Single particle TIRM experiments measure particle-surface separation distance by tracking scattered intensities. The scattered light is generated by an evanescent wave interacting with a levitating microsphere. The exponential decay of the evanescent wave, normal to the surface, results in scattered intensities that vary with separation distance. Measurement of the separation distance allows us to calculate the total potential energy profile acting on the particles. These experiments have been shown to exhibit nanometer spatial resolution and the ability to detect potentials on the order of kT with no external treatment of the particle. We find that the separation distance is a function of the decay of the evanescent wave and the size of the sphere. Different sizes of spheres, located the same distance from the surface, exhibit varying scattered intensity distributions. Single particles have been studied extensively but multiple particle experiments are needed for studies of more complex systems and surfaces. Increasing the number of colloidal particles in a TIRM experiment greatly increases the complexity of the system. Calculation of separation distances and potentials over a large group of microspheres requires that the spheres display a uniform stuck-particle intensity distribution. But, for large numbers of particles, this is not the case. In some instances, stuck-particle intensities can vary more than an order of magnitude. This research involves creating a mathematical model to study scattered intensity distributions for a large size range of polystyrene microspheres. The model is based on basic Mie theory. We compare the theoretically simulated results to the experimentally obtained results and find that scattered intensity variations in multiple particle TIRM experiments are attributed to particle polydispersity (particle size variation). This is a very important result because we know that if we can maintain a relatively uniform particle size distribution, then we will see a relatively uniform stuck-particle intensity distribution. The model can then be used to select a size range of microspheres that will exhibit a more uniform distribution so as to increase the sensitivity and feasibility of multiple particle TIRM.

<P> Polymers are one of the most prevalent types of molecules in modern life. These long macromolecules make up everything from DNA to plastics to Jell-0™. An interesting class of polymers are block copolymers, which are composed of two (or more) chains, or blocks, of chemically distinct monomers covalently bonded end-to-end to form a single polymer. Different types of polymers tend to avoid each other, but since block copolymers are joined together the polymer species can not macroscopically phase separate. Instead, they separate on the scale of the size of the polymers, forming nanostructures. For a diblock copolymer melt, which is made from two types of polymers, these nanostructures can be, depending on the ratio of the length of one block to the other, spheres, cylinders, lamellae, or the more bizarre gyroid phase. </p> <P> Self-consistent field theory (SCFT) as formulated by Helfand in 1975 has in recent years been successfully applied to the study of the phase behaviour of diblock copolymers. However, most of the studies assume that the polymers are monodisperse, while almost all polymer melts are polydisperse. This work examines the effect of polydispersity in the block lengths on phase behaviour of diblock copolymer melts, by developing the SCFT for polydisperse block copolymers. The theory is examined using a perturbation method, as well as the random-phase approximation (RPA). The perturbation parameter is the ratio K of the weight-averaged molecular weight and the number-averaged molecular weight, which is a common measure of polydispersity. </p> <P> The results show polydispersity shifts the transition from a disordered phase to an ordered phase to a higher temperature, and increases the period of the nanostructures. It is also observed that polydispersity leads to larger non-lamellar phase regions in the phase diagrams. Results from the RPA also suggest that macrophase separation occurs for large polydispersities. </p> / Thesis / Master of Science (MSc)

