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Die Dissertation verfolgt das Ziel, das Sedimentationsverhalten kolloidaler Suspensionen in Abhängigkeit von der Partikelkonzentration und den Partikelwechselwirkungen zu untersuchen und die Grenzen einer Sedimentationsanalyse im Zentrifugalkraftfeld auszuarbeiten. Um Effekte der Partikelagglomeration von den anderen Einflussfaktoren unterscheiden zu können, wurde besonderes Augenmerk auf die Gewährleistung der Suspensionsstabilität und deren messtechnischen Nachweis gerichtet. Im Submikrometerbereich gewinnen die zwischen den einzelnen Partikeln wirkenden nicht-hydrodynamischen Kräfte gegenüber Trägheits- oder Feldkräften an Bedeutung und können diese sogar dominieren. Infolgedessen ist der Zustand einer kolloidalen Suspension nicht mehr allein über die Partikelgrößenverteilung und die Partikelkonzentration definiert, sondern gleichfalls abhängig von den bestehenden Grenzflächeneigenschaften, die wiederum von solchen Eigenschaften der kontinuierlichen Phase wie pH-Wert oder Elektrolytgehalt abhängen. Im Zentrum der experimentellen Arbeiten stand die Untersuchung des Einflusses der Partikelkonzentration auf das Sedimentationsverhalten feinster Partikelsysteme. In der Literatur existiert keine einheitliche Beschreibung des Konzentrationseinflusses. In der Dissertation wurde die Suspensionsstabilität von Siliziumdioxid-Suspensionen gezielt beeinflusst, um deren Einfluss auf das Sedimentationsverhalten zu bewerten. Es wurde gezeigt, dass nur für instabile Suspensionen die erwartete Sinkgeschwindigkeitsüberhöhung existiert. Es wurde aber auch gezeigt, dass die bei unterschiedlichen Drehzahlen erhaltenen Sinkgeschwindigkeiten nicht über das Beschleunigungsvielfache skaliert werden können. Weiterhin wurde demonstriert, dass in solchen Systemen eine komplexe Abhängigkeit vom Feststoffgehalt existiert, weil die Koagulationsgeschwindigkeit auch von der Partikelkonzentration abhängt. Als geeignetes Kriterium zur Bewertung des Agglomerationszustandes monodisperser Partikelsysteme konnte die Breite der Sinkgeschwindigkeitsverteilung nachgewiesen werden. In Ergänzung zu den im Zusammenhang mit der Partikelkoagulation instabiler Suspensionen führen insbesondere bei elektrostatisch stabilisierten Partikelsystemen die mit der elektrochemischen Doppelschicht verbundenen Wechselwirkungen zu einer Beeinflussung der Suspensionsstruktur und folglich zu einer Beeinflussung hydrodynamischer Phänomene. Das wird in den bekannten Modellen nicht berücksichtigt und in der vorgelegten Arbeit am Sedimentationsverhalten monodisperser Partikelsysteme bei verschiedenen Elektrolytgehalten der kontinuierlichen Phase untersucht. Zu diesem Zweck wurden aus einer konzentrierten Suspension von 200 nm Partikeln und dem ihr zugehörigen Zentrifugat Suspensionsproben unterschiedlichen Feststoffgehaltes zubereitet. Mit einem Potenzansatz, wie er bereits von Richardson und Zaki für die Sedimentation von Mikrometerpartikeln verwendet wurde, konnte der experimentell bestimmte Zusammenhang zwischen der Sinkgeschwindigkeit und der Suspensionsporosität beschrieben werden. Dabei ist die Sedimentationsbehinderung umso ausgeprägter, je kleiner der Elektrolytgehalt, d.h. je ausgedehnter die Doppelschichtdicke ist. Die stark mit dem Elektrolytgehalt korrelierenden Werte für den Exponenten dieses Ansatzes liegen zwischen 5 und 10 im Gegensatz zu Richardson und Zaki von 4,65. Untersuchungen zum Konzentrationseinfluss mit gröberen Partikeln in elektrolytarmen Lösungsmitteln bestätigten die gegenüber dem Mikrometerbereich stärkere Sedimentationsbehinderung elektrostatisch stabilisierter kolloidaler Suspensionen. Zusätzlich zu den wissenschaftlichen Untersuchungen erfolgte die Konstruktion einer einfachen und robusten Sedimentationszelle mit optischer Messwerterfassung. Es wurde die Funktionstüchtigkeit eines zuverlässigen, partikelgrößenselektiven Messgerät z.B. für Industrielabore zur Optimierung von Sedimentationsprozessen demonstriert. / The sedimentation of stabilized suspensions strongly depends on the particle concentration. Even for dilute systems the decrease of the settling velocity compared to that of isolated particles is quite significant. This is primarily due to hydrodynamic interactions (HI), which are long-range interaction, since disturbances in the flow field decline reciprocally with the distance from the surface. The sedimentation of colloidal particles is additionally affected by the electric double layer surrounding them. The double layer leads to electro-viscous effects as well as to electrostatic repulsion between neighboring particles. Both phenomena can amplify the hydrodynamic hindrance to considerable extent. In this dissertation thesis an experimental study on the influence of double layer thickness on the sedimentation of charged colloidal particles is presented. Investigations were carried out using an optical centrifuge, in which the sedimentation velocity of monosized sub-micrometer silica particles were studied at different particle concentration and varying ionic strength. The results are discussed with regard to the applicability of theoretical models and (semi-)empirical approximations. Richardson and Zaki had determined a power law exponent of 4.65 for hard sphere systems in the 100 micrometer range. In the experimental work for sub-micrometer particles the exponent was determined between 5 (low double layer thickness at high electrolyte concentration) and 10 (high double layer thickness at low electrolyte concentration). Additionally a simple and robust device for gravitational sedimentation analysis by optical signal sensing was designed. It enables industrial application for the optimization of sedimentation processes.

