Ferrofluids are comprised of subdomain particles of magnetite or iron oxide material that can become magnetized in the presence of a magnetic field. These unique liquids are being incorporated into many new applications due to the ability to control them at a distance using magnetic fields. However, although our understanding of the dynamics of ferrofluids has evolved, many aspects of ferrohydrodynamics remain largely unexplored, especially experimentally. This study is the first to characterize the stability and internal dynamics of accumulating or dispersing ferrofluid aggregates spanning the stable, low Reynolds number behavior to unstable, higher Reynolds numbers. The dynamics of ferrofluid aggregates are governed by the interaction between the bulk flow shear stresses acting to wash away the aggregate and magnetic body forces acting to retain them at the magnet location. This interaction results in different aggregate dynamics, including the stretching and coagulation of the aggregate to Kelvin-Helmholtz shedding from the aggregate interface as identified by focused shadowgraphs. Using TRDPIV, the first time-resolved flow field measurements conducted in ferrofluids reveal the presence of a three-stage process by which the ferrofluid interacts with a pulsatile bulk flow. An expanded parametric study of the effect of Reynolds number, magnetic field strength, and flow unsteadiness reveals that the increased field results can result in the lifting and wash away of the aggregate by means of vortex strengthening. In pulsatile flow, different forms of the three-stage interaction occur based on magnetic field, flow rate, and Reynolds number. / Ph. D.

Polymerverkapselte magnetische Nanopartikel versprechen, in der Zukunft sehr erfolgreich bei Anwendungen in der Biologie und der Medizin eingesetzt werden zu können z. B. in der Krebstherapie und als Kontrastmittel bei der magnetischen Kernspinresonanztomographie. Diese Arbeit zeigt, dass durch die interdisziplinäre Kombination verschiedener Techniken Herstellungsverfahren und Eigenschaften solcher Partikel verbessert werden können. <br /> <br /> Unter Miniemulsionen versteht man wässrige Dispersionen relativ stabiler Öltröpfchen, zwischen 30 und 50 nm Größe. Ein Nanometer (nm) ist der 1.000.000.000ste Teil eines Meters. Ein Haar ist ungefähr 60.000 Nanometer breit.<br /> <br /> Hergestellt werden Miniemulsionen durch Scherung eines Systems bestehend aus Öl, Wasser, Tensid (Seife) und einer weiteren Komponente, dem Hydrophob, das die Tröpfchen stabilisieren soll. Die Polymerisation von Miniemulsionen ermöglicht die Verkapselung anorganischer Materialen z. B. magnetischer Teilchen oder Gadolinium-haltiger Komponenten. Zu Optimierung des Verkapselung, ist es notwendig, die richtige Menge eines geeigneten Tensids zu finden. <br /> <br /> Die magnetischen polymerverkapselten Nanopartikel, die in einer wässrigen Trägerflüssigkeit dispergiert sind, zeigen in Abhängigkeit von Partikelgröße, Zusammensetzung, elektronischer Beschaffenheit, etc. ein sogenanntes superpara- oder paramagnetisches Verhalten. Superpara- oder paramagnetisches Verhalten bedeutet, dass die Flüssigkeiten in Anwesenheit äußerer Magnetfeldern ihre Fließfähigkeit beibehalten. Wenn das Magnetfeld entfernt wird, haben sie keine Erinnerung mehr daran, unter dem Einfluss eines Magnetfeldes gestanden zu haben, d. h., dass sie nach Abschalten des Magnetfeldes selbst nicht mehr magnetisch sind. <br /> <br /> Die Vorteile des Miniemulsionsverfahrens sind der hohe Gehalt und die homogene Verteilung magnetischer Teilchen in den einzelnen Nanopartikeln. Außerdem ermöglicht dieses Verfahren nanostrukturierte Kompositpartikel herzustellen, wie z. B polymerverkapselte Nanopartikel mit Nanoschichten bestehend aus magnetischen Molekülen. / Combining the magnetic properties of a given material with the tremendous advantages of colloids can exponentially increase the advantages of both systems. This thesis deals with the field of magnetic nanotechnology. Thus, the design and characterization of new magnetic colloids with fascinating properties compared with the bulk materials is presented. <br /> <br /> Ferrofluids are referred to either as water or organic stable dispersions of superparamagnetic nanoparticles which respond to the application of an external magnetic field but lose their magnetization in the absence of a magnetic field. <br /> <br /> In the first part of this thesis, a three-step synthesis for the fabrication of a novel water-based ferrofluid is presented. The encapsulation of high amounts of magnetite into polystyrene particles can efficiently be achieved by a new process including two miniemulsion processes. The ferrofluids consist of novel magnetite polystyrene nanoparticles dispersed in water which are obtained by three-step process including coprecipitation of magnetite, its hydrophobization and further surfactant coating to enable the redispersion in water and the posterior encapsulation into polystyrene by miniemulsion polymerization. It is a desire to take advantage of a potential thermodynamic control for the design of nanoparticles, and the concept of &quot;nanoreactors&quot; where the essential ingredients for the formation of the nanoparticles are already in the beginning. The formulation and application of polymer particles and hybrid particles composed of polymeric and magnetic material is of high interest for biomedical applications. Ferrofluids can for instance be used in medicine for cancer therapy and magnetic resonance imaging.