I have studied the effects of polydispersity in systems of hard-sphere, colloidal PMMA particles with and without short-range attraction. In hard-sphere, colloidal systems, the parameter controlling phase behaviour is Ø , the volume fraction of colloids in the solvent. As Ø increases in polydisperse systems, theory predicts a transition from a single phase fluid to a fluid coexisting with a solid (crystal), to a fluid coexisting with multiple solid phases. By considering a volume fraction series of particles with 12% polydispersity and comparing the results with previous experimental results and predictions of the volume fractions within the coexistence regions, we concluded that this system may be exhibiting both fluid-solid and fluid-solid-solid behaviour within the experimental coexistence region. Theory also predicts that coexisting phases in polydisperse hard-sphere systems will fractionate: they will contain different particle size distributions (psds). This was investigated by directly measuring psds for one sample within the coexistence region at different time points. The results show that no statistically significant size fractionation was present after 28 days but by 120 days the solid phase contained a slightly narrower distribution of larger particles than the coexisting fluid phase. At higher than the coexistence region in this polydisperse system, the expected coexisting solids are not observed. Instead, a novel, non-equilibrium phase is present. The dynamics were probed using 3-dimensional dynamic light scattering, which confirmed the non-equilibrium nature of the phase: significant dynamical heterogeneities and anomalous ageing behaviour were present. These experimental dynamics are compared with dynamics obtained from simulations of different hard-sphere psds, including the experimental particle size distribution. The effect of adding a short-range, depletion attraction to a polydisperse colloidal system was systematically explored. Phase boundaries and the position of the metastable gas-liquid binodal were determined experimentally. The resultant phase diagram topology is qualitatively different to a system of monodisperse particles with the same attraction range. Furthermore, within the metastable binodal region, three-phase gas-liquid-solid samples were observed, which is neither an equilibrium or metastable state in monodisperse systems. The coexisting samples were again characterised using electron microscopy and also small-angle x-ray scattering, which revealed significant size fractionation in the gas-liquid separated samples but not in the samples which eventually crystallised.

No description available.

Over the past years, catastrophic dust explosion incidents have caused numerous injuries, fatalities and economical losses. Dust explosions are rapid exothermic reactions that take place when a combustible dust is mixed with air in the presence of an ignition source within a confined space. A variety of strategies are currently available to prevent dust explosion accidents. However, the recurrence of these tragic events confirms flaws in process safety for dust handling industries. This dissertation reports advances in different approaches that can be followed to prevent and mitigate dust explosions. For this research, a 36 L dust explosion vessel was designed, assembled and automated to perform controlled dust explosion experiments. First, we explored the effect of size polydispersity on the evolution of aluminum dust explosions. By modifying systematically the span of the particle size distribution we demonstrated the dramatic effect of polydispersity on the initiation and propagation of aluminum dust explosions. A semi-empirical combustion model was used to quantify the laminar burning velocity at varying particle size. Moreover, correlations between ignition sensitivity and rate of pressure rise with polydispersity were developed. Second, we analyzed the effect of particle size and crystalline levels in the decomposition reactions of explosion inhibitor agents (i.e., phosphates). We fractionated ammonium phosphate- monobasic (NH_4H_2PO_4) and dibasic ((NH_4)_2HPO_4) at different size ranges, and synthesized zirconium phosphate (Zr(HPO_4)_2·H_2O) at varying size and crystalline levels. Particle size was found to be crucial to improve the rate of heat absorption of each inhibitor. A simplified model was developed to identify factors dominating the efficiency of dust explosion inhibitors. Finally, we conducted computational fluid dynamic (CFD) simulations to predict overpressures in dust explosions vented through ducts in large scale scenarios. We particularly focused on the adverse effects caused by flow restrictions in vent ducts. Critical parameters, including ignition position, geometric configuration of the vent duct, and obstructions of outflow such as bends and panels were investigated. Comparison between simulation and experimental results elucidated potential improvements in available guidelines. The theoretical analyses complemented the experimental work to provide a better understanding of the effects of particle size on the evolution of dust explosions. Furthermore, the validation of advanced simulation tools is considered crucial to overcome current limitations in predicting dust explosions in large scale scenarios.

