577 research outputs found
Dynamics of a Semiflexible Polymer or Polymer Ring in Shear Flow
Polymers exposed to shear flow exhibit a rich tumbling dynamics. While rigid
rods rotate on Jeffery orbits, flexible polymers stretch and coil up during
tumbling. Theoretical results show that in both of these asymptotic regimes the
tumbling frequency f_c in a linear shear flow of strength \gamma scales as a
power law Wi^(2/3) in the Weissenberg number Wi=\gamma \tau, where \tau is a
characteristic time of the polymer's relaxational dynamics. For flexible
polymers these theoretical results are well confirmed by experimental single
molecule studies. However, for the intermediate semiflexible regime the
situation is less clear. Here we perform extensive Brownian dynamics
simulations to explore the tumbling dynamics of semiflexible polymers over a
broad range of shear strength and the polymer's persistence length l_p. We find
that the Weissenberg number alone does not suffice to fully characterize the
tumbling dynamics, and the classical scaling law breaks down. Instead, both the
polymer's stiffness and the shear rate are relevant control parameters. Based
on our Brownian dynamics simulations we postulate that in the parameter range
most relevant for cytoskeletal filaments there is a distinct scaling behavior
with f_c \tau*=Wi^(3/4) f_c (x) with Wi=\gamma \tau* and the scaling variable
x=(l_p/L)(Wi)^(-1/3); here \tau* is the time the polymer's center of mass
requires to diffuse its own contour length L. Comparing these results with
experimental data on F-actin we find that the Wi^(3/4) scaling law agrees
quantitatively significantly better with the data than the classical Wi^(2/3)
law. Finally, we extend our results to single ring polymers in shear flow, and
find similar results as for linear polymers with slightly different power laws.Comment: 17 pages, 14 figure
Kooperationsinitiativen an Schweizer Hochschulen : Ausgestaltung und Sicherung ihrer Nachhaltigkeit
Depuis un certain temps, les acteurs publics prônent l'établissement de coopérations entre hautes écoles. S'il y a certes des motivations diverses pour lancer une coopération, les notions d'aménagement de portefeuille ou d'efficience paraissent prédominantes. Sur cette trame, le présent travail vise à présenter une vue d'ensemble des coopérations entre hautes écoles suisses. Cet objectif se décline par l'établissement d'un inventaire des coopérations existantes et une discussion des facteurs qui influencent leur pérennisation.
Les principaux résultats de notre étude révèlent une densité élevée de coopérations qui se caractérisent par une grande diversité de formes. Pour mener à bien un projet de coopération, il importe que les partenaires développent une vision commune en termes scientifiques et institutionnels, entretiennent la confiance mutuelle et continuent à voir dans le projet une valeur ajoutée. La pérennisation d'une coopération présuppose l'intégration dans la stratégie et les structures régulières de la haute école. La condition sine qua non pour y arriver est l'intérêt des hautes écoles concernées qui ne peut émerger que sur la base d'un processus autonome et «bottom-up».
Seit geraumer Zeit ist ein steigendes Interesse an Kooperationen zwischen Hochschulen zu verzeichnen. Letztere gehen Kooperationen aus ganz unterschiedlichen Gründen ein, im öffentlichen Diskurs wird jedoch vor allem von Portfoliobereinigung oder Effizienz gesprochen. Vor diesem Hintergrund will die vorliegende Arbeit eine Übersicht über Kooperationen zwischen Schweizer Hochschulen geben. Neben der Erstellung eines Inventars existierender Kooperationen wird insbesondere diskutiert, welche Faktoren das dauerhafte Bestehen von Kooperationen beeinflussen.
Es wird deutlich, dass die Dichte an Kooperationen sehr hoch ist und die unterschiedlichsten Formen existieren. Ausschlaggebend für den Erfolg einer Kooperation sind die Entwicklung einer gemeinsamen wissenschaftlichen und institutionellen Vision, der Aufbau gegenseitigen Vertrauens und ein längerfristiger Mehrwert für die Partner. Zur Sicherung des dauerhaften Bestehens von Kooperationen ist die Integration in die Strategie und die Regelstrukturen der Hochschule erforderlich. Dies kann nur erreicht werden, wenn die betroffenen Hochschulen Interesse am Projekt entwickeln, was wiederum einen autonomen und bottom-up geführten Prozess voraussetzt
Analyzing Short-Term Noise Dependencies of Spike-Counts in Macaque Prefrontal Cortex Using Copulas and the Flashlight Transformation
Simultaneous spike-counts of neural populations are typically modeled by a Gaussian distribution. On short time scales, however, this distribution is too restrictive to describe and analyze multivariate distributions of discrete spike-counts. We present an alternative that is based on copulas and can account for arbitrary marginal distributions, including Poisson and negative binomial distributions as well as second and higher-order interactions. We describe maximum likelihood-based procedures for fitting copula-based models to spike-count data, and we derive a so-called flashlight transformation which makes it possible to move the tail dependence of an arbitrary copula into an arbitrary orthant of the multivariate probability distribution. Mixtures of copulas that combine different dependence structures and thereby model different driving processes simultaneously are also introduced. First, we apply copula-based models to populations of integrate-and-fire neurons receiving partially correlated input and show that the best fitting copulas provide information about the functional connectivity of coupled neurons which can be extracted using the flashlight transformation. We then apply the new method to data which were recorded from macaque prefrontal cortex using a multi-tetrode array. We find that copula-based distributions with negative binomial marginals provide an appropriate stochastic model for the multivariate spike-count distributions rather than the multivariate Poisson latent variables distribution and the often used multivariate normal distribution. The dependence structure of these distributions provides evidence for common inhibitory input to all recorded stimulus encoding neurons. Finally, we show that copula-based models can be successfully used to evaluate neural codes, e. g., to characterize stimulus-dependent spike-count distributions with information measures. This demonstrates that copula-based models are not only a versatile class of models for multivariate distributions of spike-counts, but that those models can be exploited to understand functional dependencies
