1,721,093 research outputs found
The role of correlations in the collective behaviour of microswimmer suspensions
No full textIn this work, we study the collective behaviour of a large number of self-propelled microswimmers immersed in a fluid. Using unprecedently large-scale lattice Boltzmann simulations, we reproduce the transition to bacterial turbulence. We show that, even well below the transition, swimmers move in a correlated fashion that cannot be described by a mean-field approach. We develop a novel kinetic theory that captures these correlations and is non-perturbative in the swimmer density. To provide an experimentally accessible measure of correlations, we calculate the diffusivity of passive tracers and reveal its non-trivial density dependence. The theory is in quantitative agreement with the lattice Boltzmann simulations and captures the asymmetry between pusher and puller swimmers below the transition to turbulence
Code and accessories for 'Subcritical instabilities in plane Poiseuille flow of an Oldroyd-B fluid'
No full textRecently, detailed experiments on visco-elastic channel flow have provided convincing evidence for a nonlinear instability scenario which we had argued for based on calculations for visco-elastic Couette flow. Motivated by these experiments we extend the previous calculations to the case of visco-elastic Poiseuille flow, using the Oldroyd-B constitutive model. Our results confirm that the subcritical instability scenario is similar for both types of flow, and that the nonlinear transition occurs for Weissenberg numbers somewhat larger than one. We provide detailed results for the convergence of our expansion and for the spatial structure of the mode that drives the instability. This also gives insight into possible similarities with the mechanism of the transition to turbulence in Newtonian pipe flow
Data and movies accompanying 'Kinetic theory of pattern formation in mixtures of microtubules and molecular motors'
No full textIn this study we formulate a theoretical approach, based on a Boltzmann-like kinetic equation, to describe pattern formation in two-dimensional mixtures of microtubular filaments and molecular motors. Following the previous work by Aranson and Tsimring [Phys. Rev. E {\bf 74}, 031915 (2006) https://doi.org/10.1103/PhysRevE.74.031915 ] we model the motor-induced reorientation of microtubules as collision rules, and devise a semi-analytical method to calculate the corresponding interaction integrals. This procedure yields an infinite hierarchy of kinetic equations that we terminate by employing a well-established closure strategy, developed in the pattern-formation community and based on a power-counting argument. We thus arrive at a closed set of coupled equations for slowly varying local density and orientation of the microtubules, and study its behaviour by performing a linear stability analysis and direct numerical simulations. By comparing our method with the work of Aranson and Tsimring, we assess the validity of the assumptions required to derive their and our theories. We demonstrate that our approximation-free evaluation of the interaction integrals and our choice of a systematic closure strategy result in a rather different dynamical behaviour than was previously reported. Based on our theory, we discuss the ensuing phase diagram and the patterns observed.See readme.txt file for details
Swimming suppresses correlations in dilute suspensions of pusher microorganisms
No full textActive matter exhibits various forms of non-equilibrium states in the absence of external forcing, including macroscopic steady-state currents. Such states are often too complex to be modelled from first principles and our understanding of their physics relies heavily on minimal models. These have mostly been studied in the case of “dry” active matter, where particle dynamics are dominated by friction with their surroundings. Significantly less is known about systems with long-range hydro- dynamic interactions that belong to “wet” active matter. Dilute suspensions of motile bacteria, modelled as self-propelled dipolar particles interacting solely through long-ranged hydrodynamic fields, are arguably the most studied example from this class of active systems. Their phenomenol- ogy is well-established: at sufficiently high density of bacteria, there appear large-scale vortices and jets comprising many individual organisms, forming a chaotic state commonly known as bac- terial turbulence. As revealed by computer simulations, below the onset of collective motion, the suspension exhibits very strong correlations between individual microswimmers stemming from the long-ranged nature of dipolar fields. Here we demonstrate that this phenomenology is captured by the minimal