Ink-jet deposition is an emerging technology that provides a more efficient, economic, scalable method of manufacturing than other traditional additive techniques by laying down droplets layer by layer to build up 3-D objects. The focus of this thesis is to investigate the material interface evolution during the droplet deposition process, which holds the key to understanding the material joining process. Droplet deposition is a complicated process and can be broken down into droplet impingement dynamics and droplet hardening. This research focuses on the study of the interface dynamics of droplet impingement. In order to study the interface dynamics, a novel metric is developed to quantify the evolving geometry of the droplet interface in both 2-D and 3-D for single and multiple droplets respectively, by measuring the similarity between the evolving droplet geometry and a desired shape. With the developed shape metric, the underlying physics of the interface evolution for single droplet impingement are examined with simulations using an experimentally validated numerical model. Results show that the Weber number determines the best achievable shape and its timing during the droplet impingement when Ohnesorge number is smaller than 1, while the Reynolds number is the determining factor when Ohnesorge number is larger than 1. A regime map is constructed with the results and an empirical splash criterion to guide the choice of process parameters for given fluid properties in order to achieve the best shape without splash for single droplet impingement. In order to study the interface dynamics for multiple droplet interaction, which is computationally prohibitive for commercial software packages, an efficient numerical model is developed based on the Lattice Boltzmann (LB) method. A new LB formulation equivalent to the phase-field model is developed with consistent boundary conditions through a multiscale analysis. The numerical model is validated by comparing its simulation results with that of commercial software COMSOL and experimental data. Results show our LB model not only has significant improvement of computational speed over COMSOL but is also more accurate. Finally, the developed numerical solver is used to study the interface evolution of multiple droplet interaction with the aid of the 3-D shape metric proposed before. Simulations are performed on a wide range of impingement conditions for two-droplet, a-line-of-droplet, and an-array-of-droplet interactions. The underlying physics of the interface coalescence and breakup coupling with the impingement dynamics are examined. For line-droplet interaction, the strategy for achieving the equilibrium shape in the shortest time is studied. An important issue is discovered for array-droplet interaction, which is the air bubble formation during the droplet interaction. The mechanism for the air bubble formation is investigated and the strategy to avoid this undesirable effect is also suggested. This thesis has largely reduced the gap between basic science of studying droplet impingement dynamics and engineering application in inkjet deposition and provided preliminary insights on the material joining process for additive manufacturing.

Dans les procédés liés aux émulsions, des écoulements turbulents et polyphasiques entrent en jeu. De tels procédés apparaissent dans des domaines variés. Dans l'industrie agro-alimentaire, la production de lait fait intervenir un homogénéisateur à haute pression, et certains produits tels que la mayonnaise sont des émulsions stables. On trouve aussi des émulsions dans le domaine de la santé : elles assurent le bon fonctionnement de nos poumons, tandis que d'autres peuvent être injectées par voie parentérale en tant que médicaments. On les retrouve aussi dans les procédés de séparation, par exemple eau-brut de pétrole dans l'industrie pétrochimique. Dans tous les cas, la fragmentation et la coalescence des bulles et gouttes doivent être maîtrisées, car elles influencent directement la distribution en taille de la phase dispersée. La fréquence d'apparition de ces phénomènes peut être prédite en utilisant des modèles adaptés. Cependant, la présence de molécules tensioactives modifie grandement cette fréquence et par conséquent la distribution en taille en sortie du procédé. Or, ce type de molécules est présent dans quasiment tous les procédés polyphasiques. L'étude des effets des tensioactifs dans ces procédés s'est alors imposée. Dans un des plus récents modèles, les bulles ou gouttes sont considérées comme des oscillateurs forcés par la turbulence de l'écoulement environnant. Il est alors nécessaire de connaître à la fois la turbulence dans le voisinage de la goutte et les propriétés dynamique de la goutte. La première peut être déterminée expérimentalement. La réponse de la goutte au forçage est alors décrite comme une somme d'harmoniques sphériques dont la dynamique est décrite pour chaque mode par une pulsation et un coefficient d'amortissement. Cette thèse aborde l’étude des effets des tensioactifs sur ces deux grandeurs. Elle s’est déroulée en collaboration entre l'IMFT et le LGC, ce qui a permis d'associer les compétences de chaque laboratoire dans les domaines de la physico-chimie, de l'hydrodynamique des phases dispersées et des écoulements turbulents diphasiques. Le projet lors de cette thèse est d'étudier numériquement les effets des tensioactifs sur les