La coacervation complexe, une séparation de phase associative principalement induite par des interactions électrostatiques, entre la B-lactoglobuline (BLG, protéine animale) et la gomme d’Acacia (AG, polysaccharide végétal) a été étudiée dans ce travail. La plus grande difficulté pour comprendre la coacervation complexe au niveau moléculaire entre BLG et AG révèle être la polymolécularité élevée d’AG. A partir de là, la motivation principale de cette thèse était de comprendre et contrôler les interactions entre la BLG et les fractions moléculaires d’AG, FI (~88% d’AG) et FII (~10% d’AG) en utilisant la titration calorimétrique isotherme, la diffusion statique et dynamique de lumière, la mobilité électrophorétique, la Granulo-Polarimétrie et la microscopie optique. Une énergie d’interaction plus forte, une stoechiométrie d’association plus faible et ainsi une complexation favorable ont étés montrées entre la BLG et FII en relation avec l’accessibilité et la densité de charges plus élevées de FII. Les résultats majeurs de cette étude ont ainsi montré des rôles différents des fractions de l’AG dans la coacervation complexe avec la BLG / The complex coacervation mechanism, an associative phase separation mainly induced by electrostatic interactions, between ?-lactoglobulin (BLG, animal protein) and Acacia gum (AG, vegetal polysaccharide) was studied in this work. The most significant difficulty to understand complex coacervation between BLG and AG at the molecular level is the molecular weight polydispersity of AG. From there, the main motivation of this research was to better understand and control the interactions between BLG and the major molecular fractions of AG, FI (~88% of AG) and FII (~10% of AG) using isothermal titration calorimetry, static and dynamic light scattering, electrophoretic mobility, Granulo-Polarimetry and optical microscopy. Higher energy of interaction, lower stoichiometry of association and then favorable complexation were shown between BLG and FII in relation with higher accessibility and density of charges for FII. The major results of this study reveal then different roles of AG fractions in complex coacervation with BLG

L'objectif des travaux présentés dans ce mémoire est de caractériser la texture et la rhéologie des milieux granulaires en fonction de la polydispersité de forme et de taille des particules mais aussi en fonction de la cohésion interne des particules lorsqu'elles sont fragmentables, en vue d'une meilleure compréhension du procédé de fabrication des compacts. Pour ces études, nous avons utilisé la méthode de Dynamique des Contacts avec un modèle de rupture des particules. Nos analyses montrent que la polydispersité de forme peut jouer un rôle important lorsque la polydispersité de taille est faible. Par exemple, la résistance au cisaillement est presque indépendante de la polydispersité de taille mais elle diminue lorsque les formes deviennent plus irrégulières. La fragmentation des particules est fortement hétérogène en raison de la redistribution des contraintes au sein du matériau. Celui-ci garde la mémoire de la distribution granulométrique initiale, mais une classe intermédiaire de tailles se développe avec une distribution en loi de puissance des tailles et un rapport d'aspect moyen proche du nombre d'argent. Au cours du cisaillement, les déformations se localisent dans des bandes avec une compacité supérieure au reste du matériau en raison de la fragmentation des particules. La fragmentation tend également à annuler la dilatance et le pic de contrainte. / The general objective of the work presented in this dissertation is to investigate the microstructure and rheology of granular materials as a function of size and shape polydispersity of the particles, and to analyze the role of particle fragmentation as a function of the internal cohesion of particles in view of a better understanding of the manufacture process of powder compacts. For this work, we used numerical simulations by means of the Contact Dynamics method with a model of particle fracture. Our results suggest that shape polydispersity may play an important role when size polydispersity is low. For example, the shear strength is nearly independent of size distribution but declines when the particles becomes increasingly more irregular in shape. The process of particle fragmentation is found to be highly inhomogenious as a result of stress redistribution. The memory of the initial size distribution is mainly conserved in the class of larger particles while a class of intermediate sizes develops with a power-law size distribution and a mean aspect ratio close to the silver number independently of the initial size distribution. During shear, the strain is localized in shear bands of large solid fraction as a consequence of particle fragmentation and enhanced size polydispersity. Particle fragmentation tends to reduce dilatancy and the peak shear strength.

