Collective behavior of colloids due to critical Casimir interactions

If colloidal solute particles are suspended in a solvent close to its critical point, they act as cavities in a fluctuating medium and thereby restrict and modify the fluctuation spectrum in a way which depends on their relative configuration. As a result effective, so-called critical Casimir forces (CCFs) emerge between the colloids. The range and the amplitude of CCFs depend sensitively on the temperature and the composition of the solvent as well as on the boundary conditions of the order parameter of the solvent at the particle surfaces. These remarkable, moreover universal features of the CCFs provide the possibility for an active control over the assembly of colloids. This has triggered a recent surge of experimental and theoretical interest in these phenomena. We present an overview of current research activities in this area. Various experiments demonstrate the occurrence of thermally reversible self-assembly or aggregation or even equilibrium phase transitions of colloids in the mixed phase below the lower consolute points of binary solvents. We discuss the status of the theoretical description of these phenomena, in particular the validity of a description in terms of effective, one-component colloidal systems and the necessity of a full treatment of a ternary solvent-colloid mixture. We suggest perspectives on the directions towards which future research in this field might develop.Comment: review, 88 pages, 19 figure

Understanding nanoparticle aggregation

Nanoparticles form the fundamental building blocks for many exciting applications in various scientific disciplines. However, the problem of the large-scale synthesis of nanoparticles remains challenging. It is necessary to understand the nanoparticle aggregation for the rational design of reactors for high-throughput synthesis of nanoparticles with well-controlled properties. Often, nanoparticle aggregation is modeled using stochastic methods based on scaling arguments and assumptions about the nanoparticle interaction potential. Therefore, a more rigorous approach is desired for understanding nanoparticle aggregation. In this dissertation, a novel framework integrating experiments and multi-scale simulations for studying nanoparticle aggregation is presented. Atomic force microscopy (AFM) was employed to measure the force between polystyrene (PS) micro- and nanoparticles. Specifically, AFM was used to directly measure the force (in air) between a 300 nm PS nanoparticle and a PS film, which was compared with the force measured between a 2 ym PS particle and a PS film. A novel approach based on layer-by-layer assembly to functionalize an AFM probe was developed and applied to the measurement of the force between nanoparticles. The nanoparticle force was deduced from the variation of force between a silica colloidal probe (5-30 ym) functionalized with a monolayer of 300 nm PS particles and a PS film as a function of the diameter of the silica particle. It was shown that continuum models are inadequate to explain the measured forces, which underlines the need for a more rigorous multiscale modeling methodology to understand nanoparticle interaction potential. In principle, nanoparticle interaction potentials can be derived from electronic structure calculations for a molecule using a multiscale modeling approach. To this end, a systematic method of coarse-graining based on force matching was implemented and applied to coarse-grain three common solvent molecules (carbon tetrachloride, benzene and water) to their center of mass. The coarse-grained potentials derived from first principles based effective fragment potential (EFP) were able to reproduce the structural properties that were in reasonable agreement with those obtained using EFP molecular dynamics while achieving a computational speed-up of four orders of magnitude. The nanoparticle interaction potential determines the morphology of corresponding aggregates. On the other hand, the aggregation kinetics are governed by the diffusivity of the aggregates. Therefore, it is essential to relate the aggregate morphology to its mobility in order to study aggregation kinetics. The diffusion of nanoparticle aggregates in the limit of infinite dilution was studied as a function of their mass (N) and fractal dimension (df) using molecular dynamics simulations in the presence of explicit solvent molecules. The diffusion coefficient (Do) for aggregates was found to scale as Do ∼ N-1/df. The ratio of the hydrodynamic radius to the radius of gyration was found to be independent of mass for aggregates of a given fractal dimension, thus enabling an estimate of the diffusion coefficient for a fractal aggregate based on its radius of gyration. The research presented in this work provides a robust framework integrating experiments and multiscale simulations for studying nanoparticle aggregation