échelles temporelles caractéristiques des oscillations, dans le cas où la goutte est immobile ou bien en mouvement dans un fluide externe. Une équation de transport des tensioactifs ainsi que l'effet Marangoni à l'interface ont été modelisés dans le code DIVA, et validés à l'aide de cas tests. Ensuite, des simulations de gouttes subissant des oscillations de forme suivant le mode 2 des harmoniques sphériques ont permis de décrire les effets des tensioactifs sur la dynamique des interfaces. Ils ont été validés par la théorie pour des oscillations linéaires. Le couplage entre le mouvement d'ascension et les oscillations de formes a aussi été étudié, afin de comprendre l'effet d'un fort effet Marangoni, généré par l'ascension de la goutte, sur les oscillations. Les viscosités de surface peuvent aussi influencer radicalement la dynamique interfaciale. Lors de cette thèse, une méthode a été développée et validée pour permettre à l'outil de simulation de prendre en compte des viscosités de surface en se basant sur le modèle de Boussinesq-Scriven. Leur effet sur la dynamique des oscillations de forme a été étudié. / In many industrial processes, such as high-pressure homogenisation or water-oil separation, turbulent and multiphase flows are involved. To optimize those processes, coalescence and fragmentation need to be controlled since they impact directly the size distribution of drops and bubbles. The occurrence of those phenomena can be predicted using adapted models. However, the presence of surfactant molecules at the interface between two fluid phases can change radically the phenomenology of drops break up and coalescence, and their effect has not been properly included in existing models yet. In one of the latest models, drops are considered as forced oscillators driven by the local turbulence of a flow. It is therefore required to know the local turbulence of the flow and the dynamic properties of the drop. It is possible to measure experimentally the local velocities in a flow to determine the local average turbulent Weber number, which represents the forcing term of the oscillator. The dynamic response of the drop is described as a series of oscillators, the principal mode of which is characterized by two time scales, the pulsation and damping coefficient. The goal of this PhD is to study numerically the impact of surfactants on the two time scales mentioned above, whether the drop is still or in motion in the surrounding fluid. An equation of transport of surfactants and the Marangoni effect at the interface have been implemented in the code, and validated. Simulations of a drop undergoing shape oscillations along its eigenmode n = 2 have allowed to study the effects of surfactants on the time scales of oscillations, which were validated with theoretical predictions for linear oscillations. Surface viscosities can also influence the time scales of oscillations. Therefore, a numerical method has been developped and validated to take surface viscosities into account. The stress jump is based on the expression of the Boussinesq-Scriven model. Their effect is studied on the dynamic of oscillations.

Regenerative chatter is a well-known machining problem that results in unstable cutting process, poor surface quality, reduced material removal rate and damage on the machine tool itself. Stability lobe diagrams supply stable depth of cut &amp / #8211 / spindle speed combinations and they can be used to avoid chatter. The main requirement for generating the stability lobe diagrams is the system dynamics information at the tool tip in the form of point frequency response function (FRF). In this work, an analytical model that uses structural coupling and modification methods for modeling the dynamics of spindle-holder-tool assemblies in order to obtain the tool point FRF is presented. The resulting FRF obtained by the model can be used in the existing analytical and numerical models for constructing the stability lobe diagrams. Timoshenko beam theory is used in the model for improved accuracy and the results are compared with those of Euler-Bernoulli beam theory. The importance of using Timoshenko beam theory in the model is pointed out, and the circumstances, under which the theory being used in the model becomes more important, are explained. The model is verified by comparing the results obtained by the model with those of a reliable finite element software for a case study. The computational superiority in using the model developed against the finite element software is also demonstrated. Then, the model is used for studying the effects of bearing and contact dynamics at the spindle-holder and holder-tool interfaces on the tool point FRF. Based on the results of the effect analysis, a new approach is suggested for the identification of bearing and interface parameters from experimental measurements, which is demonstrated on a spindle-holder-tool assembly. The model is also employed for studying the effects of design and operational parameters on the tool point FRF, from the results of which, suggestions are made regarding the design of spindles and selection of operational parameters. Finally, it is experimentally demonstrated that the stability lobe diagram of an assembly can be predicted pretty accurately by using the model proposed, and furthermore the stability lobe diagram can be modified in a predictable manner for improving chatter stability.

