985 research outputs found
Physical properties and micromorphology of till deposits from Talla Earth Observatory, Southern Uplands, Scotland
This factual report describes the 2007 field program at BGS’ Talla Earth Observatory, in the
Scottish Southern Uplands, UK. The work involved 12 trial pits with logging of pit walls, soil
sampling for particle size analysis and undisturbed sampling for thin sections and
micromorphological analysis of a till and a hard pan in moranic deposits.
The tills of the Langholm Till Formation (of McMillan & Merritt, 2012) are technically ‘coarse
soils’ from a BS5930:1999 ground engineering perspective; typically very dense/hard, very wellgraded silty sandy gravels with a matrix dominated by silt and sand. In thin section the till sandmatrix-supported gravel clasts show a preferred alignment orientated suggesting a micro-fabric
indicative of a subglacially deposited till. Clast lithology includes sandstone, siltstone and
mudstone, and are consistent with the local bedrock lithology. Cobbles and boulders are often
‘very strong’ from a geotechnical perspective, but may have weaker ‘rotten’ crust in valley floor
settings. The work provides new data on the geotechnical properties of Scottish tills and enhances
our understanding of the physical and hydrological properties of commonly encountered
Quaternary deposits that occur in the Talla Burn and nearby upland catchments
The relationship of soil and woodland cover on soil hydraulic conductivity at a hillslope scale and local flood management in the Scottish Borders
An important criteria in Natural Flood Management (NFM) is understanding and
improving the surface soil permeability (or field, saturated hydraulic conductivity,
Kfs; Talsma, 1987) of natural ground surfaces with the view of increasing rainfall
infiltration and storage capacity (Marshall et al., 2009). At the local scale infiltrability
and soil hydraulic conductivity (Ks) are key soil properties as they activate surface
and near-surface flow paths that influence runoff generation (Elsenbeer,
2001; Bonell et al., 2010)
Negative local resistance caused by viscous electron backflow in graphene
Graphene hosts a unique electron system in which electron-phonon scattering
is extremely weak but electron-electron collisions are sufficiently frequent to
provide local equilibrium above liquid nitrogen temperature. Under these
conditions, electrons can behave as a viscous liquid and exhibit hydrodynamic
phenomena similar to classical liquids. Here we report strong evidence for this
transport regime. We find that doped graphene exhibits an anomalous (negative)
voltage drop near current injection contacts, which is attributed to the
formation of submicrometer-size whirlpools in the electron flow. The viscosity
of graphene's electron liquid is found to be ~0.1 m /s, an order of
magnitude larger than that of honey, in agreement with many-body theory. Our
work shows a possibility to study electron hydrodynamics using high quality
graphene
A homogenised model for flow, transport and sorption in a heterogeneous porous medium
A major challenge in flow through porous media is to better understand the
link between microstructure and macroscale flow and transport. For idealised
microstructures, the mathematical framework of homogenisation theory can be
used for this purpose. Here, we consider a two-dimensional microstructure
comprising an array of obstacles of smooth but arbitrary shape, the size and
spacing of which can vary along the length of the porous medium. We use
homogenisation via the method of multiple scales to systematically upscale a
novel problem involving cells of varying area to obtain effective continuum
equations for macroscale flow and transport. The equations are characterised by
the local porosity, a local anisotropic flow permeability, an effective local
anisotropic solute diffusivity, and an effective local adsorption rate. These
macroscale properties depend nontrivially on the two degrees of microstructural
geometric freedom in our problem: obstacle size, and obstacle spacing. We
exploit this dependence to construct and compare scenarios where the same
porosity profile results from different combinations of obstacle size and
spacing. We focus on a simple example geometry comprising circular obstacles on
a rectangular lattice, for which we numerically determine the macroscale
permeability and effective diffusivity. We investigate scenarios where the
porosity is spatially uniform but the permeability and diffusivity are not. Our
results may be useful in the design of filters, or for studying the impact of
deformation on transport in soft porous media
Genetic recombination is targeted towards gene promoter regions in dogs
The identification of the H3K4 trimethylase, PRDM9, as the gene responsible
for recombination hotspot localization has provided considerable insight into
the mechanisms by which recombination is initiated in mammals. However,
uniquely amongst mammals, canids appear to lack a functional version of PRDM9
and may therefore provide a model for understanding recombination that occurs
in the absence of PRDM9, and thus how PRDM9 functions to shape the
recombination landscape. We have constructed a fine-scale genetic map from
patterns of linkage disequilibrium assessed using high-throughput sequence data
from 51 free-ranging dogs, Canis lupus familiaris. While broad-scale properties
of recombination appear similar to other mammalian species, our fine-scale
estimates indicate that canine highly elevated recombination rates are observed
in the vicinity of CpG rich regions including gene promoter regions, but show
little association with H3K4 trimethylation marks identified in spermatocytes.