Ein Kumulanten-Lattice-Boltzmann-Methode für LES von Dispersionsmikrosystemen

The production of nano-particles from larger aggregates is an important industrial process, especially for life-science products. In this thesis a micro-machined disperser developed by the DFG Research Group FOR 856 mikroPART is studied numerically by the cumulant lattice Boltzmann method. The aggregates are modeled as tracer particles with mass and drag coefficient. They record the history of the stresses and the relative velocity of the aggregates with respect to the fluid. For the evaluation of the velocities and stresses a compact second-order interpolation scheme is utilized. The tracer particles are implemented in a massively parallel multi-resolution lattice Boltzmann framework. The simulation of the disperser is validated against PIV and flow rate measurements from collaborators in the mikroPART Research Group. The drag coefficients of the aggregates are obtained by detailed simulations of synthetic aggregates in simple shear flow, elongational flow, and rotational flow. An empirical relation between the drag coefficient and the number of primary particles in the aggregate and its fractal dimensions is found and used in the tracer simulation of the disperser. Different measures of load on the aggregates are obtained by the simulation, for example maximal strain, exposure time to a certain strain, and relative velocity of the particles with respect to the surrounding fluid. It is assumed that ceramic aggregates break-up when they suffer a threshold strain rate. The distribution of the maximum strain rate seen by an aggregate can be condensed into a simple exponential cumulative probability distribution. Combined with a given threshold for the particle break-up this condensed model can also be used to determine the probability for aggregate breakage after n passages of the device. It is found that aggregates with realistic geometry (fractal number 1.85) usually have Stokes numbers smaller than one such that the load on these aggregates is dominated by the strain in the surrounding fluid. This is in contrast to spherical particles (fractal number 3) that have Stokes numbers in excess of one such that the load from their relative velocity with respect to the surrounding fluid is not negligible.Die Erzeugung von Nanopartikeln aus größeren Aggregaten ist ein wichtiger industrieller Prozess insbesondere in den Lebenswissenschaften. In dieser Dissertation wird ein von der DFG-Forschergruppe FOR 856 mikroPART entwickelter Dispergierkanal mit Hilfe der Kumulanten-Lattice-Boltzmann-Methode numerisch untersucht. Die Aggregate werden als Partikel mit Masse und Strömungswiderstandsbeiwert modelliert. Sie zeichnen den Verlauf der Spannungen und den der Relativgeschwindigkeit zwischen Partikel und Fluid über die Zeit auf. Die Geschwindigkeiten und Spannungen werden mit Hilfe eines kompakten Interpolationsschemas zweiter Ordnung berechnet. Die Partikelsimulation wird in ein massiv-paralleles Mehrskalen-Lattice-Boltzmann-Framework eingebettet. Zur Validierung wird die Simulation des Dispergierkanals mit PIV- und Flussratenmessungen verglichen, die von Projektpartnern innerhalb der mikroPART-Forschergruppe durchgeführt wurden. Die Strömungswiderstandsbeiwerte der Aggregate werden durch umfangreiche Simulationen synthetischer Aggregate in einfachen Scherströmungen, Dehnströmungen und Rotationsströmungen ermittelt. Es wird ein empirischer Zusammenhang zwischen dem Strömungswiderstandsbeiwert und der Anzahl der Partikel im Aggregat sowie dessen fraktaler Dimension aufgestellt. Dieser wird in der Partikelsimulation des Dispergierkanals verwendet. Die Simulation liefert verschiedene Masse für die Belastung der Aggregate, unter anderem die maximale Dehnung, die Einwirkzeit einer gegebenen Mindestdehnung und die Relativgeschwindigkeit der Partikel zu dem umgebenden Fluid. Es wird angenommen, dass keramische Aggregate brechen, wenn eine bestimmte Schwellendehnungsrate überschritten wird. Die Verteilung der maximalen von einem Aggregat erfahrene Dehnungsrate kann durch eine einfache exponentielle kumulative Wahrscheinlichkeitsverteilung ausgedrückt werden. In Verbindung mit dem Schwellenwert kann dieses reduzierte Modell zur Abschätzung der Wahrscheinlichkeit des Aggregatbruches nach n Durchquerungen des Dispergierkanals verwendet werden. Es wird festgestellt, dass bei realistischen Aggregatsgeometrien (fraktale Dimension 1.85) typischerweise Stokeszahlen kleiner als eins auftreten, so dass der dominierende Lastmechanismus die Dehnung durch das umgebende Fluid ist. Im Gegensatz dazu treten bei kugelförmigen Partikeln (fraktale Dimension 3) Stokeszahlen größer als eins auf. Daher ist die Last aus der Relativgeschwindigkeit zu dem umgebenden Fluid nicht vernachlässigbar