Metilaluminoxano, trietilalumínio e trimetilalumínio foram encapsulados em uma rede de sílica usando sol-gel não hidrolítico. Óxidos mistos foram produzidos através da combinação dos precursores da sílica com Mg(OEt)2, MgCl2 e MgCl2 .6H2O. Os xerogéis resultantes foram caracterizados por uma série de técnicas para determinar os elementos, estrutura, textura e características morfológicas dos cocatalisadores encapsulados. A natureza do cocatalisador afeta o tempo de geleificação. A quantidade de cocatalisador encapsulado não parece ser afetada pelo volume do alquilalumínio, mas a rede de sílica e a organização do xerogel foram, de acordo com os resultados do infravermelho e XRD. Uma forte redução da área específica ocorre comparando os sistemas encapsulados com as sílicas sintetizadas em condições semelhantes. Os alquilalumínios encapsulados foram avaliados na polimerização de eteno usando Cp2ZrCl2 como catalisador na presença e ausência de MAO externo. Os cocatalisadores encapsulados não foram ativos em uma relação Al/Zr de 250 a 500. Para os sistemas mistos combinando MAO homogêneo (externo) com MAO encapsulado, a adição do cocatalisador encapsulado parece não afetar a atividade catalítica. O peso molecular dos polímeros obtido usando catalisador encapsulado é inferior ao obtido a partir das reações homogêneas. No entanto, a polidispersão obtida nos polímeros usando cocatalisador encapsulado foi maior que a obtida a partir de sistemas homogêneos e impregnado. / Methylaluminoxane, tetraethylaluminum and trimethylaluminum were encapsulated within a silica-based material using a non-hydrolytic sol-gel process. Mixed oxides were also produced by combining silica precursors with Mg(OEt)2, MgCl2 and MgCl2 .6H2O. The resulting xerogels were characterized by a series of complementary techniques to determine the elemental, the structural, the textural and the morphological characteristics of the encapsulated cocatalysts. The nature of the alkyl aluminum strongly affects the gelification time. The amount of encapsulated cocatalyst did not appear to be affected by the alkylaluminum volume, but the silica network and the xerogel organization were, according to infrared and X-ray diffraction measurements. There was a large surface reduction for the encapsulated systems compared with the silica that was synthesized under the same conditions. The resulting supported alkylaluminum was evaluated in the polymerization of ethylene using Cp2ZrCl2 as the catalyst in the presence and the absence of external MAO. The encapsulated cocatalysts were not shown to be active in the Al/Zr range of 250-500. For the mixed systems that combined homogeneous MAO with encapsulated MAO, the addition of the encapsulated cocatalyst did not appear to affect the catalyst activity. The molecular weight of the polymers that were obtained by using the encapsulated cocatalyst was lower than the molecular weight of the polymers that were obtained by using the homogeneous catalyst. Nevertheless, the polydispersity of the polymers that were obtained in the presence of the encapsulated cocatalyst was greater than polydispersity of the polymers that were produced with the homogeneous catalyst or the impregnated MAO.

