3,493 research outputs found
Cascade Freezing of Supercooled Water Droplet Collectives
Surface icing affects the safety and performance of numerous processes in
technology. Previous studies mostly investigated freezing of individual
droplets. The interaction among multiple droplets during freezing is
investigated less, especially on nanotextured icephobic surfaces, despite its
practical importance as water droplets never appear in isolation, but in
groups. Here we show that freezing of a supercooled droplet leads to
spontaneous self-heating and induces strong vaporization. The resulting,
rapidly propagating vapor front causes immediate cascading freezing of
neighboring supercooled droplets upon reaching them. We put forth the
explanation that, as the vapor approaches cold neighboring droplets, it can
lead to local supersaturation and formation of airborne microscopic ice
crystals, which act as freezing nucleation sites. The sequential triggering and
propagation of this mechanism results in the rapid freezing of an entire
droplet ensemble resulting in ice coverage of the nanotextured surface.
Although cascade freezing is observed in a low-pressure environment, it
introduces an unexpected pathway of freezing propagation that can be crucial
for the performance of rationally designed icephobic surfaces
On the nature of Bose-Einstein condensation enhanced by localization
In a previous paper we established that for the perfect Bose gas and the
mean-field Bose gas with an external random or weak potential, whenever there
is generalized Bose-Einstein condensation in the eigenstates of the single
particle Hamiltonian, there is also generalized condensation in the kinetic
energy states. In these cases Bose-Einstein condensation is produced or
enhanced by the external potential. In the present paper we establish a
criterion for the absence of condensation in single kinetic energy states and
prove that this criterion is satisfied for a class of random potentials and
weak potentials. This means that the condensate is spread over an infinite
number of states with low kinetic energy without any of them being
macroscopically occupied
Exactness of the Bogoliubov approximation in random external potentials
We investigate the validity of the Bogoliubov c-number approximation in the
case of interacting Bose-gas in a \textit{homogeneous random} media. To take
into account the possible occurence of type III generalized Bose-Einstein
condensation (i.e. the occurrence of condensation in an infinitesimal band of
low kinetic energy modes without macroscopic occupation of any of them) we
generalize the c-number substitution procedure to this band of modes with low
momentum. We show that, as in the case of the one-mode condensation for
translation-invariant interacting systems, this procedure has no effect on the
exact value of the pressure in the thermodynamic limit, assuming that the
c-numbers are chosen according to a suitable variational principle. We then
discuss the relation between these c-numbers and the (total) density of the
condensate
Quantum gas microscopy of Rydberg macrodimers
A microscopic understanding of molecules is essential for many fields of
natural sciences but their tiny size hinders direct optical access to their
constituents. Rydberg macrodimers - bound states of two highly-excited Rydberg
atoms - feature bond lengths easily exceeding optical wavelengths. Here we
report on the direct microscopic observation and detailed characterization of
such macrodimers in a gas of ultracold atoms in an optical lattice. The size of
about 0.7 micrometers, comparable to the size of small bacteria, matches the
diagonal distance of the lattice. By exciting pairs in the initial
two-dimensional atom array, we resolve more than 50 vibrational resonances.