By comparison to genomic data from the Andean fox, Lycalopex culpaeus, we show
that biased gene conversion is a plausible mechanism by which the high CpG
content of the dog genome could have occurred.Comment: Updated version, with significant revision
Excess resistivity in graphene superlattices caused by umklapp electron-electron scattering
Umklapp processes play a fundamental role as the only intrinsic mechanism
that allows electrons to transfer momentum to the crystal lattice and,
therefore, provide a finite electrical resistance in pure metals. However,
umklapp scattering has proven to be elusive in experiment as it is easily
obscured by other dissipation mechanisms. Here we show that electron-electron
umklapp scattering dominates the transport properties of
graphene-on-boron-nitride superlattices over a wide range of temperatures and
carrier densities. The umklapp processes cause giant excess resistivity that
rapidly increases with increasing the superlattice period and are responsible
for deterioration of the room-temperature mobility by more than an order of
magnitude as compared to standard, non-superlattice graphene devices. The
umklapp scattering exhibits a quadratic temperature dependence accompanied by a
pronounced electron-hole asymmetry with the effect being much stronger for
holes rather than electrons. Aside from fundamental interest, our results have
direct implications for design of possible electronic devices based on
heterostructures featuring superlattices
Scaling approach to tight-binding transport in realistic graphene devices:the case of transverse magnetic focusing
Ultraclean graphene sheets encapsulated between hexagonal boron nitride crystals host two-dimensional electron systems in which low-temperature transport is solely limited by the sample size. We revisit the theoretical problem of carrying out microscopic calculations of nonlocal ballistic transport in such micron-scale devices. By employing the Landauer-Büttiker scattering theory, we propose a scaling approach to tight-binding nonlocal transport in realistic graphene devices. We test our numerical method against experimental data on transverse magnetic focusing (TMF), a textbook example of nonlocal ballistic transport in the presence of a transverse magnetic field. This comparison enables a clear physical interpretation of all the observed features of the TMF signal, including its oscillating sign
Geological structure as a control on floodplain groundwater dynamics
Groundwater in upland floodplains has an important function in regulating river flows and controlling the coupling of hillslope runoff with rivers, with complex interaction between surface waters and groundwaters throughout floodplain width and depth. Heterogeneity is a key feature of upland floodplain hydrogeology and influences catchment water flows, but it is difficult to characterise and therefore is often simplified or overlooked. An upland floodplain and adjacent hillslope in the Eddleston catchment, southern Scotland (UK), has been studied through detailed three-dimensional geological characterisation, the monitoring of ten carefully sited piezometers, and analysis of locally collected rainfall and river data. Lateral aquifer heterogeneity produces different patterns of groundwater level fluctuation across the floodplain. Much of the aquifer is strongly hydraulically connected to the river, with rapid groundwater level rise and recession over hours. Near the floodplain edge, however, the aquifer is more strongly coupled with subsurface hillslope inflows, facilitated by highly permeable solifluction deposits in the hillslope–floodplain transition zone. Here, groundwater level rise is slower but high heads can be maintained for weeks, sometimes with artesian conditions, with important implications for drainage and infrastructure development. Vertical heterogeneity in floodplain aquifer properties, to depths of at least 12 m, can create local aquifer compartmentalisation with upward hydraulic gradients, influencing groundwater mixing and hydrogeochemical evolution. Understanding the geological processes controlling aquifer heterogeneity, which are common to formerly glaciated valleys across northern latitudes, provides key insights into the hydrogeology and wider hydrological behaviour of upland floodplains
Genomics of Divergence along a Continuum of Parapatric Population Differentiation
MM received funding from the Max Planck innovation funds for this project. PGDF was supported by a Marie Curie European Reintegration Grant (proposal nr 270891). CE was supported by German Science Foundation grants (DFG, EI 841/4-1 and EI 841/6-1)
Capture and inception of bubbles near line vortices
Motivated by the need to predict vortex cavitation inception, a study has been conducted to investigate bubble capture by a concentrated line vortex of core size rcrc and circulation Γ0Γ0 under noncavitating and cavitating conditions. Direct numerical simulations that solve simultaneously for the two phase flow field, as well as a simpler one-way coupled point-particle-tracking model (PTM) were used to investigate the capture process. The capture times were compared to experimental observations. It was found that the point-particle-tracking model can successfully predict the capture of noncavitating small nuclei by a line vortex released far from the vortex axis. The nucleus grows very slowly during capture until the late stages of the process, where bubble/vortex interaction and bubble deformation become important. Consequently, PTM can be used to study the capture of cavitating nuclei by dividing the process into the noncavitating capture of the nucleus, and then the growth of the nucleus in the low-pressure core region. Bubble growth and deformation act to speed up the capture process.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87832/2/022105_1.pd
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