<br /> <br /> Superparamagnetic or paramagnetic colloids containing iron or gadolinium are also used as magnetic resonance imaging contrast agent, for example as a important tool in the diagnosis of cancer, since they enhance the relaxation of the water of the neighbouring zones. New nanostructured composites by the thermal decomposition of iron pentacarbonyl in the monomer phase and thereafter the formation of paramagnetic nanocomposites by miniemulsion polymerization are discussed in the second part of this thesis. In order to obtain the confined paramagnetic nanocomposites a two-step process was used. In the first step, the thermal decomposition of the iron pentacarbonyl was obtained in the monomer phase using oleic acid as stabilizer. In the second step, this iron-containing monomer dispersion was used for making a miniemulsion polymerization thereof. <br /> <br /> The addition of lanthanide complexes to ester-containing monomers such as butyl acrylate and subsequent polymerization leading to the spontaneous formation of highly organized layered nanocomposites is presented in the final part of this thesis. By an one-step miniemulsion process, the formation of a lamellar structure within the polymer nanoparticles is achieved. The magnetization and the NMR relaxation measurements have shown these new layered nanocomposites to be very apt for application as contrast agent in magnetic resonance imaging.

Este trabalho propõe a síntese de vários materiais nanoestruturados pelo processo sol-gel. Foram preparados sóis a base de óxido de cério, hidroxiapatita e magnetita. Filmes a base de paládio metálico foram obtidos pela técnica de decomposição térmica e redução em atmosfera de nitrogênio. A caracterização estrutural dos materiais preparados foi realizada por DRX; a caracterização morfológica foi feita mediante SAXS, MET e MEV; a caracterização química por EDS; a caracterização térmica por DSC; a caracterização magnética por RPE e a caracterização relaxometrica por IRM. No caso da hidroxiapatita, foram realizadas medidas de FTIR. Nanopartículas a base de óxido de cério e Pdo com propriedades catalíticas foram depositadas na forma de filme sobre substratos de alumínio anodizado e na região da cabeça de pistões automotivos de motor a gasolina. Os catalisadores foram suportados nos poros que os anodizados apresentam. Os pistões foram submetidos a testes catalíticos e apresentaram redução nas emissões de CO, compostos orgânicos voláteis e NOx. Além disso, verificou-se a diminuição no consumo de combustível e aumento na eficiência do motor automotivo a gasolina. A síntese de nanopartículas a base de hidroxiapatita produz um material de morfologia nanométrica, apresentando um comportamento ferromagnético atribuído ao ferro como impureza nos precursores da síntese. Também foram obtidas sóis estáveis a base de nanopartículas de hidroxiapatita revestidas por ácido oléico. A síntese e caracterização do ferrofluido à base de nanopartículas de magnetita superparamagnéticas levaram a fabricação de um agente de contraste negativo, característica principal para aplicações na IRM. / This work proposes the synthesis of several nonostructured materials by the sol-gel process. Cerium oxide, hydroxyapatite and magnetite based sols were prepared. Metallic palladium based films were obtained by the thermal decomposition technique followed by reduction under nitrogen atmosphere. Structural characterizations of the prepared materials were accomplished by XRD; morphologic characterization were carried out by SAXS, TEM and SEM, chemical characterization by EDS, thermal characterization using DSC, magnetic characterization by EPR, and the relaxometry characterization by MRI. FTIR\'s measurements were also accomplished in hydroxyapatite samples. Cerium oxide and palladium nanoparticles presenting catalytic properties were deposited as a film on anodized aluminum substrates and on automotive piston heads for gasoline motors. The catalyzers were supported by pores present in anodized aluminum surfaces. Catalytic test of the pistons have shown emission reduction of CO, organic volatile compounds, and NOx. Moreover, decrease in the fuel consumption and increase in automotive motor efficiency were verified. The synthesis of nanoparticles based on hydroxyapatite gives rise to a material of nanometric morphology. It presents a ferromagnetic behavior due to the iron as impurity in the synthesis precursors. Stable sols of hydroxyapatite nanoparticles covered by oleic acid were also obtained. Synthesis and characterization of ferrofluid based on superparamagnetic nanoparticles of magnetite lead to the production of an agent for negative contrast. It is the main characteristic for applications in MRI.