Using our spatially resolved detection, we observe the macrodimers by
correlated atom loss and demonstrate control of the molecular alignment by the
choice of the vibrational state. Our results allow for precision testing of
Rydberg interaction potentials and establish quantum gas microscopy as a
powerful new tool for quantum chemistry.Comment: 13 pages, 9 figure
High phenotypic plasticity at the dawn of the eosauropterygian radiation
The initial radiation of Eosauropterygia during the Triassic biotic recovery represents a key event in the dominance of reptiles secondarily adapted to marine environments. Recent studies on Mesozoic marine reptile disparity highlighted that eosauropterygians had their greatest morphological diversity during the Middle Triassic, with the co-occurrence of Pachypleurosauroidea, Nothosauroidea and Pistosauroidea, mostly along the margins of the Tethys Ocean. However, these previous studies quantitatively analysed the disparity of Eosauropterygia as a whole without focussing on Triassic taxa, thus limiting our understanding of their diversification and morphospace occupation during the Middle Triassic. Our multivariate morphometric analyses highlight a clearly distinct colonization of the ecomorphospace by the three clades, with no evidence of whole-body convergent evolution with the exception of the peculiar pistosauroid Wangosaurus brevirostris, which appears phenotypically much more similar to nothosauroids. This global pattern is mostly driven by craniodental differences and inferred feeding specializations. We also reveal noticeable regional differences among nothosauroids and pachypleurosauroids of which the latter likely experienced a remarkable diversification in the eastern Tethys during the Pelsonian. Our results demonstrate that the high phenotypic plasticity characterizing the evolution of the pelagic plesiosaurians was already present in their Triassic ancestors, casting eosauropterygians as particularly adaptable animals
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Surface and interstitial Ti diffusion at the rutile TiO2(110) surface
Diffusion of Ti through the TiO2 (110) rutile surface plays a key role in the growth and reactivity of TiO2. To understand the fundamental aspects of this important process, we present an analysis of the diffusion of Ti adspecies at the stoichiometric TiO2(110) surface using complementary computational methodologies of density functional theory corrected for on-site Coulomb interactions (DFT+U) and a charge equilibration (QEq) atomistic potential to identify minimum energy pathways. We find that diffusion of Ti from the surface to subsurface (and vice versa) follows an intersticialcy exchange mechanism, involving exchange of surface Ti with the 6-fold coordinated Ti below the bridging oxygen rows. Diffusion in the subsurface between layers also follows an interstitialcy mechanism. The diffusion of Ti is discussed in light of continued attempts
to understand the re-oxidation of non-stoichiometric TiO2(110) surfaces
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A double-masked placebo-controlled trial of azithromycin to prevent child mortality in Burkina Faso, West Africa: Community Health with Azithromycin Trial (CHAT) study protocol.
BACKGROUND:Biannual, mass azithromycin distribution has previously been shown to reduce all-cause child mortality in sub-Saharan Africa. Subgroup analysis suggested that the strongest effects were in the youngest children, leading to the hypothesis that targeting younger age groups might be an effective strategy to prevent mortality. We present the methods of two randomized controlled trials designed to evaluate mass and targeted azithromycin distribution for the prevention of child mortality in Burkina Faso, West Africa. METHODS/DESIGN:The Child Health with Azithromycin Treatment (CHAT) study consists of two nested, randomized controlled trials. In the first, communities are randomized in a 1:1 fashion to biannual, mass azithromycin distribution or placebo. The primary outcome is under-5 all-cause mortality measured at the community level. In the second, children attending primary healthcare facilities during the first 5-12 weeks of life for a healthy child visit (e.g., for vaccination) are randomized in a 1:1 fashion to a single orally administered dose of azithromycin or placebo. The primary outcome is all-cause mortality measured at 6 months of age. The trial commenced enrollment in August 2019. DISCUSSION:This study is expected to provide evidence on two health systems delivery approaches (mass and targeted treatment) for azithromycin to prevent all-cause child mortality. The results will inform global and national policies related to azithromycin for the prevention of child mortality. TRIAL REGISTRATION:ClinicalTrials.gov, ID: NCT03676764. Registered on 19 September 2018; prospectively registered pre results
Symmetry-breaking transitions in networks of nonlinear circuit elements
We investigate a nonlinear circuit consisting of N tunnel diodes in series,
which shows close similarities to a semiconductor superlattice or to a neural
network. Each tunnel diode is modeled by a three-variable FitzHugh-Nagumo-like
system. The tunnel diodes are coupled globally through a load resistor. We find
complex bifurcation scenarios with symmetry-breaking transitions that generate
multiple fixed points off the synchronization manifold. We show that multiply
degenerate zero-eigenvalue bifurcations occur, which lead to multistable
current branches, and that these bifurcations are also degenerate with a Hopf
bifurcation. These predicted scenarios of multiple branches and degenerate
bifurcations are also found experimentally.Comment: 32 pages, 11 figures, 7 movies available as ancillary file
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