Micro- and nano-fluidic devices represent an exciting field with a wide range of possible applications. These devices, typically made of either silica or glass, ionize when placed in contact with water. Upon the application of an electric field parallel to the wall, a flow is produced by the charged walls called the electro-osmotic flow (EOF). Since electric fields are so often used as the driving force in these devices, EOF is an extremely common phenomenon. For this reason it is highly desirable to be able to control EOF in order to optimize the functioning of these devices. One method which is quite common experimentally is the modification of the surface using polymer coatings. These coatings can be either adsorbed or grafted, and charged or neutral. The first part of this thesis looks at the role of neutral adsorbed polymer coatings for the modulation of EOF. Specifically our simulation results show that for adsorbed coatings made from a dilute polymer solution the strongest quenching of EOF is found for an adsorption strength at the phase transition for adsorption of the polymers. Further evidence is presented that shows that by using a high density of polymer solution and a polymer which has a strong attraction to the surface a very thick polymer layer can be created. Next the case of charged grafted polymer coatings is examined. The variation of the EOF with respect to several key parameters which characterize the polymer coating is investigated and compared to theory. The prediction that the electrophoretic velocity of the polymers is the same as the EOF generated by a coating made up of the same polymers is found to be false though the two values are quite close. The last section presents results which show how hydrodynamic interactions in charged polymer systems can be modeled mesoscopically without the use of explicit charges by forcing a slip between monomers and the surrounding fluid. This model is validated by simulating some surprising predictions made in the literature such as an object with no net charge having a non-zero force when subjected to an electric field, and how the velocity can even be perpendicular to the applied electric field. The thesis can be roughly divided into two topics: using polymer coatings to modulate EOF, and the free solution electrophoresis of polyelectrolytes. While EOF and free solution electrophoresis might seem unrelated it will be shown that the concepts are the same in both cases. In fact while not investigated in this thesis, the mesoscopic simulation methods for electrophoresis could be applied to the modulation of EOF with polymer coatings allowing for the simulation of longer length and time scales or more complex systems such as heterogeneously grafted colloids.

Micro- and nano-fluidic devices represent an exciting field with a wide range of possible applications. These devices, typically made of either silica or glass, ionize when placed in contact with water. Upon the application of an electric field parallel to the wall, a flow is produced by the charged walls called the electro-osmotic flow (EOF). Since electric fields are so often used as the driving force in these devices, EOF is an extremely common phenomenon. For this reason it is highly desirable to be able to control EOF in order to optimize the functioning of these devices. One method which is quite common experimentally is the modification of the surface using polymer coatings. These coatings can be either adsorbed or grafted, and charged or neutral. The first part of this thesis looks at the role of neutral adsorbed polymer coatings for the modulation of EOF. Specifically our simulation results show that for adsorbed coatings made from a dilute polymer solution the strongest quenching of EOF is found for an adsorption strength at the phase transition for adsorption of the polymers. Further evidence is presented that shows that by using a high density of polymer solution and a polymer which has a strong attraction to the surface a very thick polymer layer can be created. Next the case of charged grafted polymer coatings is examined. The variation of the EOF with respect to several key parameters which characterize the polymer coating is investigated and compared to theory. The prediction that the electrophoretic velocity of the polymers is the same as the EOF generated by a coating made up of the same polymers is found to be false though the two values are quite close. The last section presents results which show how hydrodynamic interactions in charged polymer systems can be modeled mesoscopically without the use of explicit charges by forcing a slip between monomers and the surrounding fluid. This model is validated by simulating some surprising predictions made in the literature such as an object with no net charge having a non-zero force when subjected to an electric field, and how the velocity can even be perpendicular to the applied electric field. The thesis can be roughly divided into two topics: using polymer coatings to modulate EOF, and the free solution electrophoresis of polyelectrolytes. While EOF and free solution electrophoresis might seem unrelated it will be shown that the concepts are the same in both cases. In fact while not investigated in this thesis, the mesoscopic simulation methods for electrophoresis could be applied to the modulation of EOF with polymer coatings allowing for the simulation of longer length and time scales or more complex systems such as heterogeneously grafted colloids.