L'objectif de cette thèse est de développer des bio-capteurs à base de magnétorésistance géante (GMR), ainsi que l'électronique de conditionnement associée, en vue de caractériser magnétiquement des ferrofluides biologiques. Ce travail a été réalisé en collaboration avec le Pr Sotoshi YAMADA de l'Institut « Nature and Environmental Technology» de l'Université de Kanazawa. La première partie porte sur l'état de l'art et les méthodes de mesures des propriétés magnétiques des ferrofluides et la physique de l'effet GMR. La deuxième partie concerne la mise en place d'un dispositif de mesure pour déterminer et caractériser la valeur de la sensibilité de chaque capteur. Cette sensibilité est une caractéristique cruciale pour toute application biomédicale. Sa connaissance et son optimisation permettent d'envisager des mesures précises et justes des propriétés magnétiques des ferrofluides notamment à bas niveau de signal. La troisième partie, également expérimentale, décrit les mesures de la perméabilité relative (µr) et de la susceptibilité (X) de fluides magnétiques (ferrofluides) par des capteurs GMR I, II. En outre, afin de confirmer les résultats expérimentaux obtenus avec ces capteurs, nous les avons comparés à ceux obtenus avec d'autres méthodes comme la magnétométrie à échantillon vibrant (VSM) ou à des calculs théoriques. Le quatrième et dernier chapitre présente les résultats expérimentaux de la perméabilité relative et de la susceptibilité d'un marqueur magnétique permettant la détection de la bactérie pathogène Escherichia coli O157: H7 / The intent of this thesis is to develop bio-sensors based on giant magnetoresistance (GMR) and the associated conditioning electronics, to characterize magnetically organic ferrofluids. This work was done in collaboration with Pr Sotoshi YAMADA of the Institute "Nature and Environmental Technology" at the University of Kanazawa. The first part focuses on the state of the art and the methods for magnetic properties measurements of ferrofluids and the description of the GMR effect. The second part concerns the introduction of a measuring device to determine and characterize the value of the sensitivity of each sensor. This sensitivity is a crucial parameter for any biomedical application. Its knowledge allows optimization of sensors ability to measure very low magnetic parameters of ferrofluids very precisely. The third experimental part describes measurements of relative permeability (µr) and susceptibility (X) of magnetic ferrofluids with GMR sensors I, II. In addition, to confirm the experimental results obtained with these sensors, we have compared them to those obtained with other methods such as vibrating sample magnetometer (VSM) or by theoretical calculations. The fourth and last chapter presents the experimental results of the relative permeability and susceptibility of a magnetic marker used to detect pathogenic bacteria (Escherichia coli O157: H7)

Este trabalho propõe a síntese de vários materiais nanoestruturados pelo processo sol-gel. Foram preparados sóis a base de óxido de cério, hidroxiapatita e magnetita. Filmes a base de paládio metálico foram obtidos pela técnica de decomposição térmica e redução em atmosfera de nitrogênio. A caracterização estrutural dos materiais preparados foi realizada por DRX; a caracterização morfológica foi feita mediante SAXS, MET e MEV; a caracterização química por EDS; a caracterização térmica por DSC; a caracterização magnética por RPE e a caracterização relaxometrica por IRM. No caso da hidroxiapatita, foram realizadas medidas de FTIR. Nanopartículas a base de óxido de cério e Pdo com propriedades catalíticas foram depositadas na forma de filme sobre substratos de alumínio anodizado e na região da cabeça de pistões automotivos de motor a gasolina. Os catalisadores foram suportados nos poros que os anodizados apresentam. Os pistões foram submetidos a testes catalíticos e apresentaram redução nas emissões de CO, compostos orgânicos voláteis e NOx. Além disso, verificou-se a diminuição no consumo de combustível e aumento na eficiência do motor automotivo a gasolina. A síntese de nanopartículas a base de hidroxiapatita produz um material de morfologia nanométrica, apresentando um comportamento ferromagnético atribuído ao ferro como impureza nos precursores da síntese. Também foram obtidas sóis estáveis a base de nanopartículas de hidroxiapatita revestidas por ácido oléico. A síntese e caracterização do ferrofluido à base de nanopartículas de magnetita superparamagnéticas levaram a fabricação de um agente de contraste negativo, característica principal