Correlation between surface topography and slippage: a Molecular Dynamics study

Using Molecular Dynamics simulations of a polymer liquid flowing past flat and patterned surfaces, we investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. For one-dimensional, rectangular grooves, we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length, $\delta$, and the position, $z_\textrm{h}$, at which viscous and frictional stresses are balanced according to Navier's partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction.Comment: 13 pages, 13 figure

Multi-Architecture Monte-Carlo (MC) Simulation of Soft Coarse-Grained Polymeric Materials: SOft coarse grained Monte-carlo Acceleration (SOMA)

Multi-component polymer systems are important for the development of new materials because of their ability to phase-separate or self-assemble into nano-structures. The Single-Chain-in-Mean-Field (SCMF) algorithm in conjunction with a soft, coarse-grained polymer model is an established technique to investigate these soft-matter systems. Here we present an im- plementation of this method: SOft coarse grained Monte-carlo Accelera- tion (SOMA). It is suitable to simulate large system sizes with up to billions of particles, yet versatile enough to study properties of different kinds of molecular architectures and interactions. We achieve efficiency of the simulations commissioning accelerators like GPUs on both workstations as well as supercomputers. The implementa- tion remains flexible and maintainable because of the implementation of the scientific programming language enhanced by OpenACC pragmas for the accelerators. We present implementation details and features of the program package, investigate the scalability of our implementation SOMA, and discuss two applications, which cover system sizes that are difficult to reach with other, common particle-based simulation methods

Directed transport of polymer drops on vibrating superhydrophobic substrates: A Molecular Dynamics study

Using Molecular Dynamics simulations of a coarse-grained polymer liquid we investigate the transport of droplets on asymmetrically structured (saw-tooth shaped), vibrating substrates. Due to a continuous supply of power by substrate vibrations and the asymmetry of its topography, the droplets are driven in a preferred direction. We study this directed motion as a function of the size of the droplets, the linear dimensions of the substrate corrugation, and the period of vibrations. Two mechanisms of driven transport are identified: (i) one that relies on the droplet's contact lines and (ii), in a range of vibration periods, the entire contact area contributes to the driving. In this latter regime, the set-up may be used in experiments for sorting droplets according to their size. Additionally, we show that the linear dimension of the substrate corrugation affects the flux inside the droplet. While on a substrate with a fine corrugation droplets mostly slide, on a more coarsely corrugated substrate the flux may exhibit an additional rotation pattern.Comment: 24 pages, 17 figures, 2 table

Nucleation in A/B/AB blends: Interplay between microphase assembly and macrophase separation

We study the interplay between microphase assembly and macrophase separation in A/B/AB ternary polymer blends by examining the free energy of localized fluctuation structures (micelles or droplets), with emphasis on the thermodynamic relationship between swollen micelles (microemulsion) and the macrophase-separated state, using self-consistent field theory and an extended capillary model. Upon introducing homopolymer B into a micelle-forming binary polymer blend A/AB, micelles can be swollen by B. A small amount of component B (below the A-rich binodal of macrophase coexistence) will not affect the stability of the swollen micelles. A large excess of homopolymer, B, will induce a microemulsion failure and lead to a macrophase separation. Between the binodal and the microemulsion failure concentration, macrophase separation in A/B/AB occurs by a two-step nucleation mechanism via a metastable microemulsion droplet of finite size. Our results illustrate a recently proposed argument that the two-step nucleation via a metastable intermediate is a general phenomenon in systems involving short-range attraction and long-range repulsion

The Nuclear Window to the Extragalactic Universe

We investigate two recent parameterizations of the galactic magnetic field with respect to their impact on cosmic nuclei traversing the field. We present a comprehensive study of the size of angular deflections, dispersion in the arrival probability distributions, multiplicity in the images of arrival on Earth, variance in field transparency, and influence of the turbulent field components. To remain restricted to ballistic deflections, a cosmic nucleus with energy E and charge Z should have a rigidity above E/Z=6 EV. In view of the differences resulting from the two field parameterizations as a measure of current knowledge in the galactic field, this rigidity threshold may have to be increased. For a point source search with E/Z>60 EV, field uncertainties increase the required signal events for discovery moderately for sources in the northern and southern regions, but substantially for sources near the galactic disk.Comment: 15 pages, 30 figures, few additional sentences and references as in accepted publicatio