Blood is a complex suspension of deformable particles, red blood cells, which exhibits a sophisticated dynamics when flowing in the microvasculature. Most of these complex phenomena, non-linear rheology, structuration of the suspension, heterogeneities of the hematocrit distribution, are directly connected to the rich microscopic dynamics of individual red blood cells, and their hydrodynamics interactions. We investigate a few aspects of the dynamics of red blood cells and giant vesicles - a simple model for RBCs. A study on the dynamics of very deflated vesicles, with shapes similar to those of red blood cells, shows that these objects which haven't received a lot of attention so far can exhibit richer than expected dynamics. We then mainly focus on the still unexplored problem of hydrodynamic interactions between vesicles or red blood cells and their consequences at the scale of the suspension. An experimental study of the interaction of two identical vesicles in shear flow shows that there is a net repulsion between the cells that leads to an increase of the distance between vesicles in a suspension. Scaling arguments are proposed for this interaction and a comparison with numerical results is performed and a quantitative estimation of a shear induced diffusion coefficient obtained by averaging the results for pair interactions is found. Finally, we investigate the diffusion of a cloud of red blood cells in Poiseuille flow in order to directly determine diffusion coefficients. The experiment shows that the cloud widens when traveling along the channel with a power law behaviour indicating sub-diffusion. This effect is confirmed by a theoretical analysis of a few limit cases. / Blood is a complex suspension of deformable particles, red blood cells, which exhibits a sophisticated dynamics when flowing in the microvasculature. Most of these complex phenomena, non-linear rheology, structuration of the suspension, heterogeneities of the hematocrit distribution, are directly connected to the rich microscopic dynamics of individual red blood cells, and their hydrodynamics interactions. We investigate a few aspects of the dynamics of red blood cells and giant vesicles - a simple model for RBCs. A study on the dynamics of very deflated vesicles, with shapes similar to those of red blood cells, shows that these objects which haven't received a lot of attention so far can exhibit richer than expected dynamics. We then mainly focus on the still unexplored problem of hydrodynamic interactions between vesicles or red blood cells and their consequences at the scale of the suspension. An experimental study of the interaction of two identical vesicles in shear flow shows that there is a net repulsion between the cells that leads to an increase of the distance between vesicles in a suspension. Scaling arguments are proposed for this interaction and a comparison with numerical results is performed and a quantitative estimation of a shear induced diffusion coefficient obtained by averaging the results for pair interactions is found. Finally, we investigate the diffusion of a cloud of red blood cells in Poiseuille flow in order to directly determine diffusion coefficients. The experiment shows that the cloud widens when traveling along the channel with a power law behaviour indicating sub-diffusion. This effect is confirmed by a theoretical analysis of a few limit cases.

Micro- and nano-fluidic devices represent an exciting field with a wide range of possible applications. These devices, typically made of either silica or glass, ionize when placed in contact with water. Upon the application of an electric field parallel to the wall, a flow is produced by the charged walls called the electro-osmotic flow (EOF). Since electric fields are so often used as the driving force in these devices, EOF is an extremely common phenomenon. For this reason it is highly desirable to be able to control EOF in order to optimize the functioning of these devices. One method which is quite common experimentally is the modification of the surface using polymer coatings. These coatings can be either adsorbed or grafted, and charged or neutral. The first part of this thesis looks at the role of neutral adsorbed polymer coatings for the modulation of EOF. Specifically our simulation results show that for adsorbed coatings made from a dilute polymer solution the strongest quenching of EOF is found for an adsorption strength at the phase transition for adsorption of the polymers. Further evidence is presented that shows that by using a high density of polymer solution and a polymer which has a strong attraction to the surface a very thick polymer layer can be created. Next the case of charged grafted polymer coatings is examined. The variation of the EOF with respect to several key parameters which characterize the polymer coating is investigated and compared to theory. The prediction that the electrophoretic velocity of the polymers is the same as the EOF generated by a coating made up of the same polymers is found to be false though the two values are quite close. The last section presents results which show how hydrodynamic interactions in charged polymer systems can be modeled mesoscopically without the use of explicit charges by forcing a slip between monomers and the surrounding fluid. This model is validated by simulating some surprising predictions made in the literature such as an object with no net charge having a non-zero force when subjected to an electric field, and how the velocity can even be perpendicular to the applied electric field. The thesis can be roughly divided into two topics: using polymer coatings to modulate EOF, and the free solution electrophoresis of polyelectrolytes. While EOF and free solution electrophoresis might seem unrelated it will be shown that the concepts are the same in both cases. In fact while not investigated in this thesis, the mesoscopic simulation methods for electrophoresis could be applied to the modulation of EOF with polymer coatings allowing for the simulation of longer length and time scales or more complex systems such as heterogeneously grafted colloids.

