Depuis deux dernières décennies, la séparation magnétique revient sur le tapis grâce aux applications biomédicales émergentes à la séparation de cellules ou de protéines et aux tests immunologiques. Cette thèse porte sur l’exploration détaillée de la séparation magnétique de nanoparticules à l’échelle microfluidique, amplifiée par la séparation de phase induite par le champ. Dans ce but, nous synthétisons des nanoclusters superparamagnétiques d’oxyde de fer de taille 40-70 nm, composés de nanoparticules de taille 7-9 nm. Nous faisons une étude détaillée de la cinétique de la séparation de phase de ces nanoclusters induite par le champ ainsi que de leur séparation magnétique dans des canaux microfluidiques munis de réseaux ordonnés de micropiliers magnétisables. Le taux d’agrégation de nanoclusters est principalement régi par le paramètre du couplage dipolaire et par la fraction volumique de nanoclusters, tandis que l’efficacité de capture – par le nombre de Mason. Les couches de molécules adsorbées sur la surface de nanoclusters d’habitude affaiblissent les interactions magnétiques et diminuent l’efficacité de capture, cependant, dans certains cas, elles peuvent induire des interactions colloïdales attractives et augmenter l’efficacité de capture. Les résultats de ce travail peuvent être utiles pour le développement des tests immunologiques magnéto-microfluidiques. / Magnetic separation has been gaining a new interest during two last decades thanks to emerging biomedical applications to cell or protein separation and immunoassays. This thesis is aimed at detailed exploration of magnetic nanoparticle separation in microfluidic scale enhanced by field-induce phase separation of nanoparticles. To this purpose, we synthesize superparamagnetic iron oxide nanoclusters of a size of 40-70 nm composed of numerous nanoparticles of a size 7-9 nm. We perform a detailed study of the kinetics of the field-induced phase separation of these nanoclusters as well as of their magnetic separation in microfluidic channels equipped with ordered arrays of magnetizable micropillars. The nanocluster aggregation rate is mostly governed by the dipolar coupling parameter and the nanocluster volume fraction, while the capture efficiency – by the Mason number. Molecular layers adsorbed on the nanocluster surface usually weaken magnetic interactions and decrease the capture efficiency, however, in some casesthey may induce attractive colloidal interactions and enhance the capture efficiency. The results of this work could be useful for development of magnetomicrofluidic immunoassays.

Ein erfolgversprechender Ansatz in der biomedizinischen Forschung besteht im zielgerichteten Transport von Medikamenten und deren lokaler Anreicherung im erkrankten Bereich. Aktuelle Untersuchungen auf dem Gebiet der Krebstherapie beschäftigen sich mit dem magnetischen Drug Targeting, der Kopplung von Chemotherapeutika an magnetische Nanopartikel und der Anreicherung im erkrankten Bereich unter Verwendung externer Magnetfelder. Um derartige Verfahren perspektivisch zu beherrschen sind die Grundlagen des Strömungsverhaltens der eingesetzten sogenannten Ferrofluide, Suspensionen magnetischer Nanopartikel in geeigneten Trägermedien zu ermitteln. Während von Ferrofluiden aus dem technischen Anwendungsbereich eine starke Viskositätserhöhung durch den Einfluss externer magnetischer Felder bekannt ist, gilt es diese auch für biokompatible Ferrofluide zu untersuchen. Ein besonderer Fokus liegt auf der Untersuchung der zahlreichen Einflussparameter wie dem Partikeldurchmesser, der mikroskopischen Struktur oder der magnetischen Konzentration. Weiterhin ist auch das Fließverhalten bei Verdünnung mit Blut in einer Strömungssituation möglichst nahe der medizinischen Anwendung von zentralem Interesse. Die vorliegende Arbeit beschäftigt sich mit diesen Problemstellungen. Einerseits wurden durch eine umfassende Charakterisierung wichtige Einflussparameter der Viskositätsveränderung unter dem Einfluss externer magnetischer Felder identifiziert und untersucht. Andererseits wurde ein spezielles Kapillarviskosimeter entwickelt. Dieses zeichnet sich durch die Auslegung hinsichtlich der Kapillardurchmesser sowie der Scherraten an Bereiche des menschlichen Organismus aus und es eröffnet die Möglichkeit, mit Blut verdünnte Ferrofluide unter dem Einfluss starker Magnetfelder zu untersuchen. Im Rahmen der Arbeit wurde Schafblut verwendet und es konnten Effekte gefunden werden, die eine Interaktion der Blutbestandteile mit den magnetischen Nanopartikeln vermuten lassen. Die Bildung von kettenartigen Strukturen unter dem Einfluss von Magnetfeldern, die diese Wechselwirkung verursacht, wurde mikroskopisch untersucht und ein Quantifizierungsverfahren zur Bewertung der Abhängigkeit von Magnetfeldstärke und -applikationsdauer eingeführt. Die ermittelten Resultate zeigen eine starke Beeinflussung des rheologischen und mikroskopischen Verhaltens der biokompatiblen Ferrofluide auf, welche das Potenzial besitzt die Anwendung der Flüssigkeiten zu beeinflussen und in zukünftige Forschungen, sowohl hinsichtlich der theoretischen Modellierung als auch der chemischen Synthese, einbezogen werden sollte.