Today, nanotechnology has added a new dimension to materials technology by creating opportunities to significantly enhance the properties of existing conventional materials. Polymer nanocomposites belong to one such class of materials and even though they show tremendous promise for dielectric/electrical insulation applications (“nanodielectrics” being the buzzword), the understanding related to these systems is very premature. Considering the desired research needs with respect to some of the dielectric properties of polymer nanocomposites, this study attempts to generate an understanding on some of the existing issues through a systematic and detailed experimental investigation coupled with a critical analysis of the data. An epoxy based nanocomposite system is chosen for this study along with four different choices of nano-fillers, viz. TiO2, Al2O3, ZnO and SiO2. The focus of this study is on the properties of nanocomposites at low filler loadings in the range of 0.1 - 5% by weight and the properties under investigation are the permittivity/tan delta behaviors, DC volume resistivity, AC dielectric strength and electrical discharge resistant characteristics. Significant efforts have also been directed towards addressing the interface interaction phenomena in epoxy nanocomposites and their subsequent influence on the dielectric properties of the material. The accurate characterization of the dielectric properties for polymer nanocomposites depends on the dispersion of nanoparticles in the polymer matrix and to achieve a good dispersion of nanoparticles in the epoxy matrix for the present study, a systematic design of experiments (DOE) is carried out involving two different processing methods. Consequently, a laboratory based epoxy nanocomposite processing methodology is proposed in this thesis and this process is found to be successful in dispersing nanoparticles effectively in the epoxy matrix, especially at filler concentrations lower than 5% by weight. Nanocomposite samples for the study are prepared using this method and a rigorous conditioning is performed before the dielectric measurements. The dielectric properties of epoxy nanocomposites obtained in the present study show interesting and intriguing characteristics when compared to those of unfilled epoxy and microcomposite systems and few of the results are unique and advantageous. In an unexpected observation, the addition of nanoparticles to epoxy is found to reduce the value of nanocomposite real permittivity below that of unfilled epoxy over a wide range of frequencies. Similarly, it has been observed that irrespective of the filler type, tan delta values in the case of nanocomposites are either same or lower than the value of unfilled epoxy up to a filler loading of 5% by weight, depending on the frequency and filler concentration. In fact, the nanocomposite real permittivities and tan delta values are also observed to be lower as compared to the corresponding microcomposites of the same constituent materials at the same filler loading. In another significant result, enhancements in the electrical discharge resistant characteristics of epoxy nanocomposites (with SiO2/Al2O3 nanoparticles) are observed when compared to unfilled epoxy, especially at longer durations of discharge exposures. Contrary to these encouragements observed for few of the dielectric properties, the trends of DC volume resistivity and AC dielectric breakdown strength characteristics in epoxy nanocomposites are found to be different. Irrespective of the type of filler in the epoxy matrix, it has been observed that the values of both AC dielectric strengths and DC volume resistivities are lower than that of unfilled epoxy for the filler loadings investigated. The results mentioned above seem to suggest that there has to be an interaction between the nanoparticles and the epoxy chains in the nanocomposite and therefore, glass transition temperature (Tg) measurements are performed to characterize the interaction phenomena, if any. The results of Tg for all the investigated nanocomposites also show interesting trends and they are observed to be lower than that of unfilled epoxy up to certain nanoparticle loadings. This lowering of the Tg in epoxy nanocomposites was not observed in the case of unfilled and microcomposite systems thereby strongly confirming the fact that there exists an interaction between the epoxy chains and nanoparticles in the nanocomposite. Considering the variations obtained for the nanocomposite real permittivity and Tg with respect to filler loading, a dual nanolayer interface model is utilized to explain the interaction dynamics and according to the model, interactions between epoxy chains and nanoparticles lead to the formation of two epoxy nanolayers around the nanoparticle. Analysis shows that the characteristics of the interface region have a strong influence on the dielectric behaviors of the nanocomposites and the suggested interface model seems to fit the characteristics obtained for the different dielectric/electrical insulation properties rather well. Further investigations are performed to understand the nature of interaction between nanoparticles and epoxy chains through FTIR studies and results show that there is probably an occurrence of hydrogen bonding between the epoxide groups of the epoxy resin and the free hydroxyl (OH) groups present on the nanoparticle surfaces. The results obtained for the dielectric properties of epoxy