Pour améliorer les performances des robots sous marins, l’une des approches poursuivie par les roboticiens, appelée biomimétisme, consiste à imiter ou à s’inspirer des systèmes vivants pour concevoir des robots de nouvelle génération. C’est dans ce contexte que s’est récemment déroulé un projet européen nommé ANGELS, dont l’objectif est la réalisation d’un robot bio-inspiré de l’anguille. Ce travail expérimental s’inscrit dans ce projet et est consacré à l’étude de la nage du robot. Les expériences ont été réalisées dans un canal hydraulique conçu pour cette étude. Dans un premier temps, nous avons caractérisé par traitement d’images les allures (i.e. les lois de déformation du corps) adoptées par une anguille nageant soit dans un écoulement uniforme frontal ou dans un courant traversier. Cette étude a donné lieu à l’établissement d’un modèle mathématique corrélatif de la déformation du corps de l’anguille dans ces conditions de nage. Dans un second temps, afin d’étudier les effets de la déformation du corps sur l’écoulement latéral, produit lors de la nage, des expériences par PIV ont été réalisées sur différents modèles de cylindres elliptiques rigides. Ces résultats nous ont permis de mieux comprendre et de valider une approche théorique permettant de calculer la force de propulsion en réponse à la loi de déformation du corps. Enfin, des expériences portant sur la nage anguilliforme dans un écoulement de type allée de von-Kàrmàn ont été réalisées en vue d’étudier les interactions hydrodynamiques et en particulier les mécanismes d’extraction de l’énergie de l’écoulement incident. Ces expériences ont été réalisées avec une anguille et un robot anguilliforme. Les expériences menées sur le robot montrent que pour une même loi de déformation du corps, la force de propulsion générée, en comparaison avec le cas de l’écoulement uniforme, peut sous certaines conditions être augmentée de près de 30 ont, quant à elles, permis de mettre clairement en évidence une modification de l’allure de nage de l’anguille lorsqu’elle est placée dans une allée de von-Kàrmàn. L’analyse qualitative de cette nouvelle allure nous a conduit à proposer un mode particulier d’extraction d’énergie de l’écoulement. / In order to improve the performance of the submarine robots, the robotics community has been considered a new approach known as the biomimetic. It consist on the study of a living systems such, fish, to design and construct a bio-inspired robot. In this context, recently was took place an European project called ANGELS, in which the objective is to design and construct a fish-like robot inspired from the swimming of the eel. This thesis takes place in this project and is dedicate to the study of the swimming of the robot. Experiments were carried out in a hydrodynamic test bed designed and entirely set up for this study. At first,the kinematic shapes (i.e. deformation of the body) adopted by living eel during its swimming against or slantwise a uniform flow, were characterized by mean ofan image processing analysis technique. This study has allowed the establishing of a mathematical correlative model, describing the deformation of the eel’s body in these swimming conditions. Secondly, we studied the effects of the body’s deformation on the lateral flow produced during swimming. PIV experiences were carried out on different elliptic cylinder shapes. These experiments have allowed the understanding and the validation of a theoretical approach, concerning the swimming dynamic of the fish, used to obtain the propulsion force produced in reply of the body deformation during swimming. Finally, experiments were carried out during the anguilliform swimming in a non-uniform flow such as, avon-Kàrmàn vortex street. The goal was to study the hydrodynamics interactions and in particular the mechanisms of the exploited vortices adopted by fish. These experiences were realized on the swimming of a living eel and an anguilliform robot. Experiments led on the robot show that under certain conditions, the propulsive force of the robot swimming in a von-Kàrmàn vortex street can be increased of about 30 comparison to its swimming in a uniform flow. Experiments with eel have allowed the highlighting of a particular shape of its body deformation formed when it’s swimming in a reverse von-Kàrmàn vortex street. The qualitative analysis realized on this kinematic observation led us to propose a mechanism adopted by the eel to exploited energy from altered flow.