Microscale Discrete Element Method Simulation of the Carbon Black Aggregate Fracture Behavior in a Simple Shear Flow

The shear stress induced breaking behavior of carbon black (CB) aggregates during the manufacturing process of Li‐ion batteries is investigated via microscale discrete element method (DEM) simulations. The relevant range of shear stress is chosen according to a planetary mixer and cathode slurries with high solid content. Aggregates of different sizes and shapes are modeled using a self‐written algorithm based on the tunable dimension method. Then, suitable models are chosen for representing the solid bridges between the primary particles of the CB aggregates and relevant fluid forces. The results show a correlation between aggregate size and critical shear stress which is required to initiate aggregate fracturing. Furthermore, a change in aggregate shape is linked to applied stress and initial aggregate size and shape. Hence, a recommendation for an efficient disintegration of CB aggregates during the mixing process is made

On temperature- and space-dimension dependent matter agglomeration in a mature growing stage

Model matter agglomerations, with temperature as leading control parameter, have been considered, and some of their characteristics have been studied. The primary interest has been focused on the grain volume fluctuations, the magnitude of which readily differentiates between two commonly encountered types of matter agglomeration/aggregation processes, observed roughly for high- and low-density matter organizations. The two distinguished types of matter arrangements have been described through the (entropic) potential driving system. The impact of the potential type on the character of matter agglomeration has been studied, preferentially for (low density) matter agglomeration for which a logarithmic measure of its speed has been proposed. A common diffusion as well as mechanical relaxation picture, emerging during the mature growing stage, has been drawn using a phenomenological line of argumentation. Applications, mostly towards obtaining soft agglomerates of so-called jammed materials, have been mentioned

Fractal-like structures in colloid science

The present work aims at reviewing our current understanding of fractal structures in the frame of colloid aggregation as well as the possibility they offer to produce novel structured materials. In particular, the existing techniques to measure and compute the fractal dimension df are critically discussed based on the cases of organic/inorganic particles and proteins. Then the aggregation conditions affecting df are thoroughly analyzed, pointing out the most recent literature findings and the limitations of our current understanding. Finally, the importance of the fractal dimension in applications is discussed along with possible directions for the production of new structured materials

Formation of stable clusters in colloidal suspensions

The experimental evidences and available theoretical explanations on formation of stable clusters in colloidal suspensions are reviewed. The clusters form in the parameters range intermediate between that corresponds to a stable suspension built up by singlets and that causing the irreversible coagulation of the suspension. The stable clusters can appear as a result of a competition between a short range attraction and a long range repulsion between colloidal particles or due to reversible flocculation in the shallow secondary potential well

Study of the flow field through the wall of a Diesel particulate filter using Lattice Boltzmann Methods

