Les microgels sont des particules colloïdales polymères faiblement réticulées, capables de se gonfler d’un solvant, de se déformer et de s’adsorber à des interfaces liquides. Parmi eux, les microgels de poly(N-isopropylacrylamide) (pNIPAM) sont thermosensibles et présentent une contraction en volume lorsque la température est supérieure à la température de transition de phase volumique, notée VPTT. De précédents travaux ont montré leur capacité à stabiliser des émulsions de Pickering : les émulsions peuvent être stables à des températures inférieures à la VPTT et être déstabilisées sur demande au-dessus de celle-ci. Afin d’approfondir la compréhension du mécanisme de stabilisation des émulsions, nous discutons le rôle de la structure des microgels sur leur adsorption, leur organisation à des interfaces modèles, les propriétés mécaniques des interfaces et les propriétés des émulsions résultantes en terme de stabilité (cinétique, mécanique) et de propriétés d’écoulement. Le rôle de la réticulation, de la présence de charges et de la taille est étudié pour le modèle du pNIPAM. Nous démontrons le lien existant entre conformation et propriétés macroscopiques des émulsions. En prenant en considération à la fois la structure des microgels et les procédés de formulation (voies d’émulsification), des émulsions avec des propriétés rhéologiques et des états de floculation variés peuvent être obtenues. Fort de ces connaissances, les concepts établis à l’aide des microgels de pNIPAM seront généralisés à d’autres familles de microgels, d’une part des systèmes biocompatibles avec des dérivés comportant des chaînes pendantes oligo(éthylène oxyde), d’autre part des microgels sensibles à des stimuli biologiques tels que la reconnaissance de sucre. / Microgels are soft and deformable colloidal particles that are swollen by a solvent and display the ability to deform and adsorb at liquid interfaces. The poly(N-isopropylacrylamide) (pNIPAM) microgels are thermo-sensitive and exhibit a volume contraction when the temperature is raised above the volume phase transition temperature (VPTT). These particle have shown high potential as Pickering emulsions stabilizers: emulsions could be stable at ambiant temperature, below the VPTT and destabilize on-demand above it. To get insight into understanding of the emulsion stabilization mechanism, we systematically discuss the relation between the microgel structure, their adsorption, their organization at model interfaces, the viscoelastic behavior of the interface and the resulting emulsions properties in terms of stability and flow behavior. In the present work, using pNIPAM as model microgels, we investigate the effect of their cross-linking density, their size and their charge density. Varying the microgels structures and the formulation conditions (emulsification process), we could control the emulsions flocculation state and rheological behavior. Based on this knowledge, we propose to synthesize new biocompatible microgels as emulsion stabilizers. Using ligand-modified microgels sucrose-sensitive emulsions may be obtained, enlarging the possible application domains.

Determination of the relation between the bulk or rheological properties of a particle suspension and its microscopic structure is an old and important problem in physical science. In general, the rheology of particle suspension is quite complex, and the problem becomes even more complicated if the suspending particle is deformable. Despite these difficulties, a large number of theoretical and experimental investigations have been devoted to the analysis and prediction of the rheological behavior of particle suspensions. However, among these studies there are very few investigations that focus on the role of particle deformability. A novel method for full coupling of the fluid-solid phases with sub-grid accuracy for the solid phase is developed. In this method, the flow is computed on a fixed regular 'lattice' using the lattice Boltzmann method (LBM), where each solid particle, or fiber, is mapped onto a Lagrangian frame moving continuously through the domain. The motion and orientation of the particle are obtained from Newtonian dynamics equations. The deformable particle is modeled by the lattice-spring model (LSM).The fiber deformation is calculated by an efficient flexible fiber model. The no-slip boundary condition at the fluid-solid interface is based on the external boundary force (EBF) method. This method is validated by comparing with known experimental and theoretical results. The fiber simulation results show that the rheological properties of flexible fiber suspension are highly dependent on the microstructural characteristics of the suspension. It is shown that fiber stiffness (bending ratio BR) has strong impact on the suspension rheology in the range BR < 3. The relative viscosity of the fiber suspension under shear increases significantly as BR decreases. Direct numerical simulation of flexible fiber suspension allows computation of the primary normal stress difference as a function of BR. These results show that the primary normal stress difference has a minimum value at BR ∼ 1. The primary normal stress differences for slightly deformable fibers reaches a minimum and increases significantly as BR decreases below 1. The results are explained based on the Batchelor's relation for non-Brownian suspensions. The influence of fiber stiffness on the fiber orientation distribution and orbit constant is the major contributor to the variation in rheological properties. A least-squares curve-fitting relation for the relative viscosity is obtained for flexible fiber suspension. This relation can be used to predict the relative viscosity of flexible fiber suspension based on the result of rigid fiber suspension. The unique capability of the LBM-EBF method for sub-grid resolution and multiscale analysis of particle suspension is applied to the challenging problem of platelet motion in blood flow. By computing the stress distribution over the platelet, the "blood damage index" is computed and compared with experiments in channels with various geometries [43]. In platelet simulation, the effect of 3D channel geometry on the platelet activation and aggregation is modeled by using LBM-EBF method. Comparison of our simulations with Fallon's experiments [43] shows a similar pattern, and shows that Dumont's BDI model [40] is more appropriate for blood damage investigation. It has been shown that channels with sharp transition geometry will have larger recirculation areas with high BDI values. By investigating the effect of hinge area geometry on BDI value, we intend to use this multiscale computational method to optimize the design of Bileaflet mechanical heart valves. Both fiber simulations and platelet simulations have shown that the novel LBM-EBF method is more efficient and stable compare to the conventional numerical methods. The new EBF method is a two-Cway coupling method with sub-grid accuracy which makes the platelet simulations possible. The LBM-EBF is the only method to date, to the best of author's knowledge, that can simulate suspensions with large number of deformable particles under complex flow conditions. It is hoped that future researchers may benefit from this new method and the algorithms developed here.