Appliquer un champ électrique ou un gradient de température à une solution colloïdale implique la migration des particules (soluté) en suspension. Ce déplacement n’est pas la conséquence de forces de volume comme dans le cas de la sédimentation mais de forces interfaciales agissant sur la double couche électrique présente à la surface des particules colloïdales chargées. Ces forces induisent un écoulement de surface qui à son tour engendre un champ de vitesse du fluide en 1/r³ autour des particules dans la direction opposée à leurs déplacements, où r distance au centre des particules. Dans ce travail on considère une situation différente où la suspension est confinée dans un demi-espace infini limité par une paroi rigide. Un colloïde, sous l’action d’un champ extérieur, se dépose le long de la surface rigide. Bien qu’immobile le colloïde continue de pomper le fluide environnant. Il apparaît alors un écoulement latéral le long du mur et en direction du colloïde. D’autres colloïdes insérés dans un tel écoulement subissent une force hydrodynamique de trainée à l’origine de la formation d’agrégats. De tels agrégats ont été observés aussi bien lors de déposition électrophorétique que plus récemment lors de déposition thermophorétique pour des particules micrométriques en solution aqueuse. Le champ de vitesse confiné prend une forme plus complexe que dans le cas infini : il doit satisfaire à la fois la condition limite fixée à la surface de la particule et sur le mur. Deux méthodes perturbatives, la méthode des réflexions et la méthode d’Oseen, sont utilisées pour résoudre l’équation de Stokes et trouver une solution exacte pour l’écoulement autour du colloïde confiné en puissance de e = a/h rapport du rayon de la particule sur sa distance au mur. La solution usuelle à l’ordre zéro en e donne de pauvres résultats alors que les corrections suivantes donnent de meilleurs conclusions en accord avec les récentes mesures expérimentales de potentiel hydrodynamique de paire entre colloïdes sous champ confinés le long d’un mur. / Applying a steady electric field or a constant thermal gradient to a colloidal suspension induces a finite velocity of the dispersed particles. The motion of particles is not due to a net body force like in sedimentation but to interfacial forces acting on the electric double layer at their surface. These forces involve a surface flow, which, in turn, results in a velocity field of the surrounding fluid in 1/r³ in the opposite direction of the particle displacement, with r the distance from the centre of the particle. In this work we consider a somewhat different situation, where the suspension is confined to a semi-infinite half space. The particle, under the action of the applied field, is trapped against the solid interface. Still, the creep flow remains; more precisely the particle continues to pump the fluid in the opposite direction. As a consequence there arises a lateral flow along the solid surface towards the particle. Thus others particles inserting themselves in this flow undergo drag forces and form clusters. Particles aggregation has been observed in Electrophoresis deposition and more recently in Thermophoresis deposition for micron sized polystyrene beads in aqueous solution. The total velocity field takes a form significantly more complicated than in the above mentioned unbounded cases; it must satisfy boundary conditions both at the particle surface and at the confining wall. Using the perturbative method of reflections or Oseen method based on Fourier transform we resolve the Stokes equation and find an analytic solution for the drag flow along the interface in powers of the ratio e=a/h of particle radius and wall distance. The usual solution at the zero order induces poor approximation, when following corrections in e involves better results in agreement with experimental measurements of hydrodynamic pair potential between two particles along a wall.

