169 research outputs found
Dynamics of fingering convection I: Small-scale fluxes and large-scale instabilities
Double-diffusive instabilities are often invoked to explain enhanced
transport in stably-stratified fluids. The most-studied natural manifestation
of this process, fingering convection, commonly occurs in the ocean's
thermocline and typically increases diapycnal mixing by two orders of magnitude
over molecular diffusion. Fingering convection is also often associated with
structures on much larger scales, such as thermohaline intrusions, gravity
waves and thermohaline staircases. In this paper, we present an exhaustive
study of the phenomenon from small to large scales. We perform the first
three-dimensional simulations of the process at realistic values of the heat
and salt diffusivities and provide accurate estimates of the induced turbulent
transport. Our results are consistent with oceanic field measurements of
diapycnal mixing in fingering regions. We then develop a generalized mean-field
theory to study the stability of fingering systems to large-scale
perturbations, using our calculated turbulent fluxes to parameterize
small-scale transport. The theory recovers the intrusive instability, the
collective instability, and the gamma-instability as limiting cases. We find
that the fastest-growing large-scale mode depends sensitively on the ratio of
the background gradients of temperature and salinity (the density ratio). While
only intrusive modes exist at high density ratios, the collective and
gamma-instabilities dominate the system at the low density ratios where
staircases are typically observed. We conclude by discussing our findings in
the context of staircase formation theory.Comment: 23 pages, 9 figures, submitted to JF
Dynamics of fingering convection II: The formation of thermohaline staircases
Regions of the ocean's thermocline unstable to salt fingering are often
observed to host thermohaline staircases, stacks of deep well-mixed convective
layers separated by thin stably-stratified interfaces. Decades after their
discovery, however, their origin remains controversial. In this paper we use 3D
direct numerical simulations to shed light on the problem. We study the
evolution of an analogous double-diffusive system, starting from an initial
statistically homogeneous fingering state and find that it spontaneously
transforms into a layered state. By analysing our results in the light of the
mean-field theory developed in Paper I, a clear picture of the sequence of
events resulting in the staircase formation emerges. A collective instability
of homogeneous fingering convection first excites a field of gravity waves,
with a well-defined vertical wavelength. However, the waves saturate early
through regular but localized breaking events, and are not directly responsible
for the formation of the staircase. Meanwhile, slower-growing, horizontally
invariant but vertically quasi-periodic gamma-modes are also excited and grow
according to the gamma-instability mechanism. Our results suggest that the
nonlinear interaction between these various mean-field modes of instability
leads to the selection of one particular gamma-mode as the staircase
progenitor. Upon reaching a critical amplitude, this progenitor overturns into
a fully-formed staircase. We conclude by extending the results of our
simulations to real oceanic parameter values, and find that the progenitor
gamma-mode is expected to grow on a timescale of a few hours, and leads to the
formation of a thermohaline staircase in about one day with an initial spacing
of the order of one to two metres.Comment: 18 pages, 9 figures, associated mpeg file at
http://earth.uni-muenster.de/~stellma/movie_small.mp4, submitted to JF
Thermochemical conversion of plant biomass in the energotechnological complex with heat recovery
Basic performance principles of the energotechnological complex used for thermochemical conversion of plant biomass with the influence of a magnetic field and high recovery of spent heat carrier energy have been developed. The concurrent saturation of a spent heat carrier in a loading bunker with the steam from humid biomass aimed at using a certain part of a spent heat carrier, which is clear from oxygen and nitrogen oxide, and moisture in thermochemical recovery has been considered as an important aspect of recuperation processes. A mathematical model has been developed and the results of numerical simulation have been presented for determining the distribution of temperature, velocity and pressure fields in a loading bunker. Prospective assessment of the engineering solutions developed for heat energy recovery of a double-flow spent heat carrier has been conducted
Geometry of integrable dynamical systems on 2-dimensional surfaces
This paper is devoted to the problem of classification, up to smooth
isomorphisms or up to orbital equivalence, of smooth integrable vector fields
on 2-dimensional surfaces, under some nondegeneracy conditions. The main
continuous invariants involved in this classification are the left equivalence
classes of period or monodromy functions, and the cohomology classes of period
cocycles, which can be expressed in terms of Puiseux series. We also study the
problem of Hamiltonianization of these integrable vector fields by a compatible
symplectic or Poisson structure.Comment: 31 pages, 12 figures, submitted to a special issue of Acta
Mathematica Vietnamic
Partial loss compensation in dielectric-loaded plasmonic waveguides at near infra-red wavelengths
We report on the fabrication and characterization of straight dielectric-loaded surface plasmon polaritons waveguides doped with leadsulfide quantum dots as a near infra-red gain medium. A loss compensation of ~33% (an optical gain of ~143 cm−1 ) was observed in the guided mode. The mode propagation, coupling efficiency and stimulated emission were characterized using leakage radiation microscopy. The guided mode signature was separated using spatial filters in the Fourier plane of the microscope for quantitative measurements of stimulated emission