Cellular Structures for Computation in the Quantum Regime
We present a new cellular data processing scheme, a hybrid of existing
cellular automata (CA) and gate array architectures, which is optimized for
realization at the quantum scale. For conventional computing, the CA-like
external clocking avoids the time-scale problems associated with ground-state
relaxation schemes. For quantum computing, the architecture constitutes a novel
paradigm whereby the algorithm is embedded in spatial, as opposed to temporal,
structure. The architecture can be exploited to produce highly efficient
algorithms: for example, a list of length N can be searched in time of order
cube root N.Comment: 11 pages (LaTeX), 3 figure
Stress-Energy Tensor for the Massless Spin 1/2 Field in Static Black Hole Spacetimes
The stress-energy tensor for the massless spin 1/2 field is numerically
computed outside and on the event horizons of both charged and uncharged static
non-rotating black holes, corresponding to the Schwarzschild,
Reissner-Nordstrom and extreme Reissner-Nordstr\"om solutions of Einstein's
equations. The field is assumed to be in a thermal state at the black hole
temperature. Comparison is made between the numerical results and previous
analytic approximations for the stress-energy tensor in these spacetimes. For
the Schwarzschild (charge zero) solution, it is shown that the stress-energy
differs even in sign from the analytic approximation. For the
Reissner-Nordstrom and extreme Reissner-Nordstrom solutions, divergences
predicted by the analytic approximations are shown not to exist.Comment: 5 pages, 4 figures, additional discussio
VMÖ – A new strategic transport model for Austria
For the preparation of a new traffic forecast for several time horizons up to the year 2040 and beyond (Verkehrsprognose Österreich, VPÖ 2040+) an up-to-date transport model is necessary. Currently this new national transport model Austria (Verkehrsmodell Österreich, VMÖ) is developed.
The passenger model will be disaggregated tour-based model with 5 basic steps and some extensions for special applications, like for instance tourist traffic. The number of zones will be approximately 6000. In the model a special focus is on incorporating recent trends in mode choice like “park and ride” and other multimodal chains. The freight part of the model will be an Aggregated - Disaggregated - Aggregated (ADA) model with three steps: (1) the results of an input-output-model are transformed into firm-to-firm flows, (2) the choice of shipment size and transport chain is modelled and (3) the OD relations are aggregated for the individual modes and assignment to the networks.
For individual transport with passenger cars and road freight, a quasi-dynamic road transport assignment will be developed. Public transport assignment is based on timetables. For the forecasts of travel demand for future years a pivot-point approach (with the changes) on the base matrices will be applied
Understanding visual map formation through vortex dynamics of spin Hamiltonian models
The pattern formation in orientation and ocular dominance columns is one of
the most investigated problems in the brain. From a known cortical structure,
we build spin-like Hamiltonian models with long-range interactions of the
Mexican hat type. These Hamiltonian models allow a coherent interpretation of
the diverse phenomena in the visual map formation with the help of relaxation
dynamics of spin systems. In particular, we explain various phenomena of
self-organization in orientation and ocular dominance map formation including
the pinwheel annihilation and its dependency on the columnar wave vector and
boundary conditions.Comment: 4 pages, 15 figure
On the shape of barchan dunes
Barchans are crescent-shaped sand dunes forming in aride regions with
unidirectional wind and limited sand supply. We report analytical and numerical
results for dune shapes under different environmental conditions as obtained
from the so-called `minimal model' of aeolian sand dunes. The profiles of
longitudinal vertical slices (i.e. along the wind direction) are analyzed as a
function of wind speed and sand supply. Shape transitions can be induced by
changes of mass, wind speed and sand supply. Within a minimal extension of the
model to the transverse direction the scale-invariant profile of transverse
vertical cuts can be derived analytically.Comment: to appear in J. Phys.: Condens. Matter 17 (2005
Tension Dynamics and Linear Viscoelastic Behavior of a Single Semiflexible Polymer Chain
We study the dynamical response of a single semiflexible polymer chain based
on the theory developed by Hallatschek et al. for the wormlike-chain model. The
linear viscoelastic response under oscillatory forces acting at the two chain
ends is derived analytically as a function of the oscillation frequency . We
shall show that the real part of the complex compliance in the low frequency
limit is consistent with the static result of Marko and Siggia whereas the
imaginary part exhibits the power-law dependence +1/2. On the other hand, these
compliances decrease as the power law -7/8 for the high frequency limit. These
are different from those of the Rouse dynamics. A scaling argument is developed
to understand these novel results.Comment: 23 pages, 6 figure
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