. The squeezing
can be controlled by choosing appropiate collision parameters or by
manipulating the motional states of the two components.Comment: 5 pages, 2 figures. Submitted to Phys. Rev.
Enhancing Osteogenic Differentiation of Mouse Embryonic Stem Cells by Nanofibers
Controlled differentiation of embryonic stem cells (ESC) is necessary to their use as a cell source for tissue engineering or regeneration. To date, most studies have concentrated on chemical cues to direct ESC differentiation. However, during normal embryonic development, multiple factors beyond chemical cues play a role, including the extracellular matrix (ECM) in bone development. In this study, we use nanofibrous (NF) matrices to mimic the morphology of the ECM to examine the contribution of the ECM morphology to the differentiation of mouse ESC. After 12h of differentiation culture, mouse ESC form protrusions interacting with NF matrices, while they appear not to interact with flat films. Immunofluorescence staining after 26 days of differentiation culture indicates a greater degree of differentiation for mouse ESC on NF matrices compared to flat films. Polymerase chain reaction results, also, show greater degree of osteogenic differentiation on NF matrices compared to flat films when osteogenic supplements are added to the culture. Overall, these results demonstrate that NF morphology contributes to the controlled differentiation of mouse ESC.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78135/1/ten.tea.2008.0227.pd
Castaing Instability and Precessing Domains in Confined Alkali Gases
We explore analogy between two-component quantum alkali gases and
spin-polarized helium systems. Recent experiments in trapped gases are put into
the frame of the existing theory for Castaing instability in transverse channel
and formation of homogeneous precessing domains in spin-polarized systems.
Analogous effects have already been observed in spin-polarized and
mixtures systems. The threshold effect of the confining
potential on the instability is analyzed. New experimental possibilities for
observation of transverse instability in a trap are discussed.Comment: 6 RevTex pages, no figure
Barriers and facilitators to achieving food security during the COVID-19 pandemic
The COVID-19 pandemic has considerably increased food insecurity. To identify where intervention and policy solutions are most needed, we explored barriers to obtaining food and predictors of experiencing food insecurity due to the COVID-19 pandemic. Between May and July 2020, we conducted cross-sectional online surveys with two convenience samples of U.S. adults (Study 1: n = 2,219, Study 2: n = 810). Roughly one-third of participants reported experiencing food insecurity due to the COVID-19 pandemic (Study 1: 32%, Study 2: 35%). Between one-third and half reported using the charitable food system (Study 1: 36%, Study 2: 46%). The majority of participants experienced barriers to getting food (Study 1: 84%, Study 2: 88%), of which the most commonly reported were not having enough money to buy food (Study 1: 48%; Study 2: 53%) and worrying about getting COVID-19 at the store (Study 1: 50%; Study 2: 43%). Higher education was associated with greater risk of food insecurity in both studies (all p < 0.05). Receipt of aid from SNAP buffered against the association between financial struggles and food insecurity in Study 1 (p = 0.03); there was also some evidence of this effect in Study 2 (p = 0.05). Our findings suggest that food insecurity might be reduced by mitigating financial struggles (e.g., by increasing access to SNAP) and by addressing barriers to obtaining food (e.g., by expanding accessibility of food delivery programs)
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