Contamination is becoming an important problem in great metropolitan areas. A large portion of the contaminants is emitted by the vehicle fleet. At European level, as well as in other economical areas, the regulation is becoming more and more restrictive. Euro regulations are the best example of this tendency. Specially important are the emissions of nitrogen oxide (NOx) and Particle Matter (PM). Two different strategies exist to reduce the emission of pollutants. One of them is trying to avoid their creation. Modifying the combustion process by means of different fuel injection laws or controlling the air regeneration are the typical methods. The second set of strategies is focused on the contaminant elimination. The NOx are reduced by means of catalysis and/or reducing atmosphere, usually created by injection of urea. The particle matter is eliminated using filters. This thesis is focused in this matter. Most of the strategies to reduce the emission of contaminants penalise fuel consumption. The particle filter is not an exception. Its installation, located in the exhaust duct, restricts the pass of the air. It increases the pressure along the whole exhaust line before the filter reducing the performance. Optimising the filter is then an important task. The efficiency of the filter has to be good enough to obey the contaminant normative. At the same time the pressure drop has to be as low as possible to optimise fuel consumption and performance. The objective of the thesis is to find the relation between the micro-structure and the macroscopic properties. With this knowledge the optimisation of the micro-structure is possible. The micro-structure of the filter mimics acicular mullite. It is created by procedural generation using random parameters. The relation between micro-structure and the macroscopic properties such as porosity and permeability are studied in detail. The flow field is solved using LabMoTer, a software developed during this thesis. The formulation is based on Lattice Botlzmann Methods, a new approach to simulate fluid dynamics. In addition, Walberla framework is used to solve the flow field too. This tool has been developed by Friedrich Alexander University of Erlangen Nürnberg. The second part of the thesis is focused on the particles immersed into the fluid. The properties of the particles are given as a function of the aerodynamic diameter. This is enough for macroscopic approximations. However, the discretization of the porous media has the same order of magnitude than the particle size. Consequently realistic geometry is necessary. Diesel particles are aggregates of spheres. A simulation tool is developed to create these aggregated using ballistic collision. The results are analysed in detail. The second step is to characterise their aerodynamic properties. Due to the small size of the particles, with the same order of magnitude than the separation between molecules of air, the fluid can not be approximated as a continuous medium. A new approach is needed. Direct Simulation Monte Carlo is the appropriate tool. A solver based on this formulation is developed. Unfortunately complex geometries could not be implemented on time. The thesis has been fruitful in several aspects. A new model based on procedural generation has been developed to create a micro-structure which mimics acicular mullite. A new CFD solver based on Lattice Boltzmann Methods, LabMoTer, has been implemented and validated. At the same time it is proposed a technique to optimized setup. Ballistic agglomeration process is studied in detail thanks to a new simulator developed ad hoc for this task. The results are studied in detail to find correlation between properties and the evolution in time. Uncertainty Quantification is used to include the Uncertainty in the models. A new Direct Simulation Monte Carlo solver has been developed and validated to calculate rarefied flow.La contaminación se está volviendo un gran problema para las grandes áreas metropolitanas, en gran parte debido al tráfico. A nivel europeo, al igual que en otras áreas, la regulación es cada vez más restrictiva. Una buena prueba de ello es la normativa Euro de la Unión Europea. Especialmente importantes son las emisiones de óxidos de nitrógeno (NOx) y partículas (PM). La reducción de contaminantes se puede abordar desde dos estrategias distintas. La primera es la prevención. Modificar el proceso de combustión a través de las leyes de inyección o controlar la renovación de la carda son los métodos más comunes. La segunda estrategia es la eliminación. Se puede reducir los NOx mediante catálisis o atmósfera reductora y las partículas mediante la instalación de un filtro en el conducto de escape. La presente tesis se centra en el estudio de éste último. La mayoría de as estrategias para la reducción de emisiones penalizan el consumo. El filtro de partículas no es una excepción. Restringe el paso de aire. Como consecuencia la presión se incrementa a lo largo de toda la línea reduciendo las prestaciones del motor. La optimización del filtro es de vital importancia. Tiene que mantener su eficacia a la par que que se minimiza la caída de presión y con ella el consumo de combustible. El objetivo de la tesis es encontrar la relación entre la miscroestructura y las propiedades macroscópicas del filtro. Las conclusiones del estudio podrán utilizarse para optimizar la microestructura. La microestructura elegida imita los filtros de mulita acicular. Se genera por ordenador mediante generación procedimental utilizando parámetros aleatorios. Gracias a ello se puede estudiar la relación que existe entre la microestructura y las propiedades macroscópicas como la porosidad y la permeabilidad. El campo fluido se resuelve con LabMoTer, un software desarrollado en esta tesis. Está basado en Lattice Boltzmann, una nueva aproximación para simular fluidos. Además también