6,093 research outputs found
Solvent mediated interactions between model colloids and interfaces: A microscopic approach
We determine the solvent mediated contribution to the effective potentials
for model colloidal or nano- particles dispersed in a binary solvent that
exhibits fluid-fluid phase separation. Using a simple density functional theory
we calculate the density profiles of both solvent species in the presence of
the `colloids', which are treated as external potentials, and determine the
solvent mediated (SM) potentials. Specifically, we calculate SM potentials
between (i) two colloids, (ii) a colloid and a planar fluid-fluid interface,
and (iii) a colloid and a planar wall with an adsorbed wetting film. We
consider three different types of colloidal particles: colloid A which prefers
the bulk solvent phase rich in species 2, colloid C which prefers the solvent
phase rich in species 1, and `neutral' colloid B which has no strong preference
for either phase, i.e. the free energies to insert the colloid into either of
the coexisting bulk phases are almost equal. When a colloid which has a
preference for one of the two solvent phases is inserted into the disfavored
phase at statepoints close to coexistence a thick adsorbed `wetting' film of
the preferred phase may form around the colloids. The presence of the adsorbed
film has a profound influence on the form of the SM potentials.Comment: 17 Pages, 13 Figures. Accepted for publication in Journal of Chemical
Physic
Reply to Comment on "Cosmic rays, carbon dioxide, and climate"
In our analysis [Rahmstorf et al., 2004], we arrived at two main conclusions: the data of Shaviv and Veizer [2003] do not show a significant correlation of cosmic ray flux (CRF) and climate, and the authors' estimate of climate sensitivity to CO2 based on a simple regression analysis is questionable. After careful consideration of Shaviv and Veizer's comment, we want to uphold and reaffirm these conclusions. Concerning the question of correlation, we pointed out that a correlation arose only after several adjustments to the data, including shifting one of the four CRF peaks and stretching the time scale. To calculate statistical significance, we first need to compute the number of independent data points in the CRF and temperature curves being correlated, accounting for their autocorrelation. A standard estimate [Quenouille, 1952] of the number of effective data points is
urn:x-wiley:00963941:media:eost14930:eost14930-math-0001
where N is the total number of data points and r1, r2 are the autocorrelations of the two series. For the curves of Shaviv and Veizer [2003], the result is NEFF = 4.8. This is consistent with the fact that these are smooth curves with four humps, and with the fact that for CRF the position of the four peaks is determined by four spiral arm crossings or four meteorite clusters, respectively; that is, by four independent data points. The number of points that enter the calculation of statistical significance of a linear correlation is (NEFFâ 2), since any curves based on only two points show perfect correlation; at least three independent points are needed for a meaningful result
X-Ray Diffraction and Reflectance Spectroscopy of Murchison Powders (CM2) After Thermal Analysis Under Reducing Conditions to Final Temperatures Between 300 and 1300c
The asteroids Ryugu and Bennu have spectral characteristics in common with CI/CM type carbonaceous chondrites and are target bodies for JAXAs Hayabusa2 and NASAs OSIRIS-Rex missions, respectively. Analog studies, based primarily on the Murchison CM2 chondrite, provide a pathway to separate spectral properties resulting space weathering from those inherent to parent-body, mineralogy, chemistry, and processes. Ryugu shares spectral properties with thermally metamorphosed and partly dehydrated CI/CM chondrites. We have undertaken a multidisciplinary study of the thermal decomposition of Murchison powder samples as an analog to metamorphic process that may have occurred on Ryugu. Bulk analyses include thermal And evolved gas analysis, X-ray diffraction (XRD), and VIS-NIR and Mssbauer spectroscopy; micro- to nanoscale analyses included scanning and transmission electron microscopy and electron probe micro analysisWe report here XRD and VIS-NIR analyses of pre- and post-heated Murchison powders, and in a companion paper report results from multiple electron beam techniques
The MAGIC of CINEMA: First in-flight science results from a miniaturised anisotropic magnetoresistive magnetometer
We present the first in-flight results from a novel miniaturised anisotropic
magnetoresistive space magnetometer, MAGIC (MAGnetometer from Imperial
College), aboard the first CINEMA (CubeSat for Ions, Neutrals, Electrons
and MAgnetic fields) spacecraft in low Earth orbit. An attitude-independent
calibration technique is detailed using the International Geomagnetic
Reference Field (IGRF), which is temperature dependent in the case of the
outboard sensor. We show that the sensors accurately measure
the expected absolute field to within 2% in attitude mode and 1%
in science mode. Using a simple method we are able to estimate the
spacecraft's attitude using the magnetometer only, thus characterising
CINEMA's spin, precession and nutation. Finally, we show that the
outboard sensor is capable of detecting transient physical signals
with amplitudes of ~ 20â60 nT. These include field-aligned currents
at the auroral oval, qualitatively similar to previous observations,
which agree in location with measurements from the DMSP
(Defense Meteorological Satellite Program) and POES
(Polar-orbiting Operational Environmental Satellites) spacecraft.