Le transport des micro-organismes, comme par exemple les bactéries, par un fluide se retrouve au centre de thématiques de recherche dans des domaines aussi variés que de la biologie, l’écologie, l’ingénierie et la médecine.Ce manuscrit résume mon étude expérimentale du couplage entre le mouvement microscopique de la nage des bactéries et le mouvement advectif de l’écoulement.La première partie du manuscrit porte sur la rhéologie des suspensions d’E. coli sous faible taux de cisaillement. Pour cette condition, j’ai montré que les perturbations hydrodynamiques induites par la nage réduisent fortement la viscosité. Cet effet peut-être si important pour qu’il soit suffisant pour compenser entièrement la perte visqueuse due au cisaillement.La seconde partie traite des expériences d’écoulement réalisées dans un canal capillaire. Pour cette géométrie, j’ai examiné le couplage pour des écoulements caractérisés par un plus fort taux de cisaillement. Le suivi des trajectoires et le dénombrement des bactéries m’ont permis de mettre en évidence l’existence d’une composante de vitesse normal à la direction de l’écoulement. Cette dernière montre que les bactéries suivent des trajectoires hélicoïdales qui s’enroulent autour du centre du capillaire d’une façon antihoraires. Cette nouvelle composante est corrélée à la migration préférentielle des bactéries dans une couche de localisation proche de la paroi du canal.Les couplages rhéotactiques bactéries/fluide que j’ai étudiés doivent avoir des conséquences potentielles sur le transport en géométries plus complexes qui mériteraient une étude particulière. / The question of transfer and spreading of living microorganisms, such as motile bacteria, is of interest in biology and ecology, but also in engineering and medicine.The way in which the background flow affects the behavior of these bacteria and how it impacts the bacterial transport through complex systems and on the macroscopic properties of the fluid remains unclear and little studied.In this thesis, I present an experimental investigation of the coupling between the local bacteria-driven motion and the fluid advection.In a first part, I investigate the rheological response of E. coli suspensions when subjected to weak flows (low shear rates). I show that, in particular conditions, the microscopic perturbations caused by the bacteria highly impact on the macroscopic viscosity of the suspension, leading to a striking viscosity decrease and eventually overcoming the dissipative effects due to viscous loss. I also identify the relevant time scales defining this viscosity decrease.In a second part, I perform experiments in a capillary channel and analyze the coupling for stronger flows (higher shear rates), at which bacteria were found not to impact on the macroscopic viscosity. Instead, by analyzing the bacterial trajectories under flow, I evidence a breakage of the symmetry of this trajectories which, characterized by a preferential migration, causes the localization of the bacteria in a layer that extends over a significant distance from the surface, and thus potentially influencing the bacterial transport in complex systems

Les vésicules sont utilisées d'une manière extensive comme modèle pour comprendre les dynamiques et les déformations des globules rouges au niveau individuel, mais aussi concernant les phénomènes collectives et la rhéologie. La membrane de la vésicule résiste à la flexion mais pas au cisaillement, contrairement aux globules rouges, néanmoins elles partagent plusieurs propriétés dynamiques avec les globules rouges, comme le tank-treading (mouvement en chenille de char) et le tumbling (mouvement de bascule) sous écoulement de cisaillement, ou les formes parachutes et slippers (pantoufles) sous un écoulement de Poiseuille. Les globules rouges sont connus pour former des trains de cellules (clusters) dans la microcirculation attribués à la nature attractive des interactions hydrodynamiques. Nous avons étudié numériquement plusieurs types de problème comme:(i) les dynamiques de cellules isolées, (ii) le couplage hydrodynamique entre globules rouges (en utilisant les vésicules comme modèle) soumis à un écoulement de Poiseuille sous différent confinements; (iii) l'agrégation des globules rouges et la formation de rouleaux; et (iv) le rôle des macromolécules dans la formation de clusters sous écoulement. les résultats obtenus apportent un nouveau regard à la physique des objets déformables et sont transposables au cas de l'écoulement des globules rouges dans la microcirculation. / Vesicles are extensively used as a model for understanding dynamicsand deformation of red blood cells at the individual level but also regarding collective phenomena and rheology. Vesicles' membranes withstand to bending butdo not have a shear resistance, unlike red blood cells, but they still share several dynamical properties with red blood cells, like tank-treading and tumbling under linear shear flow, or parachute and slipper shapes under Poiseuille flow. The red blood cells are known to form train of cells in the microcirculation attributed to attractive hydrodynamic interactions. We investigate numerically several kind of problems such as: (i) the dynamics of isolated cells; (ii) the hydrodynamic coupling between the red blood cells (by using vesicles as a model) subject to a Poiseuille flow under different confinements; (iii) the aggregation of red blood cells and formation of rouleaux; and (iv) the contribution of macromolecules in the formation of clusters under flow condition. The obtained results give a new insight into thephysics of deformable objects under confinement that are transposable to the flow of red blood cells in the microcirculation.