para aplicações na IRM. / This work proposes the synthesis of several nonostructured materials by the sol-gel process. Cerium oxide, hydroxyapatite and magnetite based sols were prepared. Metallic palladium based films were obtained by the thermal decomposition technique followed by reduction under nitrogen atmosphere. Structural characterizations of the prepared materials were accomplished by XRD; morphologic characterization were carried out by SAXS, TEM and SEM, chemical characterization by EDS, thermal characterization using DSC, magnetic characterization by EPR, and the relaxometry characterization by MRI. FTIR\'s measurements were also accomplished in hydroxyapatite samples. Cerium oxide and palladium nanoparticles presenting catalytic properties were deposited as a film on anodized aluminum substrates and on automotive piston heads for gasoline motors. The catalyzers were supported by pores present in anodized aluminum surfaces. Catalytic test of the pistons have shown emission reduction of CO, organic volatile compounds, and NOx. Moreover, decrease in the fuel consumption and increase in automotive motor efficiency were verified. The synthesis of nanoparticles based on hydroxyapatite gives rise to a material of nanometric morphology. It presents a ferromagnetic behavior due to the iron as impurity in the synthesis precursors. Stable sols of hydroxyapatite nanoparticles covered by oleic acid were also obtained. Synthesis and characterization of ferrofluid based on superparamagnetic nanoparticles of magnetite lead to the production of an agent for negative contrast. It is the main characteristic for applications in MRI.

Este trabalho visa a caracterização estrutural, mecânica e óptica de elastômeros de uretâno/uréia (PU/PBDO), baseados em óxido de poli(propileno) e poli(butenodiol), puro, após inflação em tolueno e dopados com ferrofluido. As proporções em massa (%) de PU e PBDO utilizadas são 40/60, 50/50, 60/40 e 80/20. As análises estruturais foram feitas através das técnicas de microscopia óptica de luz polarizada, microscopia de força atômica e magnética, e microscopia confocal de varredura laser com as quais foi verificado que os filmes são isotrópicos e autofluorescentes. A amostra após inflação com tolueno não apresenta diferença em sua textura em relação a amostra pura. As amostras dopadas apresentam coloração marrom devido aos grãos magnéticos. Também verificamos que o processo de dopagem com inflação dos filmes em solução de tolueno e ferrofluido é eficiente, uma vez que o filme não apresenta grandes agregados de grãos magnéticos. Medidas do coeficiente de absorção óptica mostraram que o tempo de inflação das amostras em tolueno não altera de forma significativa esses filmes. Também verificamos que o tempo, de imersão do filme na solução de tolueno e ferrofluido, para que as amostras incorporem a maior quantidade de grãos magnéticos aumenta com a concentração de PU. O número de grãos magnéticos na matriz elastomérica não aumenta depois que a amostra atinge o nível de saturação. Experimentos mecânicos e ópticos mostram que o processo de preparação do elastômero (casting) introduz uma anisotropia estrutural nas amostras opticamente isotrópicas. Esse fato foi evidenciado pelas medidas do módulo de Young e orientação do eixo óptico das amostras sob estiramento. A dependência da diferença de fase e consequentemente da birrefringência óptica em amostras puras, após inflação com tolueno e dopadas com ferrofluido, com a deformação é linear. O coeficiente de deformação óptico também é linear com a concentração de ferrofluido. Para todos os tempos de inflação em tolueno e ferrofluido as amostras 50/50 e 60/40 foram as que apresentaram maior birrefringência induzida por estiramento. / This work aims the structural, mechanical and optical characterization of urethane/urea elastomers (PU/PBDO), based on polypropylene oxide and polybutadiene diol, pure, after swelling in toluene and doped with ferrofluid. The ratios in weight % of PU and PBDO used are 40/60, 50/50, 60/40 and 80/20. The structural analysis were made through of polarized light microscopy, atomic and magnetic force microscopy, and confocal laser scanning microscopy techniques in such was verified that films are isotropics and autofluorescents. The sample after swelling with toluene does not present difference in its texture compared with pure sample. Doped samples present brown color due the magnetic grains. It was also verified that doping process by swelling of film in solution of ferrofluid with toluene is efficient because the film not shows large aggregates of