535 research outputs found

    Modelling Collective Opinion Formation by Means of Active Brownian Particles

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    The concept of active Brownian particles is used to model a collective opinion formation process. It is assumed that individuals in community create a two-component communication field that influences the change of opinions of other persons and/or can induce their migration. The communication field is described by a reaction-diffusion equation, the opinion change of the individuals is given by a master equation, while the migration is described by a set of Langevin equations, coupled by the communication field. In the mean-field limit holding for fast communication we derive a critical population size, above which the community separates into a majority and a minority with opposite opinions. The existence of external support (e.g. from mass media) changes the ratio between minority and majority, until above a critical external support the supported subpopulation exists always as a majority. Spatial effects lead to two critical ``social'' temperatures, between which the community exists in a metastable state, thus fluctuations below a certain critical wave number may result in a spatial opinion separation. The range of metastability is particularly determined by a parameter characterizing the individual response to the communication field. In our discussion, we draw analogies to phase transitions in physical systems.Comment: Revised text version. Accepted for publication in European Physics Journal B. For related work see http://summa.physik.hu-berlin.de/~frank/active.html and http://www.if.pw.edu.pl/~jholys

    CAST – City analysis simulation tool: an integrated model of land use, population, transport and economics

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    The paper reports on research into city modelling based on principles of Science of Complexity. It focuses on integration of major processes in cities, such as economics, land use, transport and population movement. This is achieved using an extended Cellular Automata model, which allows cells to form networks, and operate on individual financial budgets. There are 22 cell types with individual processes in them. The formation of networks is based on supply and demand mechanisms for products, skills, accommodation, and services. Demand for transport is obtained as an emergent property of the system resulting from the network connectivity and relevant economic mechanisms. Population movement is a consequence of mechanisms in the housing and skill markets. Income and expenditure of cells are self-regulated through market mechanisms and changing patterns of land use are a consequence of collective interaction of all mechanisms in the model, which are integrated through emergence

    Sustainable low liquor ratio dyeing of wool with acid dyes: Effect of auxiliaries on agglomeration of dye molecules in a dyebath and dyeing uniformity

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    The use of a low-liquor ratio (1:10 or less) in the dyeing of wool has several economic and environmental benefits, such as lower energy usage and smaller quantity of auxiliaries are required compared to the traditional dyeing method. However, the reduction in liquor ratio increases the occurrence of agglomeration of dyes causing uneven dyeing. The aim of this study is to develop a low-liquor dyeing method for wool with acid dyes that could potentially be applied to larger scale industrial practices. In this work, the feasibility of application of several chemical auxiliaries to prevent the agglomeration of dyes in dyebaths and also for improving the dyeing uniformity in the low liquor ratio dyeing of wool fabric with three acid dyes, has been investigated. Optical microscopy was used to evaluate the performance of various auxiliaries to prevent the agglomeration of dyes in a dyebath. The dyeing uniformity was assessed by measuring the colour difference in various parts of the same dyed fabric by a hand-held reflectance spectrophotometer. It was found that Teric G12A6 showed the best results in terms of prevention from the agglomeration of dyes and the dyeing uniformity produced for all three acid dyes investigated. The low liquor ratio dyeing of wool fabrics with acid dyes using Teric G12A6 as a dye agglomeration preventer is equally efficient as the traditional high liquor ratio dyeing in terms of colour strength and uniformity in dyeing. The developed method could reduce dyeing cost and environmental footprint compared to the traditional dyeing of wool

    Coordination of Decisions in a Spatial Agent Model

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    For a binary choice problem, the spatial coordination of decisions in an agent community is investigated both analytically and by means of stochastic computer simulations. The individual decisions are based on different local information generated by the agents with a finite lifetime and disseminated in the system with a finite velocity. We derive critical parameters for the emergence of minorities and majorities of agents making opposite decisions and investigate their spatial organization. We find that dependent on two essential parameters describing the local impact and the spatial dissemination of information, either a definite stable minority/majority relation (single-attractor regime) or a broad range of possible values (multi-attractor regime) occurs. In the latter case, the outcome of the decision process becomes rather diverse and hard to predict, both with respect to the share of the majority and their spatial distribution. We further investigate how a dissemination of information on different time scales affects the outcome of the decision process. We find that a more ``efficient'' information exchange within a subpopulation provides a suitable way to stabilize their majority status and to reduce ``diversity'' and uncertainty in the decision process.Comment: submitted for publication in Physica A (31 pages incl. 17 multi-part figures