model of microswimmers. We develop a kinetic theory that goes beyond the com- monly used mean-field assumption, and explicitly takes into account such correlations. Notably, these can be computed exactly within our theory. We calculate the fluid velocity variance, spatial and temporal correlation functions, the fluid velocity spectrum, and the enhanced diffusivity of tracer particles. We find that correlations are suppressed by particle self-propulsion, although the mean-field behaviour is not restored even in the limit of very fast swimming. Our theory is not perturbative and is valid for any value of the micro-swimmer density below the onset of collective motion. This work constitutes a significant methodological advance and allows us to make qual- itative and quantitative predictions that can be directly compared to experiments and computer simulations of micro-swimmer suspensions
Anode for electrochemical processes
No full textAn electrode for electrochemical processes, particularly suited for use as oxygen-evolving anode, is prepared starting froma titanium substrate by alkali pickling under controlled conditions. After the pickling step, the titanium sibstrate is preferably provided with a coating containing titanium dioxide
Electrochemistry of the chlorine-water system
No full textThe electrochemical production of disinfectant solutions containing active chlorine is usually carried out within cells, whose anode and cathode chambers are separated by a dividing wall (an ion-selective diaphragm or a membrane). The latter separator is generally present in order to avoid the cathodic reduction of active species synthesized at the anode; however, dealing with aqueous solutions containing small amounts (a few grams per liter) of chloride ions, a reasonable synthesis of active chlorine can be obtained also in undivided cells, i.e. in the absence of a diaphragm or a membrane.
In these cases, the species synthesized at the anode can be effectively consumed or partly reduced at the cathode, and the reaction products depend on the chosen electrochemical parameters (current density, temperature, anode material).
The present communication will focus on data obtained through 3-hours electrolyses at different conditions, carried out on aqueous solutions containing 5 g/l of KCl; a speciation of chlorinated species were performed every 30 minutes, by means of titrations and ion chromatography
Rate constants of the fulvenallenyl recombination with propargyl and its role in PAH formation: a theoretical and kinetic modeling study
No full textThe temperature- and pressure-dependent rate constants of the reaction of fulvenallenyl (C7H5) and propargyl (C3H3) radicals have been explored using advanced electronic structure methods and kinetics theories. The results show that the head-C3H3 + tail-C7H5 addition is the fastest, followed by the tail + tail addition, and that at typical combustion conditions of 1500 K and 1 atm, the reaction mostly results in collisional stabilization of entrance channel adducts without further cyclization, whereas the well-skipping pathway to fulvalene - a precursor to naphthalene, prevails at high temperatures. The formation of the aromatic two-ring isomers, naphthalene, methylene-indenes, and azulene, can be enhanced only at high temperatures and much lower pressures, when the collisional stabilization of the intermediate wells is not efficient. The computed phenomenological rate constants were consequently simplified using master equation-based lumping and the pseudo steady state approximation to reduce the size of the system. This post-processing confirmed fulvalene as the main two-ring aromatic product and thus, its contribution should be considered in kinetic models. While naphthalene is only a trace product of the C7H5 + C3H3 reaction, it can be formed from fulvalene via H-assisted isomerization. The impact of the updated thermochemistry and rate constants in CRECK and ITV kinetic models on mole fractions of relevant cyclic aromatic species was also analyzed. The results suggest that the relevance of the C7H5 + C3H3 reaction has been likely overestimated in the current literature and needs to be carefully reevaluated by revising the pathways involving the formation and consumption of C7H6 and C7H5 and improving predictions of their precursors, such as unsaturated C4 species in flames
Electrode for electrochemical processes and method for producing the same
No full textAn electrode for electrochemical processes with tin dioxide-containing catalytic coating is prepared from a precursor solution comprising tin(IV) hydroxychlorides. The process of deposition of tin dioxide is characterized by a high yield and an improved reproducibility
NEW ELECTRODIC MATERIALS FOR ELECTROMETALLURGIC APPLICATIONS: CATALYTIC AND ENVIRONMENTAL ASPECTS
