1,440 research outputs found
Fast and rewritable colloidal assembly via field synchronized particle swapping
We report a technique to realize reconfigurable colloidal crystals by using
the controlled motion of particle defects above an externally modulated
magnetic substrate. The transport of particles is induced by applying a uniform
rotating magnetic field to a ferrite garnet film characterized by a periodic
lattice of magnetic bubbles. For filling factor larger than one colloid per
bubble domain, the particle current arises from propagating defects where
particles synchronously exchange their position when passing from one occupied
domain to the next. The amplitude of an applied alternating magnetic field can
be used to displace the excess particles via a swapping mechanism, or to
mobilize the entire colloidal system at a predefined speed
Creating business value through agile project management and information systems development: the perceived impact of scrum
Value creation through information systems (IS) and information technology (IT) is a major IS research topic. However there still exists an ambiguity and fuzziness of the \u27IS business value\u27 concept and a lack of clarity surrounding the value creation process. This also true for organizations that develop IS/IT and for development technologies like information systems development and project management methods that are applied in the production of IS/IT. The agile method Scrum is one such technology. In the research presented here we studied productivity, quality and employee satisfaction as supported by Scrum as value creating measures. Our positive assessment is built upon subjective perceptions and goes beyond hard measures and indicators. It provides insights into individual and organisational impacts and sheds light on the value generation process. The measures we present thus deal with some of the deficiencies in current IS business value research and contribute to filling existing gaps in an IS business value research agenda
Self-Similar Log-Periodic Structures in Western Stock Markets from 2000
The presence of log-periodic structures before and after stock market crashes
is considered to be an imprint of an intrinsic discrete scale invariance (DSI)
in this complex system. The fractal framework of the theory leaves open the
possibility of observing self-similar log-periodic structures at different time
scales. In the present work we analyze the daily closures of three of the most
important indices worldwide since 2000: the DAX for Germany and the Nasdaq100
and the S&P500 for the United States. The qualitative behaviour of these
different markets is similar during the temporal frame studied. Evidence is
found for decelerating log-periodic oscillations of duration about two years
and starting in September 2000. Moreover, a nested sub-structure starting in
May 2002 is revealed, bringing more evidence to support the hypothesis of
self-similar, log-periodic behavior. Ongoing log-periodic oscillations are also
revealed. A Lomb analysis over the aforementioned periods indicates a
preferential scaling factor . Higher order harmonics are also
present. The spectral pattern of the data has been found to be similar to that
of a Weierstrass-type function, used as a prototype of a log-periodic fractal
function.Comment: 17 pages, 14 figures. International Journal of Modern Physics C, in
pres
Dust sedimentation and self-sustained Kelvin-Helmholtz turbulence in protoplanetary disk mid-planes. I. Radially symmetric simulations
We perform numerical simulations of the Kelvin-Helmholtz instability in the
mid-plane of a protoplanetary disk. A two-dimensional corotating slice in the
azimuthal--vertical plane of the disk is considered where we include the
Coriolis force and the radial advection of the Keplerian rotation flow. Dust
grains, treated as individual particles, move under the influence of friction
with the gas, while the gas is treated as a compressible fluid. The friction
force from the dust grains on the gas leads to a vertical shear in the gas
rotation velocity. As the particles settle around the mid-plane due to gravity,
the shear increases, and eventually the flow becomes unstable to the
Kelvin-Helmholtz instability. The Kelvin-Helmholtz turbulence saturates when
the vertical settling of the dust is balanced by the turbulent diffusion away
from the mid-plane. The azimuthally averaged state of the self-sustained
Kelvin-Helmholtz turbulence is found to have a constant Richardson number in
the region around the mid-plane where the dust-to-gas ratio is significant.
Nevertheless the dust density has a strong non-axisymmetric component. We
identify a powerful clumping mechanism, caused by the dependence of the
rotation velocity of the dust grains on the dust-to-gas ratio, as the source of
the non-axisymmetry. Our simulations confirm recent findings that the critical
Richardson number for Kelvin-Helmholtz instability is around unity or larger,
rather than the classical value of 1/4Comment: Accepted for publication in ApJ. Some minor changes due to referee
report, most notably that the clumping mechanism has been identified as the
streaming instability of Youdin & Goodman (2005). Movies of the simulations
are still available at http://www.mpia.de/homes/johansen/research_en.ph
Rapid planetesimal formation in turbulent circumstellar discs
The initial stages of planet formation in circumstellar gas discs proceed via
dust grains that collide and build up larger and larger bodies (Safronov 1969).
