364 research outputs found
Two-dimensional perturbations in a scalar model for shear banding
We present an analytical study of a toy model for shear banding, without
normal stresses, which uses a piecewise linear approximation to the flow curve
(shear stress as a function of shear rate). This model exhibits multiple
stationary states, one of which is linearly stable against general
two-dimensional perturbations. This is in contrast to analogous results for the
Johnson-Segalman model, which includes normal stresses, and which has been
reported to be linearly unstable for general two-dimensional perturbations.
This strongly suggests that the linear instabilities found in the
Johnson-Segalman can be attributed to normal stress effects.Comment: 16 pages, 10 figures, to appear in EPJE, available online first,
click DOI or http://www.springerlink.com/content/q1q0187385017628
Signal-to-Noise Ratio Improvement in Electron Beam Testing by Using a Dispersive Analyser
In conventional e-beam testers the potential of a device under test is measured by collecting the secondary electrons (SE\u27s) faster than a certain limit, while the slower SE\u27s are rejected. We have built an e-beam tester in which the slower SE\u27s are also collected. In this paper we will show that this can decrease the minimum measurable voltage substantially. An additional advantage of a double channel analyser is the possible reduction of the influence of fluctuations in the primary beam and in the secondary emission coefficient
Driven translocation of a polymer: role of pore friction and crowding
Force-driven translocation of a macromolecule through a nanopore is
investigated by taking into account the monomer-pore friction as well as the
"crowding" of monomers on the {\it trans} - side of the membrane which
counterbalance the driving force acting in the pore. The set of governing
differential-algebraic equations for the translocation dynamics is derived and
solved numerically. The analysis of this solution shows that the crowding of
monomers on the trans side hardly affects the dynamics, but the monomer-pore
friction can substantially slow down the translocation process. Moreover, the
translocation exponent in the translocation time - vs. - chain length
scaling law, , becomes smaller when monomer-pore
friction coefficient increases. This is most noticeable for relatively strong
forces. Our findings may explain the variety of values which were
found in experiments and computer simulations.Comment: 12 page
An Electron Beam Tester with Dispersive Secondary Electron Energy Analyser
The design principles of a new, experimental e-beam tester are described. Using the magnetic field of an immersion objective lens the secondary electrons are guided to an energy analyser between the condenser lenses and the objective lens. The latter can now have a short working distance and small aberrations. The electron energies are analysed by a combination trochoidal motion - retarding field analyser, which enables detection of the faster secondary electrons on one detector and detection of the slower secondary electrons on a second detector. The benefit of this set up is a possibility for voltage contrast isolation, normalization with respect to primary beam current and an improved signal to noise ratio in voltage measurements. The use of a variable axis immersion lens allows a large field of view
Polarizable Force Fields for CO<sub>2</sub> and CH<sub>4</sub> Adsorption in M-MOF-74
(Graph Presented) The family of M-MOF-74, with M = Co, Cr, Cu, Fe, Mg, Mn, Ni, Ti, V, and Zn, provides opportunities for numerous energy related gas separation applications. The pore structure of M-MOF-74 exhibits a high internal surface area and an exceptionally large adsorption capacity. The chemical environment of the adsorbate molecule in M-MOF-74 can be tuned by exchanging the metal ion incorporated in the structure. To optimize materials for a given separation process, insights into how the choice of the metal ion affects the interaction strength with adsorbate molecules and how to model these interactions are essential. Here, we quantitatively highlight the importance of polarization by comparing the proposed polarizable force field to orbital interaction energies from DFT calculations. Adsorption isotherms and heats of adsorption are computed for CO2, CH4, and their mixtures in M-MOF-74 with all 10 metal ions. The results are compared to experimental data, and to previous simulation results using nonpolarizable force fields derived from quantum mechanics. To the best of our knowledge, the developed polarizable force field is the only one so far trying to cover such a large set of possible metal ions. For the majority of metal ions, our simulations are in good agreement with experiments, demonstrating the effectiveness of our polarizable potential and the transferability of the adopted approach.</p
Forced translocation of a polymer: dynamical scaling vs. MD-simulation
We suggest a theoretical description of the force-induced translocation
dynamics of a polymer chain through a nanopore. Our consideration is based on
the tensile (Pincus) blob picture of a pulled chain and the notion of
propagating front of tensile force along the chain backbone, suggested recently
by T. Sakaue. The driving force is associated with a chemical potential
gradient that acts on each chain segment inside the pore. Depending on its
strength, different regimes of polymer motion (named after the typical chain
conformation, "trumpet", "stem-trumpet", etc.) occur. Assuming that the local
driving and drag forces are equal (i.e., in a quasi-static approximation), we
derive an equation of motion for the tensile front position . We show
that the scaling law for the average translocation time changes from
to (for the
free-draining case) as the dimensionless force (where , , , , are the Kuhn segment length, the chain
length, the Flory exponent, the driving force, and the temperature,
respectively) increases. These and other predictions are tested by Molecular
Dynamics (MD) simulation. Data from our computer experiment indicates indeed
that the translocation scaling exponent grows with the pulling force
) albeit the observed exponent stays
systematically smaller than the theoretically predicted value. This might be
associated with fluctuations which are neglected in the quasi-static
approximation.Comment: 17 pages, 8 figures; figure 5 is new; figures 4 and 6-8 are upgrade
Interspecific competition shapes the structural stability of mutualistic networks
Mutualistic networks have attracted increasing attention in the ecological
literature in the last decades as they play a key role in the maintenance of
biodiversity. Here, we develop an analytical framework to study the structural
stability of these networks including both mutualistic and competitive
interactions. Analytical and numerical analyses show that the structure of the
competitive network fundamentally alters the necessary conditions for species
coexistence in communities. Using 50 real mutualistic networks, we show that
when the relative importance of shared partners is incorporated via weighted
competition, the feasibility area in the parameter space is highly correlated
with May's stability criteria and can be predicted by a functional relationship
between the number of species, the network connectance and the average
interaction strength in the community. Our work reopens a decade-long debate
about the complexity-stability relationship in ecological communities, and
highlights the role of the relative structures of different interaction types.Comment: 33 pages including main text, supplementary material and figures.
Submitted for publicatio
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