483 research outputs found
Collisions of inhomogeneous pre-planetesimals
In the framework of the coagulation scenario, kilometre-sized planetesimals
form by subsequent collisions of pre-planetesimals of sizes from centimetre to
hundreds of metres. Pre-planetesimals are fluffy, porous dust aggregates, which
are inhomogeneous owing to their collisional history. Planetesimal growth can
be prevented by catastrophic disruption in pre-planetesimal collisions above
the destruction velocity threshold. We develop an inhomogeneity model based on
the density distribution of dust aggregates, which is assumed to be a Gaussian
distribution with a well-defined standard deviation. As a second input
parameter, we consider the typical size of an inhomogeneous clump. These input
parameters are easily accessible by laboratory experiments. For the simulation
of the dust aggregates, we utilise a smoothed particle hydrodynamics (SPH) code
with extensions for modelling porous solid bodies. The porosity model was
previously calibrated for the simulation of silica dust, which commonly serves
as an analogue for pre-planetesimal material. The inhomogeneity is imposed as
an initial condition on the SPH particle distribution. We carry out collisions
of centimetre-sized dust aggregates of intermediate porosity. We vary the
standard deviation of the inhomogeneous distribution at fixed typical clump
size. The collision outcome is categorised according to the four-population
model. We show that inhomogeneous pre-planetesimals are more prone to
destruction than homogeneous aggregates. Even slight inhomogeneities can lower
the threshold for catastrophic disruption. For a fixed collision velocity, the
sizes of the fragments decrease with increasing inhomogeneity.
Pre-planetesimals with an active collisional history tend to be weaker. This is
a possible obstacle to collisional growth and needs to be taken into account in
future studies of the coagulation scenario.Comment: 12 pages, 9 figures, 4 table
The four-populations model: a new classification scheme for pre-planetesimal collisions
Within the collision growth scenario for planetesimal formation, the growth
step from centimetre sized pre-planetesimals to kilometre sized planetesimals
is still unclear. The formation of larger objects from the highly porous
pre-planetesimals may be halted by a combination of fragmentation in disruptive
collisions and mutual rebound with compaction. However, the right amount of
fragmentation is necessary to explain the observed dust features in late T
Tauri discs. Therefore, detailed data on the outcome of pre-planetesimal
collisions is required and has to be presented in a suitable and precise
format. We propose and apply a new classification scheme for pre-planetesimal
collisions based on the quantitative aspects of four fragment populations: the
largest and second largest fragment, a power-law population, and a
sub-resolution population. For the simulations of pre-planetesimal collisions,
we adopt the SPH numerical scheme with extensions for the simulation of porous
solid bodies. By means of laboratory benchmark experiments, this model was
previously calibrated and tested for the correct simulation of the compaction,
bouncing, and fragmentation behaviour of macroscopic highly porous silica dust
aggregates. It is shown that previous attempts to map collision data were much
too oriented on qualitatively categorising into sticking, bouncing, and
fragmentation events. We show that the four-populations model encompasses all
previous categorisations and in addition allows for transitions. This is
because it is based on quantitative characteristic attributes of each
population such as the mass, kinetic energy, and filling factor. As a
demonstration of the applicability and the power of the four-populations model,
we utilise it to present the results of a study on the influence of collision
velocity in head-on collisions of intermediate porosity aggregates.Comment: 14 pages, 11 figures, 5 tables, to be published in Astronomy and
Astrophysic
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
ICTV Virus Taxonomy Profile: Pospiviroidae
[EN] Members of the family Pospiviroidae have single-stranded circular RNA genomes that adopt a rod-like or a quasi-rod-like conformation. These genomes contain a central conserved region that is involved in replication in the nucleus through an asymmetric RNA-RNA rolling-circle mechanism. Members of the family Pospiviroidae lack the hammerhead ribozymes that are typical of viroids classified in the family Avsunviroidae. The family Pospiviroidae includes the genera Apscaviroid, Cocadviroid, Coleviroid, Hostuviroid and Pospiviroid, with >25 species. This is a summary of the ICTV Report on the family Pospiviroidae, which is available at ictv.global/report/pospiviroidae.Production of this summary, the online chapter, and associated resources was funded by a grant from the Wellcome Trust (WT108418AIA).Di Serio, F.; Owens, RA.; Li, S.; Matousek, J.; Pallás Benet, V.; Randles, JW.; Sano, T.... (2021). ICTV Virus Taxonomy Profile: Pospiviroidae. Journal of General Virology. 102(2):1-2. https://doi.org/10.1099/jgv.0.00154312102
An Effective Amperometric Biosensor Based on Gold Nanoelectrode Arrays
A sensitive amperometric biosensor based on gold nanoelectrode array (NEA) was investigated. The gold nanoelectrode array was fabricated by template-assisted electrodeposition on general electrodes, which shows an ordered well-defined 3D structure of nanowires. The sensitivity of the gold NEA to hydrogen peroxide is 37 times higher than that of the conventional electrode. The linear range of the platinum NEA toward H2O2is from 1 × 10−6to 1 × 10−2 M, covering four orders of magnitudes with detection limit of 1 × 10−7 M and a single noise ratio (S/N) of four. The enzyme electrode exhibits an excellent response performance to glucose with linear range from 1 × 10−5to 1 × 10−2 M and a fast response time within 8 s. The Michaelis–Menten constantkm and the maximum current densityimaxof the enzyme electrode were 4.97 mM and 84.60 μA cm−2, respectively. This special nanoelectrode may find potential application in other biosensors based on amperometric signals
