9,656 research outputs found
Solvent fluctuations around solvophobic, solvophilic and patchy nanostructures and the accompanying solvent mediated interactions
Using classical density functional theory (DFT) we calculate the density
profile and local compressibility of a
simple liquid solvent in which a pair of blocks with (microscopic) rectangular
cross-section are immersed. We consider blocks that are solvophobic,
solvophilic and also ones that have both solvophobic and solvophilic patches.
Large values of correspond to regions in space where the
liquid density is fluctuating most strongly. We seek to elucidate how enhanced
density fluctuations correlate with the solvent mediated force between the
blocks, as the distance between the blocks and the chemical potential of the
liquid reservoir vary. For sufficiently solvophobic blocks, at small block
separations and small deviations from bulk gas-liquid coexistence, we observe a
strongly attractive (near constant) force, stemming from capillary evaporation
to form a low density gas-like intrusion between the blocks. The accompanying
exhibits structure which reflects the incipient gas-liquid
interfaces that develop. We argue that our model system provides a means to
understanding the basic physics of solvent mediated interactions between
nanostructures, and between objects such as proteins in water, that possess
hydrophobic and hydrophilic patches.Comment: 19 pages, 21 figure
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
The standard mean-field treatment of inter-particle attraction in classical DFT is better than one might expect
In classical density functional theory (DFT) the part of the Helmholtz free
energy functional arising from attractive inter-particle interactions is often
treated in a mean-field or van der Waals approximation. On the face of it, this
is a somewhat crude treatment as the resulting functional generates the simple
random phase approximation (RPA) for the bulk fluid pair direct correlation
function. We explain why using standard mean-field DFT to describe
inhomogeneous fluid structure and thermodynamics is more accurate than one
might expect based on this observation. By considering the pair correlation
function and structure factor of a one-dimensional model fluid,
for which exact results are available, we show that the mean-field DFT,
employed within the test-particle procedure, yields results much superior to
those from the RPA closure of the bulk Ornstein-Zernike equation. We argue that
one should not judge the quality of a DFT based solely on the approximation it
generates for the bulk pair direct correlation function.Comment: 9 pages, 3 figure
Depletion potentials in highly size-asymmetric binary hard-sphere mixtures: Comparison of accurate simulation results with theory
We report a detailed study, using state-of-the-art simulation and theoretical
methods, of the depletion potential between a pair of big hard spheres immersed
in a reservoir of much smaller hard spheres, the size disparity being measured
by the ratio of diameters q=\sigma_s/\sigma_b. Small particles are treated
grand canonically, their influence being parameterized in terms of their
packing fraction in the reservoir, \eta_s^r. Two specialized Monte Carlo
simulation schemes --the geometrical cluster algorithm, and staged particle
insertion-- are deployed to obtain accurate depletion potentials for a number
of combinations of q\leq 0.1 and \eta_s^r. After applying corrections for
simulation finite-size effects, the depletion potentials are compared with the
prediction of new density functional theory (DFT) calculations based on the
insertion trick using the Rosenfeld functional and several subsequent
modifications. While agreement between the DFT and simulation is generally
good, significant discrepancies are evident at the largest reservoir packing
fraction accessible to our simulation methods, namely \eta_s^r=0.35. These
discrepancies are, however, small compared to those between simulation and the
much poorer predictions of the Derjaguin approximation at this \eta_s^r. The
recently proposed morphometric approximation performs better than Derjaguin but
is somewhat poorer than DFT for the size ratios and small sphere packing
fractions that we consider. The effective potentials from simulation, DFT and
the morphometric approximation were used to compute the second virial
coefficient B_2 as a function of \eta_s^r. Comparison of the results enables an
assessment of the extent to which DFT can be expected to correctly predict the
propensity towards fluid fluid phase separation in additive binary hard sphere
mixtures with q\leq 0.1.Comment: 16 pages, 9 figures, revised treatment of morphometric approximation
and reordered some materia
First Canadian Breeding Record of the Dovekie (Alle alle)
The Dovekie (Alle alle) is the smallest and one of the most abundant alcids inhabiting the North Atlantic Ocean (Salomonsen, 1950; Brown et al., 1975; Roby et al., 1981). Until now there have been no documented breeding records of the Dovekie in the Canadian Arctic, though they are known to gather by the millions in northwest Baffin Bay during spring migration to breeding colonies in Northwest Greenland (Renaud et al., 1982). The Dovekie is well known to the Inuit of Baffin Island; it is called akpaliapik, in contradistinction to its larger relative, the akpa or Thick-billed Murre (uria lomvia). In August 1983, during a bowhead whale (Balaena mysticetus) study on the east coast of Baffin Island, Apak Qaqqasiq mentioned that Dovekies nested in at least two locations in Home Bay. With him and Josepi Tigullaraq, N.W.T. Wildlife Officer from Clyde, we visited one of these colonies by boat on 20 August. The colony was located on a small island (1 km long) called Abbajalik in Inuktitut (unnamed on maps), in northern Home Bay (69 02 N, 67 23 W) about 800 km south of the closest known Dovekie nesting locations in Northwest Greenland. ... The Dovekie colony appeared to be vacated and we were about to leave when Qaqqasiq detected a faint call from the moraine. After carefully moving a few boulders, we found two Dovekie eggs, and nearby an adult Dovekie. ..
