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

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 $\alpha$ in the translocation time - vs. - chain length scaling law, $\tau \propto N^{\alpha}$, becomes smaller when monomer-pore friction coefficient increases. This is most noticeable for relatively strong forces. Our findings may explain the variety of $\alpha$ values which were found in experiments and computer simulations.Comment: 12 page

An Experimental 11.5 T Nb3Sn LHC Type of Dipole Magnet

As part of the magnet development program for the LHC an experimental 1 m long 11.5 T single aperture Nb3Sn dipole magnet has been designed and is now under construction. The design is focused on full utilisation of the high current density in the powder tube Nb3Sn. A new field optimisation has led to a different winding layout and cable sizes as compared to the reference LHC design. Another important feature of the design is the implementation of a shrink fit ring collar system. An extensive study of the critical current of the Nb3Sn cables as a function of the transverse stress on the cables shows a permanent degradation by the cabling process of about 20%, still leaving a safety margin at the operation field of 11.5 T of 15%. A revised glass/mica glass insulation system is applied which improves the thermal conductivity of the windings as well as the impregnation process considerably. This paper describes various design and production details of the magnet system as well as component test

Afstand en nabijheid: een theoretische en empirische verkenning van de begrippen afstand en nabijheid in humanistisch geestelijk werk

Afstand en nabijheid zijn twee begrippen die wij allemaal kennen en gebruiken. Maar als we er goed naar kijken zijn het toch twee hele ‘losse’ begrippen waarvan de betekenis lang niet altijd duidelijk is. In deze scriptie heb ik dan ook, aan de hand van literatuuronderzoek en interviews met humanistisch geestelijke verzorgers in de zorg, een bijdrage proberen te leveren aan de theorievorming over afstand en nabijheid in humanistisch geestelijk werk. Uit de literatuur en de interviews is gebleken dat afstand en nabijheid belangrijke begrippen zijn voor geestelijke verzorging en beschouwd worden als altijd aanwezige elementen waar jij, als professional, in elke ontmoeting met de cliënt steeds weer op een goede manier mee om moet gaan. In dit onderzoek is naar voren gekomen dat afstand en nabijheid verschillende dimensies hebben en op verschillende niveau’s een rol spelen. Het is bepaald geen eendimensionaal of eenduidig begrip. Betrokkenheid, bejegening, lichamelijk contact en verantwoordelijkheid lijken de hoofdaspecten te zijn van afstand en nabijheid in geestelijke verzorging op lichamelijk, emotioneel en handelingsvlak. Verder is gebleken dat de aspecten zelfzorg en zelfreflectie, zelfonthulling, grenzen, risico’s en do’s en dont’s, lichamelijk contact, cultuurverschil en seksegerelateerde verschillen allemaal een rol spelen bij afstand en nabijheid in geestelijke verzorging. Afstand en nabijheid zijn complexe begrippen waar maar weinig over gepubliceerd is en waar men nog lang niet over uitgedacht is. Het is dan ook van belang dat er in de studie meer aandacht komt voor dit onderwerp, er meer kennis vergaart wordt door de beroepsgroep en de bewustwording wordt vergroot

On the inner workings of Monte Carlo codes

We review state-of-the-art Monte Carlo (MC) techniques for computing fluid coexistence properties (Gibbs simulations) and adsorption simulations in nanoporous materials such as zeolites and metal-organic frameworks. Conventional MC is discussed and compared to advanced techniques such as reactive MC, configurational-bias Monte Carlo and continuous fractional MC. The latter technique overcomes the problem of low insertion probabilities in open systems. Other modern methods are (hyper-)parallel tempering, Wang-Landau sampling and nested sampling. Details on the techniques and acceptance rules as well as to what systems these techniques can be applied are provided. We highlight consistency tests to help validate and debug MC codes

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 $X(t)$. We show that the scaling law for the average translocation time $$ changes from $\sim N^{2\nu}/f^{1/\nu}$ to $\sim N^{1+\nu}/f$ (for the free-draining case) as the dimensionless force ${\widetilde f}_{R} = a N^{\nu}f /T$ (where $a$, $N$, $\nu$, $f$, $T$ 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 $\alpha$ grows with the pulling force ${\widetilde f}_{R}$) albeit the observed exponent $\alpha$ 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