<p><strong><em>Dedicated to the loving memory of my mother and father,</em></strong></p> <p><strong><em>Zohreh Hojati & Mostafa Emami</em></strong></p> / <p>Commercial interests in polymeric foams continue to increase due to their unique physical characters and the new emerging applications for foamed materials. This thesis investigates the foam development process under non-pressurized conditions as applicable to rotational molding to elucidate the underlying mechanisms in the bubble transformation process and provide an accurate basis for predicting the morphological structure and macroscopic properties of the foamed materials. It was found that the foaming mechanism is comprised of four distinct stages: two stages of bubble nucleation, primary and secondary nucleation, followed by bubble growth and bubble coalescence/shrinkage. Following the nucleated bubbles during the foaming process revealed that primary nucleation was the controlling stage in determining the final cellular structure. Growth and coalescence mechanisms were dynamically active and competed during both heating and cooling cycles.</p> <p>The influence of the rheological properties on the rate of nucleation and the bubble growth mechanism were investigated. Morphological analysis was used to determine the rheological processing window in terms of shear viscosity, elastic modulus, melt strength and strain-hardening, intended for the production of foams with greater foam expansion, increased bubble density and reduced bubble size. Visualization experiments and theoretical predictions showed that higher viscosity could impede the number of nuclei generated in the foaming system. A bubble growth model and simulation scheme was also developed to describe the bubble growth phenomena that occurred in non-pressurized foaming systems. It was verified that the viscous bubble growth model was capable of depicting the growth behaviors of bubbles under various processing conditions.</p> / Doctor of Philosophy (PhD)

Cette thèse est consacrée à l’étude mathématiques et numérique d’un problème physique issu de la volcanologie. On s’intéresse à la modélisation polydisperse de croissance de bulles par exsolution, décompression et coalescence. Un modèle de croissance polydisperse a été proposé dans la litérature, mais ne prenait en compte que le volume des bulles, ce qui restreint le domaine d’application car la croissance par exsolution dépend également de la masse d’eau présente dans la bulle. Pour améliorer ce modèle, nous sommes parti d’une description monodisperse adimensionnelle de la croissance d’une bulle par décompression et exsolution, donnée par le couplage de deux EDO et une EDP. Un code numérique est proposé pour résoudre le problème monodisperse et est actuellement utilisé. Après avoir validé numériquement ce code et considéré plusieurs cas limites, nous avons étudié les solutions du problème et défini une approximation du flux qui nous permet de découpler le système d’équations. Ensuite, nous avons étendu le modèle polydisperse de une à deux dimensions. Une résolution de la coalescence est proposée et couplée avec le modèle de croissance polydisperse. La résolution de la coalescence est confrontée à d’autres schémas numériques en une et deux dimensions afin de valider le schéma numérique proposé. Les premiers test numériques appliqués au problème physique donnent de bon résultats. / This thesis is devoted to mathematical and numerical study of a physical problem coming from volcanology. We look at the polydisperse modeling of bubbles growth by exsolution, decompression and coalescence. A polydisperse growth model has been proposed in literature, but it takes into accountonly the volume of bubbles, which restrict the application field, because growth by exsolution also depends on the water mass in the bubbles. In order to upgrade this model, we start with a non-dimensional monodisperse description of the bubble growth by decompression and exsolution, given by a coupled ODE system and a PDE. A numerical code is proposed to solve the monodisperse problem and is currently used. After validating this code numerically and considering several limit cases, we studied the solutions of the system and defined a flux approximation to decouple the equations system. Next, we extend the polydispers model from one to two dimensions, the volume and the water mass of bubbles. A resolution of coalescence is proposed and coupled with the polydisperse growth model. The resolution of coalescence is confronted with others numericals schemes in one and two dimensions in order to validate the proposed numerical scheme. The first numerical tests applied to a physical problem give good results.

Part I: The present work is to calculate dimensionless three-dimensional universal phase and lift force diagrams for a microbubble or pendant drop in a static liquid on a solid surface or orifice. Studying microbubble dynamics is important due to its controlling mass, momentum, energy and concentration transfer rates encountered in micro- and nano-sciences and technologies. In this work, dimensionless phase and force diagrams are presented by applying an equation for microbubble shape to accuracy of the second order of small Bond number provided by O¡¦Brien (1991). Two dimensionless independent parameters, Bond number and contact angle (or base radius), are required to determine dimensionless phase and force diagrams governing static and dynamic states of a microbubble. The phase diagram divides the microbubble surface into three regions, the apex to inflection, inflection to neck, and neck to the edge of microbubble. The growth, collapse, departure and entrapment of a microbubble on a surface thus can be described. The lift forces include hydrostatic buoyancy, difference in gas and hydrostatic pressures at the microbubble base, capillary pressure and surface tension resulted from variation of circumference. The force to attach the microbubble to solid surface is the surface tension resulted from variation of circumference, which is not accounted for in literature. Adjusting the base radius to control static and dynamic behaviors of a microbubble is more effective than Bond number. Part II: Controlling states and growth of a microscale bubble (or pendant drop) in a static liquid on a surface by introducing general phase diagrams is proposed. Microbubbles are often used to affect transport phenomena in micro- and nano-technologies. In this work, a general phase diagram is provided by applying a perturbation solution of Young-Laplace equation for bubble shape with truncation errors of the second power of small Bond number. The three-dimensional phase diagram for a given Bond number is uniquely described by the dimensionless radius of curvature at the apex, contact angle and base radius of the microbubble. Provided that initial and end states are chosen, adjusting two of them gives the desired states and growth, decay and departure of the bubble described by path lines in the phase diagram. A universal three-dimensional phase diagram for a microbubble is also introduced.