Suspensions floculées modèles : de la caractérisation à la modélisation

Abstract : Suspensions of colloidal particles suspended in a Newtonian fluid are encountered in many fields of daily life or industrial sectors. Complex behavior of colloidal suspensions comes from the interplay between hydrodynamical and several physico-chemical interactions between the particles. Even if a lot of works have been devoted to these topics, the complex relationships between the interparticle forces and the suspension's overall macroscopic behavior are far from being fully understood. This work aims to understand and characterize, at different scales, and through several techniques, the link between the flocculation state and some rheological properties (elastic modulus and yield stress) of colloidal suspensions.As in real system diversity in sizes, shapes, chemical constituents combined with chemical reaction make the system hard to control we work with model suspensions (spherical and crushed silica particles suspended in water).The suspensions being highly thixotropic, we characterize the evolution of their rheological properties (elastic modulus: G’ and yield stress: τc) with time.We observe for both materials that yield stress and elastic modulus increase with volume fraction, ionic strength and resting time.The two quantities G’ and τc depend on the microstructure ie the floculation state but are linked to two different kinds of response when a mechanical loading is apply to the system. Of course, both quantities depend on the flocculation state of the system but there is not clear evidence that these two quantities must be linked one to the other. However all the data G’=f (τc) collapse onto a simple curve for a given type and a given size of particle. This result proves that elastic shear modulus can be used as a “structural parameter” to account for the influence of flocculation on the yield stress.Moreover we focus on the effect of particle size on the measured properties and we observe that the yield stress scales with the inverse of the squared particle size and the elastic modulus with the inverse of particle size. The models existing in literature do not predict this scaling which would suggest that the interactions controlling macroscopic behaviour are still not fully understood.The observations performed with cofocal microscopy do not reveal any structural organisations when suspensions are left at rest. Then thixotropy of suspensions is assumed to be linked to nano scale change in microstructure. But different microstructure are observed regarding the suspension formulation. A primary microstructure is instantaneously formed once preshear is stopped depending on the intensity of interactions and volume fraction. After that the evolution of rheometric quantities with resting time are attributed to very local rearrangement not detectable with microscopy
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Jennifer Fusier. Suspensions floculées modèles : de la caractérisation à la modélisation. Matériaux. Université Paris-Est, 2016. Français. ⟨NNT : 2016PESC1114⟩. ⟨tel-01545550⟩

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