magnetic grains. Measurements of optical absorption coefficient show that the swelling time of the samples in toluene does not modify significantly these films. We verified that the immersion time of the film in the solution of ferrofluid with toluene, to incorporate in the greatest quantity of magnetic grains in samples, increases with the PU concentration. The number of magnetic grains in the elastomeric matrix does not increase after the samples reach the saturation level. Mechanical and optical experiments show that the elastomer preparation procedure (casting) introduces a structural anisotropy in the optically isotropic sample. This result was evidenced by the measurements of the Young\'s module and orientation of the sample\'s optic axis under stretching. The dependence of phase shift, and, consequently the optical birefringence, with strain in pure, after swelling in toluene and doped with ferrofluid samples, is linear. The strain-optic coefficient is linear with the concentration of ferrofluid. For all the swelling time in toluene and ferrofluid the 50/50 and 60/40 samples were those that present the greatest induced birefringence under stretching.

<p>This thesis will investigate previously unexplored concepts in magnetic manipulation including controlling the assembly of magnetic and nonmagnetic particles either in bulk fluid or near a substrate. Both uniform glass interfaces and substrates with magnetic microstructures are considered. The main goal of this work is to discuss new strategies for implementing magnetic assembly systems that are capable of exquisitely controlling the positions and orientations of single-component as well as multi-component particle suspensions, including both magnetic and non-magnetic particles. This work primarily focuses on controlling spherical particles; however, there are also several demonstrations of controlling anisotropically shaped particles, such as microrods and Janus colloids. </p><p> Throughout this work, both conventional magnetophoresis and inverse magnetophoresis techniques were employed, the latter relying on ferrofluid, i.e. a suspension of magnetic nanoparticles in a nonmagnetic carrier fluid, which provides a strong magnetic permeability in the surrounding fluid in order to manipulate effectively non-magnetic materials. In each system it was found that the dimensionless ratio between magnetic energy and thermal energy could be successfully used to describe the degree of control over the positions and orientations of the particles. One general conclusion drawn from this work is that the ferrofluid can be modeled with a bulk effective permeability for length scales on the order of 100 nm. This greatly reduces modeling requirements since ferrofluid is a complex collection of discrete nanoparticles, and not a homogenous fluid. It was discovered that the effective magnetic permeability was often much larger than expected, and this effect was attributed to particle aggregation which is inherent in these systems. In nearly all cases, these interactions caused the ferrofluid to behave as though the nanoparticles were clustered with an effective diameter about twice the real diameter.</p><p> The principle purpose of this thesis is to present novel systems which offer the ability to manipulate and orient multi-component spherical or anisotropic particle suspensions near surfaces or in the bulk fluid. First, a novel chip-based technique for transport and separation of magnetic microparticles is discussed. Then, the manipulation of magnetic nanoparticles, for which Brownian diffusion is a significant factor, is explored and modeled. Parallel systems of nonmagnetic particles suspended in ferrofluid are also considered in the context of forming steady state concentration gradients. Next, systems of particles interacting with planar glass interfaces are analyzed, modeled, and a novel application is developed to study the interactions between antigen-antibody pairs by using the self-repulsion of non-magnetic beads away from a ferrofluid/glass interface. This thesis also focuses on studying the ability to manipulate particles in the bulk fluid. First, simple dipole-dipole aggregation phenomenon is studied in suspensions of both nonmagnetic polystyrene particles and endothelial cells. For the sizes of particles considered in these studies, currently accepted diffusion limited aggregation models could not explain the observed behavior, and a new theory was proposed. Next, this thesis analyzed the interactions that exist in multi-component magnetic and nonmagnetic particle suspensions, which led to a variety of novel and interesting colloidal assemblies. This thesis finally discusses the manipulation of anisotropic particles, namely, the ability to control the orientation of particles including both aligning nonmagnetic rods in ferrofluid as well as achieving near-holonomic control of Janus particles with optomagnetic traps. General conclusions of the viability of these techniques are outlined and future studies are proposed in the final chapter.</p> / Dissertation