Made available in DSpace on 2014-12-17T14:08:31Z (GMT). No. of bitstreams: 1 GustavoHVPP.pdf: 1012393 bytes, checksum: d85648d797f788ab43d046f3e32bca54 (MD5) Previous issue date: 2008-11-03 / The new oil reservoirs discoveries in onshore and ultra deep water offshore fields and complex trajectories require the optimization of procedures to reduce the stops operation during the well drilling, especially because the platforms and equipment high cost, and risks which are inherent to the operation. Among the most important aspects stands out the drilling fluids project and their behavior against different situations that may occur during the process. By means of sedimentation experiments, a correlation has been validated to determe the sedimentation particles velocity in variable viscosity fluids over time, applying the correction due to effective viscosity that is a shear rate and time function. The viscosity evolution over time was obtained by carrying out rheologic tests using a fixed shear rate, small enough to not interfere in the fluid gelling process. With the sedimentation particles velocity and the fluid viscosity over time equations an iterative procedure was proposed to determine the particles displacement over time. These equations were implemented in a case study to simulate the cuttings sedimentation generated in the oil well drilling during stops operation, especially in the connections and tripping, allowing the drilling fluid project in order to maintain the cuttings in suspension, avoiding risks, such as stuck pipe and in more drastic conditions, the loss of the well / As novas descobertas de reservat?rios de petr?leo em campos onshore e offshore em l?minas d ?guas ultra-profundas e de trajet?rias complexas demandam a otimiza??o dos processos de perfura??o para reduzir as opera??es de paradas durante a perfura??o de um po?o, especialmente devido ao elevado custo das plataformas, equipamentos e dos riscos que s?o inerentes ? opera??o. Dentre os aspectos mais importantes destaca-se o projeto de fluidos de perfura??o e o estudo de seu comportamento frente a diferentes situa??es que podem ocorrer durante o processo. Atrav?s de experimentos de sedimenta??o, foi validada uma correla??o para determina??o da velocidade de sedimenta??o de part?culas em fluidos de viscosidade vari?vel com o tempo, aplicando-se as devidas corre??es para viscosidade efetiva que ? fun??o da taxa de deforma??o e do tempo. A evolu??o da viscosidade com o tempo foi obtida atrav?s de ensaios reol?gicos utilizando uma taxa de deforma??o fixa, pequena o suficiente para n?o interferir no processo de gelifica??o do fluido. Com as equa??es de velocidade de sedimenta??o de part?culas e da viscosidade do fluido com o tempo foi proposto um procedimento iterativo capaz de determinar o deslocamento das part?culas com o tempo. Essas equa??es constitutivas foram aplicadas no estudo de caso para simula??o da sedimenta??o dos cascalhos gerados na perfura??o de um po?o de petr?leo durante paradas operacionais, especialmente as conex?es e manobras, possibilitando o projeto do fluido de perfura??o de maneira a manter os cascalhos em suspens?o, evitando riscos, como por exemplo, a pris?o da coluna de perfura??o e em condi??es mais dr?sticas, a perda do po?o

Equilibrium statistical mechanics has a remarkable property: the steady state probability distribution can be calculated by a procedure independent of the detailed dynamics of the system under consideration. The partition function contains the complete thermodynamics of the system. The calculation of the partition function itself might be a daunting task and one might need to resort to approximate methods in practice. But there is no problem in principle on how to do the statistical mechanics of a system that is at thermal equilibrium. Nonequilibrium statistical mechanics is a completely different story. There is no general formalism, even in principle, the application of which is guaranteed to yield the probability distribution, even for stationary states, without explicit consideration of the dynamics of the system. Instead, there are several methods of wide applicability drawn from experience which work for particular classes of systems. Frequently, one writes down phenomenological equations of motion based on general principles of conservation and symmetry and attempts to extract the dynamical response and correlations. The motivation for studying nonequilibrium systems is the very simple fact that they are ubiquitous in nature and exhibit very rich, diverse and often counter-intuitive phenomenon. We ourselves are an example of a very complex nonequilibrium system. This thesis examines three problems which illustrate the generic features of a typical driven system maintained out of thermal equilibrium. The first chapter provides a very brief discussion of nonequilibrium systems. We outline the tools that are commonly employed in the theoretical description of driven systems, and discuss the response of physical systems to applied perturbations. Chapter two considers a very simple model for a single self-propelled particle with an internal asymmetry, and nonequilibrium energy input in the form of Gaussianwhite noise. Our model connects three key nonequilibrium quantities – drift velocity, mean internal force and position-velocity correlations. We examine this model in detail and solve it using perturbative, numerical and exact methods. We begin chapter three with a brief introduction to the sedimentation of particle-fluid suspensions. Some peculiarities of low Reynolds number hydrodynamics are discussed with particular emphasis on the sedimentation of colloidal particles in a viscous fluid. We then introduce the problem of velocity fluctuations in steady sedi-mentation. The relevance of the current study to an earlier model and improvements made in the present work are then discussed. A physical understanding of our model and the conclusions that result from its analysis are an attempt to resolve the old problem of divergent velocity fluctuations in steadily sedimentating suspensions. The fourth chapter is a study to probe the nature of the fluctuations in a driven suspension of point-particles. Fluctuation relations that characterise large-deviations are a current topic of intense study. We show in this chapter that the random dynamics of suspended particles in a driven suspension occasionally move against the driving force, and that the probability of such rare events obeys a steady state fluctuation relation. In the final chapter, we summarise the models studied and point out the common features that they display. We conclude by pointing out some ways in which the problems discussed in this thesis can be extended upon in the future.