Nowadays, colloidal rods can be synthesized in large amounts. The rods are typically cylindrically and their length ranges from several nanometers to a few micrometers. In solution, systems of colloidal rodlike molecules or aggregates can form liquid-crystalline phases with long-range orientational and spatial order. In the present work, we investigate structure formation and fractionation in systems of rodlike colloids with the help of Monte Carlo simulations in the NPT ensemble. Repulsive interactions can successfully be mimicked by the hard rod model, which has been studied extensively in the past. In many cases, attractive interactions like van der Waals or depletion forces cannot be neglected, however. In the first part of this work, the phase behavior of monodisperse attractive rods is characterized for different interaction strengths. Phase diagrams as a function of rod length and pressure are presented. Most systems of synthesized mesoscopic rods have a polydisperse length distribution as a consequence of the longitudinal growth process of the rods. For many technical and research applications, a rather small polydispersity is desired in order to have well defined material properties. The polydispersity can be reduced by a spatial demixing (fractionation) of long and short rods. Fractionation and structure formation is studied in a tridisperse and a polydisperse bulk suspension of rods. We observe that the resulting structures depend distinctly on the interaction strength. The fractionation in the system is strongly enhanced with increasing interaction strength. Suspensions are typically confined in a container. We also examine the influence of adjacent substrates in systems of tridisperse and polydisperse rod suspensions. Three different substrate types are studied in detail: a planar wall, a corrugated substrate, and a substrate with rectangular cavities. We analyze the fluid structure close to the substrate and substrate controlled fractionation. The spatial arrangement of long and short rods in front of the substrate depends sensitively on the substrate structure and the pressure. Rods with a predefined length are segregated at substrates with rectangular cavities. / Kolloidale Stäbchen können mittlerweile in großen Mengen hergestellt werden. Die Form der Stäbchen ist in der Regel zylinderförmig und ihre Länge reicht von einigen Nanometern bis hin zu wenigen Mikrometern. Systeme aus kolloidalen stäbchenförmigen Molekülen oder Aggregaten können in Lösung flüssigkristalline Phasen mit langreichweitiger Orientierungs- und Raumordnung ausbilden. Im Rahmen dieser Arbeit werden Strukturbildung und Fraktionierung in Systemen aus stäbchenförmigen Kolloiden mittels Monte Carlo Simulationen im NPT Ensemble untersucht. Replusive Wechselwirkungen können erfolgreich durch harte Stäbchen modelliert werden. Dieses Modell wurde in der Vergangenheit bereits ausgiebig untersucht. Oft jedoch können attraktive Wechselwirkungen, wie z.~B. van der Waals- oder Depletionskräfte, nicht vernachlässigt werden. Im ersten Teil dieser Arbeit wird das Phasenverhalten von monodispersen attraktiven Stäbchen bei unterschiedlichen Wechselwirkungsstärken charakterisiert. Es werden Phasendiagramme bezüglich der Parameter Druck und Stäbchenlänge präsentiert. Die überwiegende Mehrzahl von Systemen aus synthetisierten mesoskopischen Stäbchen weist eine polydisperse Längenverteilung aufgrund des Längswachstums auf. Für eine Reihe technischer und wissenschaftlicher Anwendungen sind hingegen schmale Längenverteilungen wünschenswert, um wohl definierte Materialeigenschaften zu haben. Die Polydispersität kann durch räumliche Trennung (Fraktionierung) langer und kurzer Stäbchen reduziert werden. Fraktionierung und Strukturbildung werden in einer tridispersen und einer polydispersen Suspension untersucht. Wir beobachten, dass die entstehenden Strukturen ganz wesentlich von der Wechselwirkungsstärke abhängen. Der Grad der Fraktionierung wird durch Attraktivität stark erhöht. Suspensionen befinden sich typischerweise in Gefäsen. Wir untersuchen daher auch den Einfluss von begrenzenden Substraten auf Systeme aus tridispersen und polydispersen Stäbchensuspensionen. Drei verschiedene Substratstrukturen werden genauer betrachtet: Eine planare Wand, ein riefenförmiges Substrat und Substrate mit rechteckigen Aussparungen. Wir untersuchen die Flüssigkeitsstruktur in Substratnähe und substratinduzierte Fraktionierung. Die räumliche Anordnung von langen und kurzen Stäbchen hängt sehr sensibel von der Substratstruktur und dem Druck ab. Stäbchen mit einer festgelegten Länge werden an Substraten mit rechteckigen Aussparungen abgesondert.

Rheological properties of idealised models which exhibit all the non-Newtonian flow phenomenology commonly seen in dense suspensions are investigated by particulate-dynamics computer-simulations. The objectives of these investigations are: (i) to establish the origins of various aspects of dense suspension rheology such as shear-thinning, shear thickening and dilatancy; (ii) to elucidate the different regions of a typical dense suspension rheogram by examining underlying structures and shear induced anisotropies in kinetic energy, diffusivity and pressure; (iii) to investigate the scaling of the simplest idealised model suspension; i.e. the hard-sphere model in Newtonian media and its relationship to the isokinetic flow curves obtained through non-equilibrium molecular dynamics (NEMD) simulations; (iv) to preliminarily determine the effect of perturbations present in all real colloidal suspensions, namely particle size polydispersity and a slight 'softness' of the interparticle potential. Non-equilibrium isokinetic simulations have been performed upon ;systems of particles interacting through the classical hard-sphere potential and a perturbation thereof, in which the hard-core is surrounded by a 'slightly soft' repulsive skin. The decision to base the present work upon isokinetic studies was made in order to obtain a better under- standing of suspension rheology by making a direct connection with previous NEMD studies of thermal systemst(93). These studies have shown that the non-linear behaviour exhibited by these systems under shear is atttributable to a shear-induced perturbation of the equilibrium phase behaviour. The present study shows this behaviour to correspond to the high shear region of the generalised suspension flow curve.