No full textElectrometallurgy in aqueous media is largely based on the use of Pb and Pb alloy anodes, which exhibit reasonable stability and very low costs. However, because of the new and more strict environmental legislation, there is at present a tendency to reduce their use, due to their strong polluting character, and this prompts, in turn, the research on alternative anode materials. These materials have to fulfill the requirement of a long service life, together with a reasonable catalytic activity toward the oxygen evolution reaction (o.e.r.). Ordinary dimensionally stable anodes (DSA®), classically based on RuO2-containing mixed-oxide films, although exhibiting a quite good catalytic activity toward o.e.r., have a very poor service life, which excludes any interest for solutions based on them. At variance, electrode materials based on IrO2 seem more promising from the viewpoint of stability and maintain good catalytic properties. At this point, although a number of papers have been already published (e.g.: [1, 2]), the goal of the optimization of the properties of IrO2-based film electrodes has not been attained yet. Scope of the present work was the preparation and characterization of IrO2-SnO2 films deposited on Titanium, elucidating in particular the influence of the electrode film composition and noble-metal-oxide loading on catalytic activity toward the o.e.r. and service life. Accordingly, several electrode compositions were chosen between 1 and 70 mol% of IrO2, and the preparation was carried out by oxidative pyrolysis of precursor salt mixtures deposited onto Ti plate, making use of IrCl4 and SnCl4 precursor solutions, mixed in the appropriate ratios. The microstructural investigation, carried out by wide-angle X-ray scattering, has shown that a solid solution between the IrO2 and SnO2 rutile structures is formed across the whole phase diagram, and the Vegard’s law is followed. In all cases, an expansion of the cell volume was also observed, in agreement with observations made in other papers, concerning other oxides prepared by thermal methods or RF reactive sputtering [3].
Interestingly, the average crystallite size, estimated by the Scherrer method, was found to be quite small (of the order of 3-6 nm) and relatively unaffected by the film composition. These data support the expectation that the electrode materials, under such a dispersed morphology, should exhibit high charge storage capacity as well as high catalytic activity toward “difficult” reactions, like o.e.r. is. In fact, both expectations have been fully confirmed by experiments. Voltammetric charges may be as high as 0.15 C cm-2 and extended linearity regions with slopes of 43-46 mV in Tafel plots have been found, supporting the hypothesis that the rearrangements of the electroactive / catalytically active sites in the electrode films take place with low activation energies. These features are accompanied by a very long service life, under accelerated test conditions (solution: 2 N H2SO4, J=30,000 A m-2, T = 60 °C), which can attain values as high as ~107 Ah m-2, allowing the conclusion that electrode films of IrO2 stabilized with SnO2 are promising materials for industrial applications where stability and catalytic activity under o.e.r. are required
A 1-dimensional statistical mechanics model for nucleosome positioning on genomic DNA
No full textThe first level of folding of DNA in eukaryotes is provided by the so-called “10-nm chromatin fibre”, where DNA wraps around histone proteins (∼10 nm in size) to form nucleosomes, which go on to create a zig-zagging bead-on-a-string structure. In this work we present a 1-dimensional statistical mechanics model to study nucleosome positioning within one such 10 nm fibre. We focus on the case of genomic sheep DNA, and we start from effective potentials valid at infinite dilution and determined from high-resolution in vitro salt dialysis experiments. We study positioning within a polynucleosome chain, and compare the results for genomic DNA to that obtained in the simplest case of homogeneous DNA, where the problem can be mapped to a Tonks gas [1]. First, we consider the simple, analytically solvable, case where nucleosomes are assumed to be point-like. Then, we perform numerical simulations to gauge the effect of their finite size on the nucleosomal distribution probabilities. Finally we compare nucleosome distributions and simulated nuclease digestion patterns for the two cases (homogeneous and sheep DNA), thereby providing testable predictions of the effect of sequence on experimentally observable quantities in experiments on polynucleosome chromatin fibres reconstituted in vitro
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