1,185 research outputs found
The Many Faces of Heterogeneous Ice Nucleation: Interplay Between Surface Morphology and Hydrophobicity
What makes a material a good ice nucleating agent? Despite the importance of
heterogeneous ice nucleation to a variety of fields, from cloud science to
microbiology, major gaps in our understanding of this ubiquitous process still
prevent us from answering this question. In this work, we have examined the
ability of generic crystalline substrates to promote ice nucleation as a
function of the hydrophobicity and the morphology of the surface. Nucleation
rates have been obtained by brute-force molecular dynamics simulations of
coarse-grained water on top of different surfaces of a model fcc crystal,
varying the water-surface interaction and the surface lattice parameter. It
turns out that the lattice mismatch of the surface with respect to ice,
customarily regarded as the most important requirement for a good ice
nucleating agent, is at most desirable but not a requirement. On the other
hand, the balance between the morphology of the surface and its hydrophobicity
can significantly alter the ice nucleation rate and can also lead to the
formation of up to three different faces of ice on the same substrate. We have
pinpointed three circumstances where heterogeneous ice nucleation can be
promoted by the crystalline surface: (i) the formation of a water overlayer
that acts as an in-plane template; (ii) the emergence of a contact layer
buckled in an ice-like manner; and (iii) nucleation on compact surfaces with
very high interaction strength. We hope that this extensive systematic study
will foster future experimental work aimed at testing the physiochemical
understanding presented herein.Comment: Main + S
Understanding corrosion inhibition with van der Waals DFT methods: the case of benzotriazole
The corrosion of materials is an undesirable and costly process affecting many areas of technology and everyday life. As such, considerable effort has gone into understanding and preventing it. Organic molecule based coatings can in certain circumstances act as effective corrosion inhibitors. Although they have been used to great effect for more than sixty years, how they function at the atomic-level is still a matter of debate. In this work, computer simulation approaches based on density functional theory are used to investigate benzotriazole (BTAH), one of the most widely used and studied corrosion inhibitors for copper. In particular, the structures formed by protonated and deprotonated BTAH molecules on Cu(111) have been determined and linked to their inhibiting properties. It is found that hydrogen bonding, van der Waals interactions and steric repulsions all contribute in shaping how BTAH molecules adsorb, with flat-lying structures preferred at low coverage and upright configurations preferred at high coverage. The interaction of the dehydrogenated benzotriazole molecule (BTA) with the copper surface is instead dominated by strong chemisorption via the azole moiety with the aid of copper adatoms. Structures of dimers or chains are found to be the most stable structures at all coverages, in good agreement with scanning tunnelling microscopy results. Benzotriazole thus shows a complex phase behaviour in which van der Waals forces play an important role and which depends on coverage and on its protonation state and all of these factors feasibly contribute to its effectiveness as a corrosion inhibitor
Coupled cluster benchmarks of water monomers and dimers extracted from DFT liquid water: the importance of monomer deformations
To understand the performance of popular density-functional theory (DFT)
exchange-correlation (xc) functionals in simulations of liquid water, water
monomers and dimers were extracted from a PBE simulation of liquid water and
examined with coupled cluster with single and double excitations plus a
perturbative correction for connected triples [CCSD(T)]. CCSD(T) reveals that
most of the dimers are unbound compared to two gas phase equilibrium water
monomers, largely because monomers within the liquid have distorted geometries.
Of the three xc functionals tested, PBE and BLYP systematically underestimate
the cost of the monomer deformations and consequently predict too large
dissociation energies between monomers within the dimers. This is in marked
contrast to how these functionals perform for an equilibrium water dimer and
other small water clusters in the gas phase, which only have moderately
deformed monomers. PBE0 reproduces the CCSD(T) monomer deformation energies
very well and consequently the dimer dissociation energies much more accurately
than PBE and BLYP. Although this study is limited to water monomers and dimers,
the results reported here may provide an explanation for the overstructured
radial distribution functions routinely observed in BLYP and PBE simulations of
liquid water and are of relevance to water in other phases and to other
associated molecular liquids.Comment: 10 pages, 8 figures, Submitted to Journal of Chemical Physics,
Related information can be found in http://www.fhi-berlin.mpg.de/th
Statistical and neural classifiers in estimating rain rate from weather radar measurements
Weather radars are used to measure the electromagnetic radiation backscattered by cloud raindrops. Clouds that backscatter more electromagnetic radiation consist of larger droplets of rain and therefore they produce more rain. The idea is to estimate rain rate by using weather radar as an alternative to rain-gauges measuring rainfall on the ground. In an experiment during two days in June and August 1997 over the Italian-Swiss Alps, data from weather radar and surrounding rain-gauges were collected at the same time. The statistical KNN and the neural SOM classifiers were implemented for the classification task using the radar data as input and the rain-gauge measurements as output. The proposed system managed to identify matching pattern waveforms and the rainfall rate on the ground was estimated based on the radar reflectivities with a satisfactory error rate, outperforming the traditional <i>Z</i>/<i>R</i> relationship. It is anticipated that more data, representing a variety of possible meteorological conditions, will lead to improved results. The results in this work show that an estimation of rain rate based on weather radar measurements treated with statistical and neural classifiers is possible
