Analyzing and reconstructing reticulation networks under timing constraints

Reticulation networks are now frequently used to model the history of life for various groups of organisms whose evolutionary past is likely to include reticulation events like horizontal gene transfer or hybridization. However, the reconstructed networks are rarely guaranteed to be temporal. If a reticulation network is temporal, then it satisfies the two biologically motivated timing constraints of instantaneously occurring reticulation events and successively occurring speciation events. On the other hand, if a reticulation network is not temporal, it is always possible to resolve this issue by adding a number of additional unsampled or extinct taxa. In the first half of the paper, we show that deciding whether a given number of additional taxa is sufficient to transform a non-temporal reticulation network into a temporal one is an NP-complete problem. As one is often given a set of gene trees instead of a network in the context of hybridization, this motivates the second half of the paper which provides an algorithm for reconstructing a temporal hybridization network that simultaneously explains the ancestral history of two trees or indicates that no such network exists. We highlight two practical applications of this algorithm and illustrate the second application on a grass data set

Minimal model for aeolian sand dunes

We present a minimal model for the formation and migration of aeolian sand dunes. It combines a perturbative description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The latter are shown to provide the characteristic length scale. The model can explain the origin of important features of dunes, such as the formation of a slip face, the broken scale invariance, and the existence of a minimum dune size. It also predicts the longitudinal shape and aspect ratio of dunes and heaps, their migration velocity and shape relaxation dynamics. Although the minimal model employs non-local expressions for the wind shear stress as well as for the sand flux, it is simple enough to serve as a very efficient tool for analytical and numerical investigations and to open up the way to simulations of large scale desert topographies.Comment: 19 pages, 22 figure

Minimal Model for Sand Dunes

We propose a minimal model for aeolian sand dunes. It combines an analytical description of the turbulent wind velocity field above the dune with a continuum saltation model that allows for saturation transients in the sand flux. The model provides a qualitative understanding of important features of real dunes, such as their longitudinal shape and aspect ratio, the formation of a slip face, the breaking of scale invariance, and the existence of a minimum dune size.Comment: 4 pages, 4 figures, replaced with publishd versio

A Continuum Saltation Model for Sand Dunes

We derive a phenomenological continuum saltation model for aeolian sand transport that can serve as an efficient tool for geomorphological applications. The coupled differential equations for the average density and velocity of sand in the saltation layer reproduce both known equilibrium relations for the sand flux and the time evolution of the sand flux as predicted by microscopic saltation models. The three phenomenological parameters of the model are a reference height for the grain-air interaction, an effective restitution coefficient for the grain-bed interaction, and a multiplication factor characterizing the chain reaction caused by the impacts leading to a typical time or length scale of the saturation transients. We determine the values of these parameters by comparing our model with wind tunnel measurements. Our main interest are out of equilibrium situations where saturation transients are important, for instance at phase boundaries (ground/sand) or under unsteady wind conditions. We point out that saturation transients are indispensable for a proper description of sand flux over structured terrain, by applying the model to the windward side of an isolated dune, thereby resolving recently reported discrepancies between field measurements and theoretical predictions.Comment: 11 pages, 7 figure

Atomic Force Spectroscopy on Ionic Liquids

Ionic liquids have become of significant relevance in chemistry, as they can serve as environmentally-friendly solvents, electrolytes, and lubricants with bespoke properties. In particular for electrochemical applications, an understanding of the interface structure between the ionic liquid and an electrified interface is needed to model and optimize the reactions taking place on the solid surface. As with ionic liquids, the interplay between electrostatic forces and steric effects leads to an intrinsic heterogeneity, as the structure of the ionic liquid above an electrified interface cannot be described by the classical electrical double layer model. Instead, a layered solvation layer is present with a structure that depends on the material combination of the ionic liquid and substrate. In order to experimentally monitor this structure, atomic force spectroscopy (AFS) has become the method of choice. By measuring the force acting on a sharp microfabricated tip while approaching the surface in an ionic liquid, it has become possible to map the solvation layers with sub-nanometer resolution. In this review, we provide an overview of the AFS studies on ionic liquids published in recent years that illustrate how the interface is formed and how it can be modified by applying electrical potential or by adding impurities and solvents

Mechanism of action of polytetrafluoroethylene binder on the performance and durability of high-temperature polymer electrolyte fuel cells