Este trabalho visa a caracterização estrutural, mecânica e óptica de elastômeros de uretâno/uréia (PU/PBDO), baseados em óxido de poli(propileno) e poli(butenodiol), puro, após inflação em tolueno e dopados com ferrofluido. As proporções em massa (%) de PU e PBDO utilizadas são 40/60, 50/50, 60/40 e 80/20. As análises estruturais foram feitas através das técnicas de microscopia óptica de luz polarizada, microscopia de força atômica e magnética, e microscopia confocal de varredura laser com as quais foi verificado que os filmes são isotrópicos e autofluorescentes. A amostra após inflação com tolueno não apresenta diferença em sua textura em relação a amostra pura. As amostras dopadas apresentam coloração marrom devido aos grãos magnéticos. Também verificamos que o processo de dopagem com inflação dos filmes em solução de tolueno e ferrofluido é eficiente, uma vez que o filme não apresenta grandes agregados de grãos magnéticos. Medidas do coeficiente de absorção óptica mostraram que o tempo de inflação das amostras em tolueno não altera de forma significativa esses filmes. Também verificamos que o tempo, de imersão do filme na solução de tolueno e ferrofluido, para que as amostras incorporem a maior quantidade de grãos magnéticos aumenta com a concentração de PU. O número de grãos magnéticos na matriz elastomérica não aumenta depois que a amostra atinge o nível de saturação. Experimentos mecânicos e ópticos mostram que o processo de preparação do elastômero (casting) introduz uma anisotropia estrutural nas amostras opticamente isotrópicas. Esse fato foi evidenciado pelas medidas do módulo de Young e orientação do eixo óptico das amostras sob estiramento. A dependência da diferença de fase e consequentemente da birrefringência óptica em amostras puras, após inflação com tolueno e dopadas com ferrofluido, com a deformação é linear. O coeficiente de deformação óptico também é linear com a concentração de ferrofluido. Para todos os tempos de inflação em tolueno e ferrofluido as amostras 50/50 e 60/40 foram as que apresentaram maior birrefringência induzida por estiramento. / This work aims the structural, mechanical and optical characterization of urethane/urea elastomers (PU/PBDO), based on polypropylene oxide and polybutadiene diol, pure, after swelling in toluene and doped with ferrofluid. The ratios in weight % of PU and PBDO used are 40/60, 50/50, 60/40 and 80/20. The structural analysis were made through of polarized light microscopy, atomic and magnetic force microscopy, and confocal laser scanning microscopy techniques in such was verified that films are isotropics and autofluorescents. The sample after swelling with toluene does not present difference in its texture compared with pure sample. Doped samples present brown color due the magnetic grains. It was also verified that doping process by swelling of film in solution of ferrofluid with toluene is efficient because the film not shows large aggregates of magnetic grains. Measurements of optical absorption coefficient show that the swelling time of the samples in toluene does not modify significantly these films. We verified that the immersion time of the film in the solution of ferrofluid with toluene, to incorporate in the greatest quantity of magnetic grains in samples, increases with the PU concentration. The number of magnetic grains in the elastomeric matrix does not increase after the samples reach the saturation level. Mechanical and optical experiments show that the elastomer preparation procedure (casting) introduces a structural anisotropy in the optically isotropic sample. This result was evidenced by the measurements of the Young\'s module and orientation of the sample\'s optic axis under stretching. The dependence of phase shift, and, consequently the optical birefringence, with strain in pure, after swelling in toluene and doped with ferrofluid samples, is linear. The strain-optic coefficient is linear with the concentration of ferrofluid. For all the swelling time in toluene and ferrofluid the 50/50 and 60/40 samples were those that present the greatest induced birefringence under stretching.