Intravenously Injected Amyloid-β Peptide With Isomerized Asp7 and Phosphorylated Ser8 Residues Inhibits Cerebral β-Amyloidosis in AβPP/PS1 Transgenic Mice Model of Alzheimer’s Disease
Cerebral β-amyloidosis, an accumulation in the patient’s brain of aggregated amyloid-β (Aβ) peptides abnormally saturated by divalent biometal ions, is one of the hallmarks of Alzheimer’s disease (AD). Earlier, we found that exogenously administrated synthetic Aβ with isomerized Asp7 (isoD7-Aβ) induces Aβ fibrillar aggregation in the transgenic mice model of AD. IsoD7-Aβ molecules have been implied to act as seeds enforcing endogenous Aβ to undergo pathological aggregation through zinc-mediated interactions. On the basis of our findings on zinc-induced oligomerization of the metal-binding domain of various Aβ species, we hypothesize that upon phosphorylation of Ser8, isoD7-Aβ loses its ability to form zinc-bound oligomeric seeds. In this work, we found that (i) in vitro isoD7-Aβ with phosphorylated Ser8 (isoD7-pS8-Aβ) is less prone to spontaneous and zinc-induced aggregation in comparison with isoD7-Aβ and intact Aβ as shown by thioflavin T fluorimetry and dynamic light scattering data, and (ii) intravenous injections of isoD7-pS8-Aβ significantly slow down the progression of institutional β-amyloidosis in AβPP/PS1 transgenic mice as shown by the reduction of the congophilic amyloid plaques’ number in the hippocampus. The results support the role of the zinc-mediated oligomerization of Aβ species in the modulation of cerebral β-amyloidosis and demonstrate that isoD7-pS8-Aβ can serve as a potential molecular tool to block the aggregation of endogenous Aβ in AD
Boosting Local Field Enhancement by on-Chip Nanofocusing and Impedance-Matched Plasmonic Antennas
Strongly confined surface plasmon-polariton modes can be used for efficiently
delivering the electromagnetic energy to nano-sized volumes by reducing the
cross sections of propagating modes far beyond the diffraction limit, i.e., by
nanofocusing. This process results in significant local-field enhancement that
can advantageously be exploited in modern optical nanotechnologies, including
signal processing, biochemical sensing, imaging and spectroscopy. Here, we
propose, analyze, and experimentally demonstrate on-chip nanofocusing followed
by impedance-matched nanowire antenna excitation in the end-fire geometry at
telecom wavelengths. Numerical and experimental evidences of the efficient
excitation of dipole and quadrupole (dark) antenna modes are provided,
revealing underlying physical mechanisms and analogies with the operation of
plane-wave Fabry-P\'erot interferometers. The unique combination of efficient
nanofocusing and nanoantenna resonant excitation realized in our experiments
offers a major boost to the field intensity enhancement up to ,
with the enhanced field being evenly distributed over the gap volume of
, and promises thereby a variety of useful
on-chip functionalities within sensing, nonlinear spectroscopy and signal
processing
Efficient unidirectional nanoslit couplers for surface plasmons
Plasmonics is based on surface plasmon polariton (SPP) modes which can be
laterally confined below the diffraction limit, thereby enabling ultracompact
optical components. In order to exploit this potential, the fundamental
bottleneck of poor light-SPP coupling must be overcome. In established SPP
sources (using prism, grating} or nanodefect coupling) incident light is a
source of noise for the SPP, unless the illumination occurs away from the
region of interest, increasing the system size and weakening the SPP intensity.
Back-side illumination of subwavelength apertures in optically thick metal
films eliminates this problem but does not ensure a unique propagation
direction for the SPP. We propose a novel back-side slit-illumination method
based on drilling a periodic array of indentations at one side of the slit. We
demonstrate that the SPP running in the array direction can be suppressed, and
the one propagating in the opposite direction enhanced, providing localized
unidirectional SPP launching.Comment: 13 pages, 4 figure
Graphene-protected copper and silver plasmonics
Plasmonics has established itself as a branch of physics which promises to
revolutionize data processing, improve photovoltaics, increase sensitivity of
bio-detection. A widespread use of plasmonic devices is notably hindered (in
addition to high losses) by the absence of stable and inexpensive metal films
suitable for plasmonic applications. This may seem surprising given the number
of metal compounds to choose from. Unfortunately, most of them either exhibit a
strong damping of surface plasmons or easily oxidize and corrode. To this end,
there has been continuous search for alternative plasmonic materials that are,
unlike gold, the current metal of choice in plasmonics, compatible with
complementary metal oxide semiconductor technology. Here we show that copper
and silver protected by graphene are viable candidates. Copper films covered
with one to a few graphene layers show excellent plasmonics characteristics
surpassing those of gold films. They can be used to fabricate plasmonic devices
and survive for at least a year, even in wet and corroding conditions. As a
proof of concept, we use the graphene-protected copper to demonstrate
dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon
resonances. Our results are likely to initiate a wide use of graphene-protected
plasmonics.Comment: 22 pages, 5 figure
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