How this process continues from metre-sized boulders to kilometre-scale
planetesimals is a major unsolved problem (Dominik et al. 2007): boulders stick
together poorly (Benz 2000), and spiral into the protostar in a few hundred
orbits due to a head wind from the slower rotating gas (Weidenschilling 1977).
Gravitational collapse of the solid component has been suggested to overcome
this barrier (Safronov 1969, Goldreich & Ward 1973, Youdin & Shu 2002). Even
low levels of turbulence, however, inhibit sedimentation of solids to a
sufficiently dense midplane layer (Weidenschilling & Cuzzi 1993, Dominik et al.
2007), but turbulence must be present to explain observed gas accretion in
protostellar discs (Hartmann 1998). Here we report the discovery of efficient
gravitational collapse of boulders in locally overdense regions in the
midplane. The boulders concentrate initially in transient high pressures in the
turbulent gas (Johansen, Klahr, & Henning 2006), and these concentrations are
augmented a further order of magnitude by a streaming instability (Youdin &
Goodman 2005, Johansen, Henning, & Klahr 2006, Johansen & Youdin 2007) driven
by the relative flow of gas and solids. We find that gravitationally bound
clusters form with masses comparable to dwarf planets and containing a
distribution of boulder sizes. Gravitational collapse happens much faster than
radial drift, offering a possible path to planetesimal formation in accreting
circumstellar discs.Comment: To appear in Nature (30 August 2007 issue). 18 pages (in referee
mode), 3 figures. Supplementary Information can be found at 0708.389
Colloidal transport on magnetic garnet filmsw
This article reports several recent discoveries related to the controlled transport of paramagnetic colloidal particles above magnetic garnet films. The garnet films are thin uniaxial ferromagnetic films in which ferromagnetic domains can be organized into symmetric patterns consisting of stripes or bubbles and generate strong local magnetic field gradients. Application of an external homogeneous magnetic field on a larger scale compared to the spatial periodicity of the magnetic pattern in the film modulates the potential generated at its surface and induces the controlled motion of colloidal particles placed above the film. Several novel dynamical regimes are observed and reported, from localized trajectories to direct particle transport, depending on the geometry of the underlying magnetic pattern and on the parameters, which control the external driving field, such as frequency, strength and direction. Moreover, we show that this strategy allows separation and sorting of bi-disperse particle systems based on the particle size as well as the transport of chemical or biological cargoes attached to the colloidal carriers. Controlled transport of micro-sized cargoes (chemical or biological) by colloidal particle carriers in a microfluidic environment can bring significant contributions in several fields from targeted drug delivery to the realization of precise fluid-based micro-scale devices
Associations between atrial cardiopathy and cerebral amyloid: The ARIC-PET study
Background Atrial fibrillation (AF) is a risk factor for cognitive decline, possibly from silent brain infarction. Left atrial changes in structure or function (atrial cardiopathy) can lead to AF but may impact cognition independently. It is unknown if AF or atrial cardiopathy also acts on Alzheimer disease-specific mechanisms, such as deposition of Ī²-amyloid. Methods and Results A total of 316 dementia-free participants from the ARIC (Atherosclerosis Risk in Communities) study underwent florbetapir positron emission tomography, electrocardiography, and 2-dimensional echocardiography. Atrial cardiopathy was defined as ā„1: (1) left atrial volume index \u3e34 mL/
Tetherin antagonism by SARS-CoV-2 ORF3a and spike protein enhances virus release
The antiviral restriction factor, tetherin, blocks the release of several different families of enveloped viruses, including the Coronaviridae. Tetherin is an interferonāinduced protein that forms parallel homodimers between the host cell and viral particles, linking viruses to the surface of infected cells and inhibiting their release. We demonstrate that SARSāCoVā2 infection causes tetherin downregulation and that tetherin depletion from cells enhances SARSāCoVā2 viral titres. We investigate the potential viral proteins involved in abrogating tetherin function and find that SARSāCoVā2 ORF3a reduces tetherin localisation within biosynthetic organelles where Coronaviruses bud, and increases tetherin localisation to late endocytic organelles via reduced retrograde recycling. We also find that expression of Spike protein causes a reduction in cellular tetherin levels. Our results confirm that tetherin acts as a host restriction factor for SARSāCoVā2 and highlight the multiple distinct mechanisms by which SARSāCoVā2 subverts tetherin function
- ā¦