    Quantifying criticality, information dynamics and thermodynamics of collective motion

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    Active matter consists of self-propelled particles whose interactions give rise to coherent collective motion. Well-known examples include schools of fish, flocks of birds, swarms of insects and herds of ungulates. On the micro-scale, cells, enzymes and bacteria also move collectively as active matter, inspiring engineering of artificial materials and devices. These diverse systems exhibit similar collective behaviours, including gathering, alignment and quick propagation of perturbations, which emerge from relatively simple local interactions. This phenomenon is known as self-organisation and is observed in active matter as well as in many other complex collective phenomena, including urban agglomeration, financial crises, ecosystems dynamics and technological cascading failures. Some open challenges in the study of self-organisation include (a) how the information processing across the collective and over time gives rise to emergent behaviour, (b) how to identify the regimes in which different collective behaviours exist and their phase transitions, and (c) how to quantify the thermodynamics associated with these phenomena. This thesis aims to investigate these topics in the context of active matter, while building a rigorous theoretical framework. Specifically, this thesis provides three main contributions. Firstly, the question of how to formally measure information transfer across the collective is addressed and applied to a real system, i.e., a school of fish. Secondly, general relations between statistical mechanical and thermodynamical quantities are analytically derived and applied to a model of active matter, resulting in the formulation of the concept of “thermodynamic efficiency of computation during collective motion”. This concept is then extended to the domain of urban dynamics. Thirdly, this thesis provides a rigorous quantification of the non-equilibrium entropy production associated with the collective motion of active Brownian particles

    Reactor design, reaction engineering and cocatalyst development for photocatalytic water splitting half-reactions