Suspensionen magnetischer Nanopartikel – sogenannte Ferrofluide – haben in den vergangenen Jahren große Bedeutung bezüglich ihrer technischen Anwendung gewonnen. Parallel zur Entwicklung des technischen Einsatzes wird auch seit langer Zeit die Möglichkeit einer Verwendung in der Krebstherapie diskutiert. Allerdings haben Tierversuche gezeigt, dass für den erfolgreichen Übergang in klinische Studien noch wesentliche offene Fragen geklärt werden müssen, wobei die Biodistribution der magnetischen Partikel im Tumor und im gesamten behandelten Organismus eine der zu klärenden Kernfragen darstellt. Normalerweise werden hierfür histologische Schnitte durchgeführt, die jedoch nur lokale, zweidimensionale Informationen liefern. Einen detaillierten Einblick in die Verteilung bietet die Röntgen-Mikrotomografie, deren Einsatz bereits eine Reihe wesentlicher Erkenntnisse in diesem Zusammenhang erbracht hat. / Suspensions of magnetic nanoparticles – commonly called ferrofluids – are nowadays widely used in technical applications. Parallel to this development, it has been discussed for a long time whether these fluids could be used in cancer treatment. In this context, animal experiments have shown that there are still a number of fundamental questions to be clarified before proceeding to clinical tests. One of these points concerns determination of the biodistribution of the magnetic particles, both in the tumour tissue and in the organism as a whole. The standard determination method involves histological sections, but this provides only local, two-dimensional information. A much more detailed insight into the distribution of the particles can be obtained by means of x-ray microtomography, a method which has meanwhile already provided extensive and valuable information in this context.

No description available.

Nanotechnology has made it possible to create materials with unique properties. This development offers new opportunities and overcomes challenges for many thermal transport applications. Yet, it requires a more fundamental scientific understanding of nanoscale transport. This thesis emphasizes how simulation, mathematical, and numerical methods can lead to more grounded studies of nanoscale thermal transport for biological and physical applications. For instance, magnetic fluid hyperthermia (MFH), an emerging cancer treatment, is a noninvasive method to selectively destroy a tumor by heating a ferrofluid-impregnated malignant tissue with minimal damage to the surrounding healthy tissue. We model the problem by considering an idealized spherical tumor that is surrounded by healthy tissue. The dispersed magnetic nanoparticles in the tumor are excited by an AC magnetic field to generate heat. The temperature distribution during MFH is investigated through a bioheat transfer relation which indicates that the P\'eclet, Joule, and Fourier numbers are the more influential parameters that determine the heating during such a thermotherapy. Thus, we show that a fundamental parametric investigation of the heating of soft materials can provide pathways for optimal MFH design. Since ferrofluid materials themselves play a key role in heating, we examine six materials that are being considered as candidates for MFH use. These are simulated to investigate the heating of ferrofluid-loaded tumors. We show that iron-platinum, magnetite, and maghemite are viable MFH candidates since they are able to provide the desired heating of a tumor which will destroy it while keeping the surrounding healthy tissues at a relatively safe temperature. Recent advances in the synthesis and nanofabrication of electron devices have lead to diminishing feature sizes. This has in turn increased the power dissipation per unit area that is required to cool the devices, leading to a serious thermal management challenge. The phonon thermal conductivity is an important material property because of its role in thermal energy transport in semiconductors. A higher thermal conductivity material is capable of removing more heat since higher frequency phonons are able to travel through it. In this thesis, the effects of surface stress on the lattice thermal conductivity are presented for a silicon nanowire. Based on a continuum approach, a phonon dispersion relation is derived for a nanowire that is under surface stress and the phonon relaxation time is employed to subsequently determine its thermal conductivity. The surface stress is found to significantly influence the phonon dispersion and thus the Debye temperature. Consequently, the phonon thermal conductivity decreases with increasing surface stress. Different magnitudes of surface stress could arise from various material coatings and through different nanofabrication processes, effects of which are generally unclear and not considered. Our results show how such variations in surface stress can be gainfully used in phonon engineering and to manipulate the thermal conductivity of a nanomaterial. The thermal transport during thermoelectric cooling is also an important property since thermoelectric devices are compact, reliable, easy to control, use no refrigerants and require lower maintenance than do more traditional refrigeration devices. We focus on the Thomson effect that occurs when there is a current flow in the presence of a temperature gradient in the material, and investigate its influence on an intrinsic silicon nanowire cooler. The temperature dependence of the Thomson effect has a significant influence on the cooling temperature. We also consider thermal nonequilibrium between electrons and phonons over the carrier cooling length in the nanowire. The results show that a strong energy exchange between electrons and phonons lowers the cooling performance, suggesting useful strategies for thermoelectric device design. / Ph. D.

