Deformable particles migrate in a sheared suspension. A source of this phenomena would be the collective interactions between particles which rheologically means the emergence of normal stresses generated by direct contact between particles. In this thesis the particle normal stress has been experimentally investigated as an origin of migration of deformable particles. Non-Brownian droplets and microcapsules were used as a model system of deformable particles. A membrane emulsification process was adapted to produce uniform suspensions of microcapsules at high throughput. Then an automatic microfluidic system was developed to measure the elasticity of their shell. In microcapsule suspensions, it was found that the contact viscosities show a shear-thinning behavior. In high volume fraction, the contact viscosity, which originate from normal stress, is the principal contributor to the apparent viscosity. Migration and normal stress of buoyant particles were inferred by measuring the vertical concentration profile in viscous resuspension experiments in a vertical Couette cell. The normal stress varies linearly with the Shields number for droplets and non-linearly with a 0.7 power law for microcapsules as rigid particles. The dependency of the normal stress on the volume fraction is in agreement with the models proposed in the literature for solid particles. All of these results suggest that the migration of deformable particles in dense suspension under shear can be explained by the same rheological laws as for rigid particles, regardless of their deformability (for Ca < 0.4). Furthermore, in the case of capsules, these rheological laws are governed by the solid contacts between the particles.
Supervised by Hugues Bodiguel (LRP), Marc Léonetti (LRP) and Clément de Loubens (LRP).
Particles migrate in the transverse direction of the flow due to the existence of normal stress anisotropy in weakly viscoelastic liquids. We test the ability of theoretical predictions to predict the transverse velocity migration of particles in a confined Poiseuille flow according to the viscoelastic constitutive parameters of dilute polymer solutions. First, we carefully characterize the viscoelastic properties of two families of dilute polymer solutions at various concentrations using shear rheometry and capillary breakup experiments. Second, we develop a specific three-dimensional particle tracking velocimetry method to measure with a high accuracy the dynamics of particles focusing in flow for Weissenberg numbers Wi ranging from 10-2 and 10-1, and particle confinement β of 0.1 and 0.2. The results show unambiguously that the migration velocity scales as Wiβ2, as expected theoretically for weakly elastic flows of an Oldroyd-B liquid. We conclude that classic constitutive viscoelastic laws are relevant to predict particle migration in dilute polymer solutions whereas detailed analysis of our results reveals that theoretical models overestimate by a few tenths the efficiency of particle focusing.
Supervised by Hugues Bodiguel (LRP) and Clément de Loubens (LRP).Publications
Deformable particles such as cells, vesicles and microcapsules exhibit abundant spatiotemporal dynamics in flows. In particular, it is commonly accepted that the membrane mechanical properties and flow types govern these dynamics, for example global deformation and shape oscillation. There also exist locally self-organized shape modulations in response to the flows, for example wrinkling and breakup instabilities. The objective of this thesis is to understand the emergence of such instabilities on microcapsules . The challenge comes to the tunability and control of the membrane rheological properties. We first develop a new formulation of assembling microcapsules made of a thin membrane with widely tunable properties. We describe an original visualization set-up that images microcapsules in orthogonal views, allowing a 3D characterization of pattern formation and the first measurement of wrinkles wavelength. The wrinkling instability is characterized by various scaling laws to highlight the salient parameters. Especially, wrinkling pattern appears above a unique critical capillary number regardless of membrane properties. Wrinkles-to-folds transition is observed if the capillary number is greater than the second critical capillary number. However, under extremely high capillary number, microcapsules surface become stable again, prior to breakup. A phase diagram of capsules breakup in extensional flow is also established and compared to the case of droplets.
Supervised by Marc Léonetti (LRP), Marc Jaeger (M2P2) and Clément de Loubens (LRP).Publications
This work sought to determine the factors influencing mixing and mass transfer in the small intestine. Specifically, the work was focussed on the gut periphery (i.e. perivillous region) of the terminal ileum in the brushtail possum (Trichusurus vulpecula). The salient questions to answer were; 1. What are the microrheological properties and disposition of mucus in the perivillous space? 2. What are the disposition and movements of the mucosa and the associated villi during postprandial gut motility patterns of pendular contractions? 3. Are villi rigid structures during physiological levels of lumen flow? The following three main experimental works of this thesis were all conducted using live gut wall samples maintained ex vivo. In addition, computational models were developed incorporating the novel findings detailed in this thesis to assist in visualizing mixing and mass transfer in the perivillous space. 1. The properties of the perivillous fluid environment were assessed by multiple-particle-tracking of the Brownian motion of fluorescent microbeads on gut samples. 2. The movements and disposition of the mucosal surface and associated villi during pendular contractions were observed for whole lengths of everted gut samples. 3. Flow velocities in the perivillous space of gut samples were determined by microparticle-image-velocimetery of microbeads. T he movement of villi in response to physiological levels of lumen flow were quantified by image analysis. The following are the main findings and implications of the work. 1. The perivillous fluid environment consisted of discrete viscoelastic bodies dispersed within a watery Newtonian phase. Such characteristics of the fluid environment were thought to be conducive for mixing and mass transfer, and likened to the processes of gel filtration. 2. Gut pendular contractions generated transient mucosal microfolds, which resulted in the formation of periodic congregation and separation of villous tips. Such a mechanism was predicted (using computational simulations) to augment mixing and mass transfer of nutrients at the gut periphery. 3. Villi were rigid structures, which were more prone to pivot than to bend, while intervillous fluid was predicted to be quasi-static during physiological levels of lumen flow. Such a feature of villi supports a perivillous mixing and mass transfer mechanism driven by mucosal microfolding In conclusion, mixing and mass transfer in the perivillous space are governed by more complex dynamics than previously assumed and by factors previously unknown.
Supervised by Roger Lentle (Masey Univeristy), Bill Williams (Massey Univeristy) and Clément de Loubens (LRP)Publications