Une brosse de polymères est une matrice dense de macromolécules greffées à une surface donnée. Au-delà des brosses synthétiques réalisées en laboratoire, on trouve des exemples très variés dans la nature: un exemple emblématique est le glycocalyx endothélial, décorant la surface interne des vaisseaux sanguins des mammifères. L'interaction de cette structure avec le plasma et les cellules sous écoulement n'est encore que très partiellement explorée. La présente thèse propose, grâce à des simulations de "Dissipative Particle Dynamics", un modèle coarse-grained pour une analyse auto-cohérente d'une brosse polymérique dense sous écoulement parabolique. Cette étude mésoscopique met en évidence l'importance des effets collectifs entre molécules, entraînée par l'hydrodynamique, et propose des nouvelles interprétations à la phénoménologie du système brosse-écoulement. Des résultats préliminaires sont également produits pour l'interaction sous écoulement entre un objet mésoscopique déformable (prototype d'un globule rouge) et les polymères greffés. / Polymer brushes are dense matrices of grafted macromolecules. In addition to brushes finely designed in laboratory, various examples are offered by Nature, as the endothelial glycocalyx, decorating the lumen of mammalian blood vessels. The interaction of such network with the flowing plasma and cells is still partially unknown.The present thesis, by mean of Dissipative Particle Dynamics simulations, proposes a coarse-grained model for the self-consistent analysis of a dense polymer brush under parabolic flow. Our mesoscale investigation highlights the relevance of collective effects, driven by hydrodynamics, and proposes novel interpretations regarding the rich phenomenology of the brush-flow system.Preliminary results are also provided for the interplay between a mesoscopic deformable flowing object (prototype of a red blood cell) and the grafted polymers.

Dans cette thèse, les propriétés rhéologiques du sang sont étudiées suivant deux approches differentes. Les propriétés de l'écoulement du plasma sont analysées selon trois modes différents : sous cisaillement, en extension et en constriction. Jusqu'à présent, le plasma était considéré comme un fluide newtonien, et le comportement complexe du sang était simplement attribué à la présence des globules rouges. Les expériences menées ont montré un comportement visco-élastique du plasma, que doit désomais être pris en compte dans les études futures. La deuxième axe traite des globules rouges. Leur assemblage en agrégats rectilignes est à l'origine du comportement rhéofluidifiant, mais les causes de la formation des agrégats restent encore vagues. L'énergie d'interaction entre deux cellules et la distribution des tailles des clusters dans des canaux microfluidiques ont été mesurées en présence de dextran et de fibrinogène. Comme les agrégats sont normalement cassés à des taux de cisaillement élevés, on a cru qu'ils ne jouaient pas de rôle dans l'écoulement du sang. Mais le fait que le nombre de clusters augmente à des concentrations physiologiques de fibrinogène, même pour des taux de cisaillement correspondant à ceux du système microvasculaire, il est clair que l'agrégation ne peut pas être négligée dans la description de l'écoulement du sang en le réseau capillaire. / In this work, the rheological properties of human blood are investigated by two different approaches. The flow properties of plasma, the liquid component of blood, is analyzed under three different conditions: shear flow, elongational flow and contraction flow. Up to now, the plasma was considered as a Newtonian fluid, while the non-Newtonian properties of blood were only attributed to the red blood cells. The performed experiments reveal a viscoelastic behavior of the plasma which has to be considered in future studies. In addition to the plasma, also diluted polymer solutions are analyzed in order to find a good model solution for plasma. The second part concerns the red blood cells. Their adhesion to linear aggregates is held responsible for the well-known shear thinning behavior of blood but the reason for the cluster formation is still not clear. The interaction energy between two red blood cells and the distribution of different sized clusters flowing through narrow channels are measured under the influence of the two macromolecules dextran and fibrinogen. As the aggregates are actually broken at high shear rates, the current understanding is that they would not play a role for the properties of blood flow. However, an increased amount of clusters at physiological fibrinogen concentrations can be shown, even at shear rates which are common in the microvascular system, which clarifies that the aggregation cannot be neglected in the description of blood flow through the capillary network.