La séparation magnétique est utilisée dans le domaine de la purification de l’eau. Des nanoparticules magnétiques fonctionnalisées ou nues sont en charge de capter un polluant ciblé puis sont enlevées de l’eau par l’application d’un champ magnétique. Nous avons montré que l’utilisation d’un ferrofluide aqueux classique constitué de nanoparticules d’oxyde de fer recouvertes par une double couche de surfactant d’acide oléique permet une adsorption efficace des cations métalliques (ion nickel Ni2+) puis leur enlèvement en utilisant des microparticules magnétiques. Nous avons montré que des interactions magnétiques suffisamment fortes entre les nanoparticules induisent une transition de phase colloïdale. Il existe trois régimes de captage qui dépendent de deux paramètres, le premier paramètre est lié à l’intensité du champ magnétique et le second est la concentration en nanoparticules. En présence de l’écoulement de la suspension des nanoparticules, le captage des nanoparticules est toujours régi par la concentration en nanoparticules et également par le nombre de Mason. Nous avons mené une expérience de captage des nanoparticules par un milieu poreux. L’efficacité de filtration est liée à la compétition entre les forces hydrodynamiques et magnétiques ainsi que de la séparation de phase au sein de la suspension. Les théories développées lors de ce travail sont en accord raisonnable avec les résultats expérimentaux et montrent l’augmentation de l’efficacité de captage des nanoparticules avec l’augmentation de l’intensité du champ magnétique et de la concentration en nanoparticules ainsi que la diminution de l’efficacité de captage avec l’augmentation du nombre de Mason. / Magnetic separation is used in the domain of water purification. In these systems, functionalized or bare magnetic nanoparticles are used to capture a target pollutant then they are extracted from water by an applied magnetic field. We have shown that a classical aqueous ferrofluid composed by iron oxide nanoparticles covered with an oleic acid double layer allows efficient adsorption of metallic cations (nickel ion Ni2+) followed by nanoparticle extraction with the help of magnetic microparticles. We have shown that strong enough magnetic interactions between nanoparticles can induce a colloidal phase transition. There exists three capture regimes which depend on two parameters, the first one is related to the magnetic field intensity and the second is the nanoparticle concentration. In the presence of the flow of nanoparticle suspension, the nanoparticle capture is always governed by the nanoparticle concentration and also by Mason number. We have carried out an experiment on nanoparticle capture in a porous medium. The filtration efficiency is related to the competition between the hydrodynamic and magnetic forces, as well as to the phase separation in the suspension bulk. The theories developed in the frames of this work show a reasonable agreement with the experimental results. They predict an increase of the capture efficiency with an increase of the magnetic field intensity and the nanoparticle concentration as well as a decrease of the capture efficiency with the growth of the Mason number.

This thesis studies a droplet of ferrofluid impacting a liquid water pool. The ferrofluid is oil-based and therefore immersible in water. The shape of the ferrofluid drop at impact is changed by using an electromagnet underneath the liquid pool. The magnet is turned off by an external trigger just before the drop collides with the liquid pool surface, to stop the magnetic interaction. The prolate or oblate shape of the drop has an influence on the cavity formation and evolution after the impact. The experiments look specifically at the maximum depth and diameter of the cavity, as a function of the drop impact shape for the same impact velocity. This is done over a range of impact velocities. The prolate drops generate deeper cavities than spherical or oblate drops. Furthermore, a study is conducted on the jet formation that occurs during the cavity collapse to investigate the influence of droplet shape on the jet velocity.