Thus, we demonstrate and discuss the potential science capabilities
of the MAGIC instrument onboard a CubeSat platform
Stochastic transitions: Paths over higher energy barriers can dominate in the early stages
The time evolution of many physical, chemical, and biological systems can be
modelled by stochastic transitions between the minima of the potential energy
surface describing the system of interest. We show that in cases where there
are two (or more) possible pathways that the system can take, the time
available for the transition to occur is crucially important. The well-known
results of reaction rate theory for determining the rates of the transitions
apply in the long-time limit. However, at short times, the system can instead
choose to pass over higher energy barriers with much higher probability, as
long as the distance to travel in phase space is shorter. We construct two
simple models to illustrate this general phenomenon. We also present an
extension of the gMAM algorithm of Vanden-Eijnden and Heymann [J. Chem. Phys.
{\bf 128}, 061103 (2008)] to determine the most likely path at both short and
long times.Comment: 7 pages, 5 Figure
Higher education, mature students and employment goals: policies and practices in the UK
This article considers recent policies of Higher Education in the UK, which are aimed at widening participation and meeting the needs of employers. The focus is on the growing population of part-time students, and the implications of policies for this group. The article takes a critical perspective on government policies, using data from a major study of mature part-time students, conducted in two specialist institutions in the UK, a London University college and a distance learning university. Findings from this study throw doubt on the feasibility of determining a priori what kind of study pathway is most conducive for the individual in terms of employment gains and opportunities for upward social mobility. In conclusion, doubts are raised as to whether policies such as those of the present UK government are likely to achieve its aims. Such policies are not unique to the UK, and lessons from this country are relevant to most of the developed world
Dynamical density functional theory for the evaporation of droplets of nanoparticle suspension
We develop a lattice gas model for the drying of droplets of a nanoparticle
suspension on a planar surface, using dynamical density functional theory
(DDFT) to describe the time evolution of the solvent and nanoparticle density
profiles. The DDFT assumes a diffusive dynamics but does not include the
advective hydrodynamics of the solvent, so the model is relevant to highly
viscous or near to equilibrium systems. Nonetheless, we see an equivalent of
the coffee-ring stain effect, but in the present model it occurs for
thermodynamic rather the fluid-mechanical reasons. The model incorporates the
effect of phase separation and vertical density variations within the droplet
and the consequence of these on the nanoparticle deposition pattern on the
surface. We show how to include the effect of slip or no-slip at the surface
and how this is related to the receding contact angle. We also determine how
the equilibrium contact angle depends on the microscopic interaction
parameters.Comment: 35 pages, 10 figure
Generation of defects and disorder from deeply quenching a liquid to form a solid
We show how deeply quenching a liquid to temperatures where it is linearly
unstable and the crystal is the equilibrium phase often produces crystalline
structures with defects and disorder. As the solid phase advances into the
liquid phase, the modulations in the density distribution created behind the
advancing solidification front do not necessarily have a wavelength that is the
same as the equilibrium crystal lattice spacing. This is because in a deep
enough quench the front propagation is governed by linear processes, but the
crystal lattice spacing is determined by nonlinear terms. The wavelength
mismatch can result in significant disorder behind the front that may or may
not persist in the latter stage dynamics. We support these observations by
presenting results from dynamical density functional theory calculations for
simple one- and two-component two-dimensional systems of soft core particles.Comment: 25 pages, 11 figure
Contrasting responses of DMS and DMSP to ocean acidification in Arctic waters
Increasing atmospheric CO2 is decreasing ocean pH most rapidly in colder regions such as the Arctic. As a component of the EPOCA pelagic mesocosm experiment off Spitzbergen in 2010, we examined the consequences of decreased pH and increased pCO2 on the concentrations of dimethylsulphide (DMS). DMS is an important reactant and contributor to aerosol formation and growth in the Arctic troposphere. In the nine mesocosms with initial pH 8.3 to 7.5, equivalent to pCO2 of 180 to 1420 ÎŒatm, highly significant but inverse responses to acidity (hydrogen ion concentration [H+]) occurred following nutrient addition. Compared to ambient [H+], average concentrations of DMS during the most representative phase of the 30 d experiment were reduced by approximately 60% at the highest [H+] and by 35% at [H+] equivalent to 750 ÎŒatm pCO2, as predicted for 2100. In contrast, concentrations of dimethylsulphoniopropionate (DMSP), the precursor of DMS, were elevated by approximately 50% at the highest [H+] and by 30% at [H+] corresponding to 750 ÎŒatm pCO2. Measurements of the specific rate of synthesis of DMSP by phytoplankton indicate increased production at high [H+], in parallel to rates of inorganic carbon fixation. The elevated DMSP production at high [H+] was largely a consequence of increased dinoflagellate biomass and in particular, the increased abundance of the species Heterocapsa rotundata. We discuss both phytoplankton and bacterial processes that may explain the reduced ratios of DMS:DMSPt at higher [H+]. The experimental design of eight treatment levels provides comparatively robust empirical relationships of DMS and DMSP concentration, DMSP production and dinoflagellate biomass versus [H+] in Arctic waters
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