se ha utilizado el framework Walberla desarrollado por la universidad Friedrich Alexander de Erlangen Nürnberg. La segunda parte de la tesis se centra en las partículas suspendidas en el fluido. Sus propiedades vienen dadas en función del diámetro aerodinámico. Es una buena aproximación desde un punto de vista macroscópico. Sin embargo éste no es el caso. El tamaño de la discretización requerida para calcular el medio poroso es similar al tamaño de las partículas. En consecuencia se necesita simular geometrías realistas. Las partículas Diesel son agregados de esferas. El proceso de aglomeración se ha simulado mediante colisión balística. Los resultados se han analizado con detalle. El segundo paso es la caracterización aerodinámica de los aglomerados. Debido a que el tamaño de las partículas precursoras es similar a la distancia entre moléculas el fluido no puede ser considerado un medio continuo. Se necesita una nueva aproximación. La herramienta apropiada es la Simulación Directa Monte Carlo (DSMC). Por ello se ha desarrollado un software basado en esta formulación. Desafortunadamente no ha habido tiempo suficiente como para implementar condiciones de contorno sobre geometrías complejas. La tesis ha sido fructífera en múltiples aspectos. Se ha desarrollado un modelo basado en generación procedimental capaz de crear una microestructura que aproxime mulita acicular. Se ha implementado y validado un nuevo solver CFD, LabMoTer. Además se ha planteado una técnica que optimiza la preparación del cálculo. El proceso de aglomeración se ha estudiado en detalle gracias a un nuevo simulador desarrollado ad hoc para esta tarea. Mediante el análisis estadístico de los resultados se han planteado modelos que reproducen la población de partículas y su evolución con el tiempo. Técnicas de Cuantificación de Incertidumbre se han empleado para modelar la dispersión de datos. Por último, un simulador basadoLa contaminació s'està tornant un gran problema per a les grans àrees metropolitanes, en gran part degut al tràfic. A nivell europeu, a l'igual que en atres àrees, la regulació és cada volta més restrictiva. Una bona prova d'això és la normativa Euro de l'Unió Europea. Especialment importants són les emissions d'òxits de nitrogen (NOX) i partícules (PM). La reducció de contaminants se pot abordar des de dos estratègies distintes. La primera és la prevenció. Modificar el procés de combustió a través de les lleis d'inyecció o controlar la renovació de la càrrega són els mètodos més comuns. La segona estratègia és l'eliminació. Se pot reduir els NOX mediant catàlisis o atmòsfera reductora i les partícules mediant l'instalació d'un filtre en el vas d'escap. La present tesis se centra en l'estudi d'este últim. La majoria de les estratègies per a la reducció d'emissions penalisen el consum. El filtre de partícules no és una excepció. Restringix el pas d'aire. Com a conseqüència la pressió s'incrementa a lo llarc de tota la llínea reduint les prestacions del motor. L'optimisació del filtre és de vital importància. Ha de mantindre la seua eficàcia a la par que que es minimisa la caiguda de pressió i en ella el consum de combustible. L'objectiu de la tesis és trobar la relació entre la microescritura i les propietats macroscòpiques del filtre. Les conclusions de l'estudi podran utilisar-se per a optimisar la microestructura. La microestructura elegida imita els filtres de mulita acicular. Se genera per ordenador mediant generació procedimental utilisant paràmetros aleatoris. Gràcies ad això es pot estudiar la relació que existix entre la microestructura i les propietats macroscòpiques com la porositat i la permeabilitat. El camp fluït se resol en LabMoTer, un software desenrollat en esta tesis. Està basat en Lattice Boltzmann, una nova aproximació per a simular fluïts. Ademés també s'ha utilisat el framework Walberla, desentollat per l'Universitat Friedrich Alexander d'Erlangen Nürnberg. La segona part de la tesis se centra en les partícules suspeses en el fluït. Les seues propietats venen donades en funció del diàmetro aerodinàmic. És una bona aproximació des d'un punt de vista macroscòpic. No obstant este no és el cas. El tamany de la discretisació requerida per a calcular el mig porós és similar al tamany de les partícules. En conseqüència es necessita simular geometries realistes. Les partícules diésel són agregats d'esferes. El procés d'aglomeració s'ha simulat mediant colisió balística. Els resultats s'han analisat en detall. El segon pas és la caracterisació aerodinàmica dels aglomerats. Degut a que el tamany de les partícules precursores és similar a la distància entre molècules el fluït no pot ser considerat un mig continu. Se necessita una nova aproximació. La ferramenta apropiada és la Simulació Directa Monte Carlo (DSMC). Per això s'ha desenrollat un software basat en esta formulació. Malafortunadament no ha hagut temps suficient com per a implementar condicions de contorn sobre geometries complexes. La tesis ha segut fructífera en múltiples aspectes. S'ha desenrollat un model basat en generació procedimental capaç de crear una microestructura que aproxime mulita acicular. S'ha implementat i validat un nou solver CFD, LabMoTer. Ademés s'ha plantejat una tècnica que optimisa la preparació del càlcul. El procés d'aglomeració s'ha estudiat en detall gràcies a un nou simulador desenrollat ad hoc per ad esta tasca. Mediant l'anàlisis estadístic dels resultats s'han plantejat models que reproduixen la població de partícules i la seua evolució en el temps. Tècniques de Quantificació d'Incertea s'han empleat per a modelar la dispersió de senyes. Per últim, un simulador basat en DSMC s'ha desenrollat per a calcular fluïts rarificats.García Galache, JP. (2017). Study of the flow field through the wall of a Diesel particulate filter using Lattice Boltzmann Methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90413TESI