Amplifier spurious input current components in electrode-electrolyte interface impedance measurements
BACKGROUND: In Impedance Microbiology, the time during which the measuring equipment is connected to the bipolar cells is rather long, usually between 6 to 24 hrs for microorganisms with duplication times in the order of less than one hour and concentrations ranging from 10(1 )to 10(7 )[CFU/ml]. Under these conditions, the electrode-electrolyte interface impedance may show a slow drift of about 2%/hr. By and large, growth curves superimposed on such drift do not stabilize, are less reproducible, and keep on distorting all over the measurement of the temporal reactive or resistive records due to interface changes, in turn originated in bacterial activity. This problem has been found when growth curves were obtained by means of impedance analyzers or with impedance bridges using different types of operational amplifiers. METHODS: Suspecting that the input circuitry was the culprit of the deleterious effect, we used for that matter (a) ultra-low bias current amplifiers, (b) isolating relays for the selection of cells, and (c) a shorter connection time, so that the relays were maintained opened after the readings, to bring down such spurious drift to a negligible value. Bacterial growth curves were obtained in order to test their quality. RESULTS: It was demonstrated that the drift decreases ten fold when the circuit remained connected to the cell for a short time between measurements, so that the distortion became truly negligible. Improvement due to better-input amplifiers was not as good as by reducing the connection time. Moreover, temperature effects were insignificant with a regulation of ± 0.2 [°C]. Frequency did not influence either. CONCLUSION: The drift originated either at the dc input bias offset current (I(os)) of the integrated circuits, or in discrete transistors connected directly to the electrodes immersed in the cells, depending on the particular circuit arrangement. Reduction of the connection time was the best countermeasure
Enabling political legitimacy and conceptual integration for climate change adaptation research within an agricultural bureaucracy: a systemic inquiry
The value of using systems approaches, for situations framed as ‘super wicked’, is examined from the perspective of research managers and stakeholders in a state-based climate change adaptation (CCA) program (CliChAP). Polycentric drivers influencing the development of CCA research pre-2010 in Victoria, Australia are reflected on, using Soft Systems Methodology (SSM) to generate a boundary critique of CCA research as a human activity system. We experienced the complexity of purpose with research practices pulling in different directions, reflected on the appropriateness of agricultural bureaucracies’ historical new public management (NPM) practices, and focused on realigning management theory with emerging demands for adaptation research skills and capability. Our analysis conceptualised CliChAP as a subsystem, generating novelty in a wider system, concerned with socio-ecological co-evolution. Constraining/enabling conditions at the time dealing with political legitimacy and conceptual integration were observed as potential catalysts for innovation in research management towards better handling of uncertainty as a social process using systemic thinking in practice (StiP)
Integrated Methane Inversion (IMI 1.0): a user-friendly, cloud-based facility for inferring high-resolution methane emissions from TROPOMI satellite observations
We present a user-friendly, cloud-based facility for quantifying methane emissions with 0.25∘ × 0.3125∘
(≈ 25 km × 25 km) resolution by inverse analysis of satellite observations from the TROPOspheric Monitoring
Instrument (TROPOMI). The facility is built on an Integrated Methane Inversion optimal estimation workflow (IMI 1.0) and supported for use on the
Amazon Web Services (AWS) cloud. It exploits the GEOS-Chem chemical transport model and TROPOMI data already resident on AWS, thus avoiding
cumbersome big-data download. Users select a region and period of interest, and the IMI returns an analytical solution for the Bayesian optimal
estimate of period-average emissions on the 0.25∘ × 0.3125∘ grid including error statistics, information content, and
visualization code for inspection of results. The inversion uses an advanced research-grade algorithm fully documented in the literature. An
out-of-the-box inversion with rectilinear grid and default prior emission estimates can be conducted with no significant learning curve. Users can
also configure their inversions to infer emissions for irregular regions of interest, swap in their own prior emission inventories, and modify
inversion parameters. Inversion ensembles can be generated at minimal additional cost once the Jacobian matrix for the analytical inversion has been
constructed. A preview feature allows users to determine the TROPOMI information content for their region and time period of interest before
actually performing the inversion. The IMI is heavily documented and is intended to be accessible by researchers and stakeholders with no expertise
in inverse modelling or high-performance computing. We demonstrate the IMI's capabilities by applying it to estimate methane emissions from the US
oil-producing Permian Basin in May 2018.</p
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