Grid Integration Costs of Fluctuating Renewable Energy Sources

The grid integration of intermittent Renewable Energy Sources (RES) causes costs for grid operators due to forecast uncertainty and the resulting production schedule mismatches. These so-called profile service costs are marginal cost components and can be understood as an insurance fee against RES production schedule uncertainty that the system operator incurs due to the obligation to always provide sufficient control reserve capacity for power imbalance mitigation. This paper studies the situation for the German power system and the existing German RES support schemes. The profile service costs incurred by German Transmission System Operators (TSOs) are quantified and means for cost reduction are discussed. In general, profile service costs are dependent on the RES prediction error and the specific workings of the power markets via which the prediction error is balanced. This paper shows both how the prediction error can be reduced in daily operation as well as how profile service costs can be reduced via optimization against power markets and/or active curtailment of RES generation.Comment: Accepted for SUSTECH 2014, Portland, Oregon, USA, July 201

Das geflügelte Krokodil. Codierung von Totenbuch-Vignetten

Das Totenbuch zählt zu den bedeutendsten und am häufigsten belegten religiösen Texten des Alten Ägypten. Obwohl die Hälfte aller Handschriften neben den Sprüchen auch Vignetten besitzt, die einen erheblichen inhaltlichen und formalen Anteil der Handschriften ausmachen, konzentrierte sich die Forschung lange auf die Texte. So werden die Vignetten selbst in aktuellen Editionen oftmals nur beschrieben. Dagegen vermisst man, dass Stil und Ikonografie wie die Texte anhand von Parallelen analysiert werden. Besonders wenig Aufmerksamkeit wird den Vignetten der spätzeitlichen Totenbücher geschenkt, weil sie im Vergleich zu ihren Vorläufern im Neuen Reich als standardisiert gelten. Als Referenz verlassen sich viele Wissenschaftler deshalb auf Papyrus Turin 1791. Dieses Manuskript wurde 1842 von Richard Lepsius für damalige Verhältnisse mustergültig ediert. Noch heute dient es beim Text- und Bildvergleich als bedeutende Parallele. Dabei ist die Stellung von P.Turin 1791 noch unklar. Denn nur ein Bruchteil der etwa 1400 spätzeitlichen Handschriften mit Vignetten ist bisher publiziert, so dass das Spektrum der Varianten weitgehend unbekannt ist. Eine Übersicht über das Material und die kritische Durchsicht der Edition macht deutlich, dass man sich nicht allein auf P.Turin 1791 verlassen kann. Es gibt nämlich weit mehr Varianten als bisher angenommen wurde, und P.Turin 1791 gibt auch nicht immer den am häufigsten belegten Typ wieder. Der Artikel hinterfragt also die Verwendung von P.Turin 1791 als Referenz und zeigt die Vielfältigkeit der Vignetten in den Totenbüchern der Saitischen Rezension. Dabei wird demonstriert, dass die unterschiedlichen Varianten mit lokalen Traditionen erklärt werden können. Darüber hinaus wirft der Beitrag die Frage auf, wie die Vignetten von 1400 Handschriften überhaupt sinnvoll aufgearbeitet werden können. Der Artikel ist die schriftliche Abfassung des Vortrags „Das geflügelte Krokodil“ vom 28. Juni 2008 anlässlich der 40. Ständigen Ägyptologenkonferenz in Würzburg und des Vortrags „Encoding Vignettes of the Book of the Dead“ vom 10. Juli 2008 anlässlich eines Treffens der Arbeitsgruppe Informatik und Ägyptologie in Wien. Erstveröffentlichung: Marcus Müller-Roth, Das geflügelte Krokodil. Codierung von Totenbuch-Vignetten, in: Nigel Strudwick (ed.), Information Technology and Egyptology in 2008. Proceedings of the meeting of the Computer Working Group of the International Association of Egyptologists (Informatique et Egyptologie), Vienna, 8–11 July 2008, Bible in Technology Supplement 1, Piscataway 2008, 49-70

Flat Histogram Method of Wang-Landau and N-fold Way

We present a method for estimating the density of states of a classical statistical model. The algorithm successfully combines the Wang-Landau flat histogram method with the N-fold way in order to improve efficiency of the original single spin flip version. We test our implementation of the Wang-Landau method with the two-dimensional nearest neighbor Ising model for which we determine the tunneling time and the density of states. Furthermore, we show that our new algorithm performs correctly at right edges of an energy interval over which the density of states is computed. This removes a disadvantage of the original single spin flip Wang-Landau method where results showed systematically higher errors in the density of states at right boundaries. We compare our data with the detailed numerical tests presented in a study by Wang and Swendsen where the original Wang-Landau method was tested against various other methods. Finally, we apply our method to a thin Ising film of size $32\times 32\times 6$ with antisymmetric surface fields. With the density of states obtained from the simulations we calculate canonical averages related to the energy such as internal energy, Gibbs free energy and entropy, but we also sample microcanonical averages during simulations in order to determine canonical averages of the susceptibility, the order parameter and its fourth order cumulant. We compare our results with simulational data obtained from a conventional MC algorithm.Comment: Latex, 19 pages, 12 encapsulated Postscript figure