Magnetism, FeS colloids, and Origins of Life

A number of features of living systems: reversible interactions and weak bonds underlying motor-dynamics; gel-sol transitions; cellular connected fractal organization; asymmetry in interactions and organization; quantum coherent phenomena; to name some, can have a natural accounting via $physical$ interactions, which we therefore seek to incorporate by expanding the horizons of `chemistry-only' approaches to the origins of life. It is suggested that the magnetic 'face' of the minerals from the inorganic world, recognized to have played a pivotal role in initiating Life, may throw light on some of these issues. A magnetic environment in the form of rocks in the Hadean Ocean could have enabled the accretion and therefore an ordered confinement of super-paramagnetic colloids within a structured phase. A moderate H-field can help magnetic nano-particles to not only overcome thermal fluctuations but also harness them. Such controlled dynamics brings in the possibility of accessing quantum effects, which together with frustrations in magnetic ordering and hysteresis (a natural mechanism for a primitive memory) could throw light on the birth of biological information which, as Abel argues, requires a combination of order and complexity. This scenario gains strength from observations of scale-free framboidal forms of the greigite mineral, with a magnetic basis of assembly. And greigite's metabolic potential plays a key role in the mound scenario of Russell and coworkers-an expansion of which is suggested for including magnetism.Comment: 42 pages, 5 figures, to be published in A.R. Memorial volume, Ed Krishnaswami Alladi, Springer 201