nanocomposites in this study have widened the scope of applications of these functional materials in the electrical sector. The occurrence of lower values of real permittivity for nanocomposites is definitely unique and unexpected and this result has huge potential in electronic component packaging applications. Further, the advantages related to tan delta and electrical discharge resistance for these materials carry lot of significance since, electrical insulating materials with enhanced electrical aging properties can be designed using nanocomposite technology. Although the characteristics of AC dielectric strengths and DC volume resistivities are not found to be strictly advantageous for epoxy nanocomposites at the investigated filler loadings, these properties can be optimized when designing insulation systems for practical applications. In spite of all these advantages, serious and systematic research efforts are still desired before these materials can be successfully utilized in electrical equipment.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-06-29T13:52:59Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) tese_final.pdf: 5635071 bytes, checksum: b300efb627e9ece412ad5936ab67e8e2 (MD5) / Made available in DSpace on 2016-06-29T13:52:59Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) tese_final.pdf: 5635071 bytes, checksum: b300efb627e9ece412ad5936ab67e8e2 (MD5) Previous issue date: 2013-08-26 / CNPq / No presente trabalho duas classes de problemas são abordadas. Primeiramente, são apresentados estudos computacionais sobre o modelo O(n) de spins na rede quadrada, e em seguida apresentamos novas soluções exatas para a dinâmica de bolhas na célula de Hele-Shaw. O estudo do modelo O(n) é feito utilizando sua representação em laços (cadeias fechadas), a qual é obtida a partir de uma expansão para altas temperaturas. Nesse representação, a função de partição do modelo possui uma expansão diagramática em que cada termo depende do número e comprimento total de laços e do número de (auto)interseções entre esses laços. Propriedades críticas do modelo de laços O(n) são obtidas através de conceitos oriundos da teoria de percolação. Para executar as simulações Monte Carlo, usamos o eficiente algoritmo WORM, o qual realiza atualizações locais através do movimento da extremidade de uma cadeia aberta denominada de verme e não sofre com o problema de "critical slowing down". Para implementar esse algoritmo de forma eficiente para o modelo O(n) na rede quadrada, fazemos uso de um nova estrutura de dados conhecida como listas satélites. Apresentamos estimativas para o ponto crítico do modelo para vários valores de n no intervalo de 0 < n ≤ 2. Usamos as estatísticas de laços e vermes para extrair, respectivamente, os expoentes críticos térmicos e magnéticos do modelo. No estudo de dinâmica de interfaces, apresentamos uma solução exata bastante geral para um arranjo periódico de bolhas movendo-se com velocidade constante ao longo de uma célula de Hele-Shaw. Usando a periodicidade da solução, o domínio relevante do problema pode ser reduzido a uma célula unitária que contém uma única bolha. Nenhuma imposição de simetria sobre forma da bolha é feita, de modo que a solução é capaz de produzir bolhas completamente assimétricas. Nossa solução é obtida por métodos de transformações conformes entre domínios duplamente conexos, onde utilizamos a transformação de Schwarz-Christoffel generalizada para essa classe de domínios. / In this thesis two classes of problems are discussed. First, we present computational studies of the O(n) spin model on the square lattice and determine its critical properties, whereas in the second part of the thesis we present new exact solutions for bubble dynamics in a Hele-Shaw cell. The O(n) model is investigated by using its loop representation which is obtained from a high-temperature expansion of the original model. In this representation, the partition function admits an diagrammatic expansion in which each term depends on the number and total length of loops (closed graphs) as well as on the number of intersections between these loops. Critical properties of the O(n) model are obtained by employing concepts from percolation theory. To perform Monte Carlo simulations of the model, we use the WORM algorithm, which is an efficient algorithm that performs local updates through the motion of one of the ends (called head) of an open chain (called worm) and hence does not suffer from “critical slowing down”. To implement this algorithm efficiently for the O(n) model on the square lattice, we make use of a new data structure known as a satellite list. We present estimates for the critical point of the model for various values of n in the range 0 < n ≤ 2. We use the statistics about the loops and the worm to extract the thermal and magnetic critical exponents of the model, respectively. In our study about interface dynamics, we present a rather general exact solution for a periodic array of bubbles moving with constant velocity in a Hele-Shaw cell. Using the periodicity of the solution, the relevant domain of the problem can be reduced to a unit cell containing a single bubble. No symmetry requirement is imposed on the bubble shape, so that the solution is capable of generating completely asymmetrical bubbles. Our solution is obtained by using conformal mappings between doubly-connected domains and employing the generalized Schwarz-Christoffel formula for this class of domains.