Negative spatial regulation of the lineage specific CyIIIa actin gene in the sea urchin embryo
The CyIIIa·CAT fusion gene was injected into Strongylocentrotus purpuratus eggs, together with excess ligated competitor sequences representing subregions of the CyIIIa regulatory domain. In this construct, the chloramphenicol acetyltransferase (CAT) reporter gene is placed under the control of the 2300 nucleotide upstream regulatory domain of the lineage-specific CyIIIa cytoskeletal actin gene. CAT mRNA was detected by in situ hybridization in serial sections of pluteus stage embryos derived from the injected eggs. When carrier DNA lacking competitor CyIIIa fragments was coinjected with CyIIIa.CAT, CAT mRNA was observed exclusively in aboral ectoderm cells, i.e. the territory in which the CyIIIa gene itself is normally expressed (as also reported by us previously). The same result was obtained when five of seven different competitor subfragments bearing sites of DNA-protein interaction were coinjected. However, coinjection of excess quantities of either of two widely separated, nonhomologous fragments of the CyIIIa regulatory domain produced a dramatic ectopic expression of CAT mRNA in the recipient embryos. CAT mRNA was observed in gut, mesenchyme cells and oral ectoderm in these embryos. We conclude that these fragments contain regulatory sites that negatively control spatial expression of the CyIIIa gene
Intraoperative Neurophysiological Monitoring for Endoscopic Endonasal Approaches to the Skull Base: A Technical Guide.
Intraoperative neurophysiological monitoring during endoscopic, endonasal approaches to the skull base is both feasible and safe. Numerous reports have recently emerged from the literature evaluating the efficacy of different neuromonitoring tests during endonasal procedures, making them relatively well-studied. The authors report on a comprehensive, multimodality approach to monitoring the functional integrity of at risk nervous system structures, including the cerebral cortex, brainstem, cranial nerves, corticospinal tract, corticobulbar tract, and the thalamocortical somatosensory system during endonasal surgery of the skull base. The modalities employed include electroencephalography, somatosensory evoked potentials, free-running and electrically triggered electromyography, transcranial electric motor evoked potentials, and auditory evoked potentials. Methodological considerations as well as benefits and limitations are discussed. The authors argue that, while individual modalities have their limitations, multimodality neuromonitoring provides a real-time, comprehensive assessment of nervous system function and allows for safer, more aggressive management of skull base tumors via the endonasal route
Interfacial and wetting properties of a binary point Yukawa fluid
We investigate the interfacial phase behavior of a binary fluid mixture
composed of repulsive point Yukawa particles. Using a simple approximation for
the Helmholtz free energy functional, which yields the random phase
approximation (RPA) for the pair direct correlation functions, we calculate the
equilibrium fluid density profiles of the two species of particles adsorbed at
a planar wall. We show that for a particular choice (repulsive exponential) of
the wall potentials and the fluid pair-potential parameters, the Euler-Lagrange
equations for the equilibrium fluid density profiles may be transformed into a
single ordinary differential equation and the profiles obtained by a simple
quadrature. For certain other choices of the fluid pair-potential parameters
fluid-fluid phase separation of the bulk fluid is observed. We find that when
such a mixture is exposed to a planar hard-wall, the fluid exhibits complete
wetting on the species 2 poor side of the binodal, i.e. we observe a thick film
of fluid rich in species 2 adsorbed at the hard-wall. The thickness of the
wetting film grows logarithmically with the concentration difference between
the fluid state-point and the binodal and is proportional to the bulk
correlation length of the intruding (wetting) fluid phase. However, for state
points on the binodal that are further from the critical point, we find there
is no thick wetting film. We determine the accompanying line of first-order
(pre-wetting) surface phase transitions which separate a thin and thick
adsorbed film. We show that for some other choices of repulsive wall potentials
the pre-wetting line is still present, but its location and extent in the phase
diagram is strongly dependent on the wall-fluid interaction parameters.Comment: 11 pages, 7 figures. Accepted for publication in Journal of Chemical
Physic
Development of an Extreme High Temperature n-type Ohmic Contact to Silicon Carbide
We report on the initial demonstration of a tungsten-nickel (75:25 at. %) ohmic contact to silicon carbide (SiC) that performed for up to fifteen hours of heat treatment in argon at 1000 C. The transfer length method (TLM) test structure was used to evaluate the contacts. Samples showed consistent ohmic behavior with specific contact resistance values averaging 5 x 10-4 -cm2. The development of this contact metallization should allow silicon carbide devices to operate more reliably at the present maximum operating temperature of 600 C while potentially extending operations to 1000 C. Introduction Silicon Carbide (SiC) is widely recognized as one of the materials of choice for high temperature, harsh environment sensors and electronics due to its ability to survive and continue normal operation in such environments [1]. Sensors and electronics in SiC have been developed that are capable of operating at temperatures of 600 oC. However operating these devices at the upper reliability temperature threshold increases the potential for early degradation. Therefore, it is important to raise the reliability temperature ceiling higher, which would assure increased device reliability when operated at nominal temperature. There are also instances that require devices to operate and survive for prolonged periods of time above 600 oC [2, 3]. This is specifically needed in the area of hypersonic flight where robust sensors are needed to monitor vehicle performance at temperature greater than 1000 C, as well as for use in the thermomechanical characterization of high temperature materials (e.g. ceramic matrix composites). While SiC alone can withstand these temperatures, a major challenge is to develop reliable electrical contacts to the device itself in order to facilitate signal extractio
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