In this work, new insights into impacts of the polytetrafluoroethylene (PTFE) binder on high temperature polymer electrolyte fuel cells (HT-PEFCs) are provided by means of various characterizations and accelerated stress tests. Cathodes with PTFE contents from 0 wt% to 60 wt% were fabricated and compared using electrochemical measurements. The results indicate that the cell with 10 wt% PTFE in the cathode catalyst layer (CCL) shows the best performance due to having the lowest mass transport resistance and cathode protonic resistance. Moreover, cyclic voltammograms show that Pt (100) edge and corner sites are significantly covered by PTFE and phosphate anions when the PTFE content is higher than 25 wt%. Open-circuit and low load-cycling conditions are applied to accelerate degradation processes of the HT-PEFCs. The PTFE binder shows a network structure in the pores of the catalyst layer, which reduces phosphoric acid leaching during the aging tests. In addition, the high binder HT-PEFCs more easily suffer from a mass transport problem, leading to more severe performance degradation

Electrochemical Impedance Spectroscopy on Metallic Materials for Bipolar Plate Application in HT-PEFC

High-temperature polymer electrolyte fuel cells (HT-PEFCs) operate in the range of 120 – 180 °C and currently employ phosphoric acid doped polybenzimidazole membranes. This in turn represents a quite aggressive environment for the components of a fuel cell. For this reason bipolar plates are presently made of graphitic composite materials. Indeed, they underlie moderate degradation due to corrosion phenomena, but otherwise they exhibit high mass and volume as well as a high effort of manufacturing. Therefore metallic materials come into play as promising candidates for bipolar plates. They provide some crucial advantages, like enhanced volumetric and gravimetric power density for stacks, increased ductility, high mechanical stability, high electronic and thermal conductivity and the capability of low-cost mass production. However, we still need further understanding of the electrochemical corrosion at the interface metal (bipolar plate) and electrolyte (phosphoric acid) at elevated temperatures.In the present work, we focus on the investigation of the fundamental corrosion mechanism of metals in phosphoric acid that depends strongly on passive layer formation of the respective metallic material. Using an electrochemical cell, which was specially designed for these experimental requirements, we analyzed various stainless steels and Ni-based alloys by means of steady-state voltammetry and impedance spectroscopy. Additionally, ex-situ characterizations using SEM, XPS and ICP-OES were carried out in order to investigate the metallic surface and the corresponding dissolved ion species in the electrolyte. A characteristic interfacial impedance as a function of applied potential illustrates the corrosion resistance due to passivation of the surface in the domain of 0.3 – 1.2 V (vs. SHE). The corrosion resistance of Ni-based alloys in the passive region has been found to be higher in comparison to ferrous stainless steels at 130 °C. This effect is attributed to the diminished charge transfer reaction in nickel oxides and phosphates in contrast to analogous iron specimen. Negative potentials and an excess of 1.2 V, which are quite destructive for the oxide layer, induce consequently low impedance values and rising corrosion rates. Notable is the lower charge transfer resistance at the free corrosion potential, which indicates a thinner passive layer

Electrochemical Corrosion Study of Metallic Materials in Phosphoric Acid as Bipolar Plates für HT-PEFCs

This work investigates various commercially available austenitic stainless steels and Ni-based alloys as possible metallic bipolar plates for high-temperature polymer electrolyte fuel cells (HT-PEFCs). The dynamic formation and depletion of passivation layers that depends strongly on applied potential was analyzed in 85 wt% phosphoric acid at RT and 130°C by means of cyclic voltammetry and steady-state polarization. All materials showed a beneficial passivation in the potential window 0.3–1.2 V, which is mainly based on a stable inner Cr2O3 layer, and a reduction of the passive layers at cathodic polarization. Alloy 2.4869 (Cronix 80) with 80 wt% Ni and 20 wt% Cr reveals the lowest corrosion rates of 16.1 μA cm−2 at 130°C in the passive region at 0.6 V and a free corrosion potential Ecor of 235 mV. The improvement of passivity was achieved by the dominant superficial Ni3(PO4)2 layer. Alloying additions Mo and Ti revealed a characteristic increase of passive current densities due to instability of these passivation components at 130°C. Passivation based on Fe oxides and phosphates in stainless steels shows to be beneficial at RT, but less efficient at 130°C