Microscopic Mechanism and Kinetics of Ice Formation at Complex Interfaces: Zooming in on Kaolinite
Most ice in nature forms thanks to impurities which boost the exceedingly low
nucleation rate of pure supercooled water. However, the microscopic details of
ice nucleation on these substances remain largely unknown. Here, we have
unraveled the molecular mechanism and the kinetics of ice formation on
kaolinite, a clay mineral playing a key role in climate science. We find that
the formation of ice at strong supercooling in the presence of this clay is
twenty orders of magnitude faster than homogeneous freezing. The critical
nucleus is substantially smaller than that found for homogeneous nucleation
and, in contrast to the predictions of classical nucleation theory (CNT), it
has a strong 2D character. Nonetheless, we show that CNT describes correctly
the formation of ice at this complex interface. Kaolinite also promotes the
exclusive nucleation of hexagonal ice, as opposed to homogeneous freezing where
a mixture of cubic and hexagonal polytypes is observed
Cell-type phylogenetics and the origin of endometrial stromal cells
SummaryA challenge of genome annotation is the identification of genes performing specific biological functions. Here, we propose a phylogenetic approach that utilizes RNA-seq data to infer the historical relationships among cell types and to trace the pattern of gene-expression changes on the tree. The hypothesis is that gene-expression changes coincidental with the origin of a cell type will be important for the function of the derived cell type. We apply this approach to the endometrial stromal cells (ESCs), which are critical for the initiation and maintenance of pregnancy. Our approach identified well-known regulators of ESCs, PGR and FOXO1, as well as genes not yet implicated in female fertility, including GATA2 and TFAP2C. Knockdown analysis confirmed that they are essential for ESC differentiation. We conclude that phylogenetic analysis of cell transcriptomes is a powerful tool for discovery of genes performing cell-type-specific functions
Term admissions to neonatal units in England: a role for transitional care? A retrospective cohort study
Objective
To identify the primary reasons for term admissions to neonatal units in England, to determine risk factors for admissions for jaundice and to estimate the proportion who can be cared for in a transitional setting without separation of mother and baby.
Design
Retrospective observational study using neonatal unit admission data from the National Neonatal Research Database and data of live births in England from the Office for National Statistics.
Setting
All 163 neonatal units in England 2011–2013.
Participants
133 691 term babies born ≥37 weeks gestational age and admitted to neonatal units in England.
Primary and secondary outcomes
Primary reasons for admission, term babies admitted for the primary reason of jaundice, patient characteristics, postnatal age at admission, total length of stay, phototherapy, intravenous fluids, exchange transfusion and kernicterus.
Results
Respiratory disease was the most common reason for admission overall, although jaundice was the most common reason for admission from home (22% home vs 5% hospital). Risk factors for admission for jaundice include male, born at 37 weeks gestation, Asian ethnicity and multiple birth. The majority of babies received only a brief period of phototherapy, and only a third received intravenous fluids, suggesting that some may be appropriately managed without separation of mother and baby. Admission from home was significantly later (3.9 days) compared with those admitted from elsewhere in the hospital (1.7 days) (p<0.001).
Conclusion
Around two-thirds of term admissions for jaundice may be appropriately managed in a transitional care setting, avoiding separation of mother and baby. Babies with risk factors may benefit from a community midwife postnatal visit around the third day of life to enable early referral if necessary. We recommend further work at the national level to examine provision and barriers to transitional care, referral pathways between primary and secondary care, and community postnatal care
Communication: Truncated non-bonded potentials can yield unphysical behavior in molecular dynamics simulations of interfaces
Non-bonded potentials are included in most force fields and therefore widely
used in classical molecular dynamics simulations of materials and interfacial
phenomena. It is commonplace to truncate these potentials for computational
efficiency based on the assumption that errors are negligible for reasonable
cutoffs or compensated for by adjusting other interaction parameters. Arising
from a metadynamics study of the wetting transition of water on a solid
substrate, we find that the influence of the cutoff is unexpectedly strong and
can change the character of the wetting transition from continuous to first
order by creating artificial metastable wetting states. Common cutoff
corrections such as the use of a force switching function, a shifted potential,
or a shifted force do not avoid this. Such a qualitative difference urges
caution and suggests that using truncated non-bonded potentials can induce
unphysical behavior that cannot be fully accounted for by adjusting other
interaction parameters
Association of geopotential height patterns with heavy rainfall events in Cyprus
Dynamically induced rainfall is strongly connected with
synoptic atmospheric circulation patterns at the upper levels. This study
investigates the relationship between days of high precipitation volume
events in the eastern Mediterranean and the associated geopotential height
patterns at 500 hPa. To reduce the number of different patterns and to
simplify the statistical processing, the input days were classified into
clusters of synoptic cases having similar characteristics, by utilizing
Kohonen Self Organizing Maps (SOM) architecture. Using this architecture,
synoptic patterns were grouped into 9, 18, 27 and 36 clusters which were
subsequently used in the analysis. The classification performance was tested
by applying the method to extreme rainfall events in the eastern
Mediterranean. The relationship of the synoptic upper air patterns (500 hPa
height) and surface features (heavy rainfall events) was established, while
the 36 member classification proved to be the most efficient
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