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    Global warming concerns have brought energy conversion into the spotlight. The conversion of renewable energy into chemical energy carriers has required keen inventiveness of the scientific community to find feasible solutions within today´s global economy. The success of such solutions requires collective efforts of multiple stakeholders, but from a purely technical perspective, this translates to the search for materials that can readily split water using a renewable energy input. For example, by using the right combination of light absorbing and catalytically active materials — or simply photocatalysts — that can simultaneously harvest sunlight and catalyze water splitting (aka artificial photosynthesis). An efficient water splitting photocatalyst aims to transform as much power of the solar spectrum as possible into chemical energy stored in the form of hydrogen and oxygen. The efficiency of this conversion is the result of multiple steps ultimately related to the sequence of light absorption, charge separation and transport, and electron transfer reactions. A photocatalyst is a semiconductor material with properties (i.e., optical band gap and crystallinity) that facilitate that sequence. Photocatalyst optimization is the process of tweaking the rate of those multiple steps (i.e., through material properties) such that the losses along the sequence are minimized. This work focuses on the optimization of the photocatalytic performance of TiO2, WO3, and covalent organic frameworks (COFs). Energy conversion efficiencies using these, and state of the art photocatalysts remain far from the target set for commercial feasibility. However, since the first water splitting experience on TiO2, various materials have been also demonstrated promising photocatalytic properties for water splitting half reactions, like WO3 and COFs. While both WO3 and TiO2 (band gap ~ 2.75 and 3.2 eV, respectively) are n-type semiconductors with valence bands that provide enough thermodynamic driving force for the oxygen evolution reaction (OER), WO3 allows additional harvesting of the visible solar spectrum. COFs are crystalline organic semiconductors that can be synthesized from earth abundant elements which have demonstrated the photocatalytic hydrogen evolution reaction (HER). Differently to the existing myriad of inorganic HER photocatalysts, the superior chemical tunability of COFs allows rational design and almost unlimited options for the tailoring of their photocatalytic properties. Multiple strategies can be found in the literature to optimize the photocatalytic performance of TiO2, WO3 and COFs by the modification of the light harvester material properties. The workflow presented herein differs from those, because it zooms to other aspects that are equally crucial to explain photocatalyst performance but that are typically less explored by material researchers. These are the increase of material photocatalytic performance upon decoration with cocatalysts (HER or OER electrocatalyst), and the intricate interplay between that performance and the nanoparticulate suspensions' multiphysics (optics, transport phenomena, and colloidal suspension stabilization). The latter rationalizes the photoreactor design presented along this work, which simplifies persisting instrumental problems and uncertainties of the artificial photosynthesis field related to reaction modeling, and the accuracy, reproducibility, and sensitivity of the quantification of photocatalyst performance. Commercial TiO2 (P25) is a standardized photocatalyst with the potential to benchmark photocatalytic OER rates among different laboratories, but it requires the addition of an OER catalyst to overcome water oxidation kinetic limitations. In this work a RuOx cocatalyst is developed in-situ on P25 for such purpose. With the instrumentals developed for sensitive O2 detection, the P25@RuO2 benchmark is optimized in terms of activity and reproducibility (at simulated sunlight, AM1.5G) and its resulting external (0.2%) and internal photonic efficiency (16%) is presented. Along with the establishment of this OER benchmark, this work also drafts good practices for reporting OER rates (i.e., adventitious O2 control), and innovative photoreactor engineering and optical modelling for the disentangling of the multiple factors determining photocatalysis physics. Using the same instrumentals for OER detection and a more elaborated cocatalyst tuning approach, a novel 2D RuOx electrocatalyst (ruthenium oxide nanosheet, RONS) is added to WO3 nanoparticles to enhance photocatalytic OER rates. First, the tuning of a top-down method to produce size-controlled unilamellar RONS is developed. Then, the composites resulting from RONS impregnation on WO3 are compared to conventionally impregnated RuO2 nanoparticles (RONP) on WO3, the former displaying a 5-fold increase in photonic efficiency. These results are explained from the electrocatalytic properties at the RONS edges, and the optical properties of the resulting 2D/0D morphology of the RONS/WO3 that decreases the optical losses due to parasitic cocatalyst light absorption. COFs have enormous potential as photocatalysts by design. In this work the photocatalytic performance of a TpDTz COF is analyzed in terms of its interaction with a molecular HER cocatalyst (Ni-ME) and reaction modeling. The TpDTz COF/Ni-ME system, which is one of the few existing COF-molecular cocatalyst known to date that can produce hydrogen, shows relatively high HER photocatalytic activity (~1 mmol h-1 g-1, AM1.5G) compared to other organic visible light responsive semiconductor benchmarks (i.e., like g-C3N4) and it operates in aqueous suspension (containing triethanolamine as electron donor). The TpDTz COF/Ni-ME surprisingly overperforms Pt modified TpDTz COF. Nonetheless, the COFs' charge transport properties are not well understood and most likely short-ranged. This blurs the experimental access to COFs' photocatalytic performance bottlenecks, including the prominent case of the TpDTz COF/Ni-ME system. Regardless of such difficulties, this work deepens the HER reaction understanding of the TpDTz COF/Ni-ME by analyzing dynamic HER reaction trends detected using the aforesaid photoreactor designs and instrumentals. From the modeled HER cycle kinetics and rapid dark step, the HER rate limiting step of the TpDTz COF/Ni-ME is placed at the electron transfer to the resting Ni-ME state. These HER mechanisms on COFs are experimentally challenging to access and are herein partially accessed in-situ from a reaction engineering and modelling perspective. On the whole, this work is the culmination of a multidisciplinary effort to find new opportunities to understand and optimize materials used for energy conversion processes, ranging from fundamental material research, solid-state and optics physics, applied catalysis, to reactor engineering

    Development of a Novel Green Surfactant - Low Salinity Nanofluid for Enhanced Oil Recovery Application

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    Natural surfactants have been considered part of the EOR processes, given their non-toxic and environment-friendly nature. In this project, two novel natural surfactants have been extracted. Furthermore, the physical-chemical properties of novel saponins, foamability and foam stability, interfacial tension (IFT), and wettability between saponins and low salinity water (LSW) and nanoparticles have been investigated. On the other hand, the interactions of the particles (mechanisms) between saponins, salt, nanoparticles, crude oil, and formation rocks have been examined
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