Estruturas líquidas cristalinas estão presentes em materiais compostos por moléculas com anisotropia de forma e são caracterizadas por uma ordem orientacional de longo alcance. Na mesofase nemática as moléculas alongadas tendem a se alinhar paralelas entre si, a direção média de orientação define é caracterizada por um vetor unitário, chamado de diretor n. Os cristais líquidos liotrópicos são obtidos usualmente a partir da dispersão de moléculas anfílicas em água. Essas moléculas anfílicas, por terem uma parte polar e uma parte apolar, tendem a formar agregados anisotrópicos, com a parte polar da molécula na superfície. Esse tipo de estrutura esta presente em todos os seres vivos e o exemplo mais clássico é a membrana da célula. A existência de uma superfície de contorno ou uma fronteira modifica as propriedades de um material na vizinhança dessa fronteira. Isso é particularmente verdadeiro para os cristais líquidos, que são muitos sensíveis as condições de contorno, mesmos quando estas não são tão intensas. De fato essa propriedade e bastante útil para a fabricação de dispositivos eletro-ó pticos. Um dos efeitos de superfície mais evidente é a mudança no ordenamento das moléculas devido a quebra de simetria na superfície. Nas fases nemáticas esse efeito microscópico resulta na formação de uma camada superficial com uma ordem posicional que se estende ao longo de uma certa distância no volume. 0 outro efeito resulta da origem a mudanças significativas no estado orientacional das moléculas no volume devido a presença de forças superficiais. Esse efeito é conhecido como ancoramento. Na ausência de campos extemos ou condições de contorno qualquer orientação é igualmente provável. Neste trabalho serão apresentados os resultados de investigação das propriedades de cristais líquidos liotrópicos na interface com um substrata. Estudamos o comportamento dinâmico da camada superficial quando o campo magnético é aplicado e induz uma re­ orientação do diretor. Nesse estudo são utilizadas amostras de cristal líquidos liotrópicos. / Liquid crystalline structures are found in materials made from molecules which are anisotropic in shape; such material are characterized mainly by a long range orientational order. In the nematic phase, the rodlike molecules tend to align parallel to each other. The average orientation defines a unit vector, called director n. Lyotropic liquid crystals are usually obtained by the dispersion of amphiphilic molecules in water. Due the fact that these molecules present a polar head and a non-polar tail, they tend to form aggregates with the polar part at the aggregate surface. This type of struc­ ture is present in all living being; the most classical example is the cell membrane. The existence of a boundary surface affects the properties of a material close to this boundary. This particularly true for liquid crystals, which are very sensitive to boundary conditions, even when weak. Indeed, this property is quite useful in the fabrication of electro-optic devices. One of the most obvious surface effect is the change in the molecules organization due to a breaking of the symmetry at the boundary surface. In the nematic phases, this microscopic effect results in the formation of a surface la yer with a positional order that extends through the bulk up to a certain distance from the surface. The other effect of the surface results in a change in the orientational state of the molecules in the volume due to the sur face forces. This macroscopic effect is known as anchoring. In the absence of any external field or boundary conditions, all molecular orientation is equally probable. In this work, we will present some results of the investigation of the properties of lyotropic liquid crystals at the interface with a solid substrate. We have studied the dy­ namic of the surface layer when a applied magnetic field induces a reorientation of the nematic director. In this study different systems were used: lyotropic liquid crystals in the nematic phase, ferronematics (nematic phase doped with ferrofluid) and filled nematics (nematic phase doped with silica nano-spheres). By means of transmittance measurements.

Le Génie Electrique, en général, et l'électronique de puissance, en particulier, prend une part de plus en plus importante dans les systèmes embarqués. La fiabilité des systèmes électroniques dépend fortement de la gestion de leur température. Les systèmes de refroidissement actuels sont lourds, volumineux, et consommateurs d'énergie, ce qui est en désaccord avec les systèmes embarqués. Il est donc nécessaire de chercher de nouveaux systèmes, plus fiables, plus légers, et moins énergivores. Le sujet de cette thèse porte sur l'utilisation des ferrofluides, suspensions colloïdales magnétiques, à basse température de Curie, dont les propriétés magnétiques varient fortement avec la température entre l'ambiante et une centaine de degrés Celsius afin de les utiliser comme liquide caloporteur dans les systèmes de refroidissement. Les propriétés magnétiques fortement dépendantes de la température d'un tel fluide permettent la mise en mouvement de ce dernier par l'action couplée d'un champ magnétique et d'un gradient de température alors que toutes les pièces solides restent fixes. Le système de refroidissement n'est alors plus assujetti à l'usure de la pompe permettant la circulation du fluide caloporteur. Le système est ainsi globalement plus fiable et moins consommateur d'énergie. L'énergie de mise en mouvement du ferrofluide étant extraite directement des pertes des composants. Le comportement des ferrofluides est trop méconnu à l'heure actuelle pour concevoir et optimiser une pompe statique magnétothermique. Un effort important de modélisation et de caractérisation doit être mené. Ce manuscrit présente une étude pratique vérifiant le principe de création de pression hydrostatique par couplage magnétothermique. Une modélisation de la distribution des forces locales mettant en mouvement le ferrofluide ainsi que la mise en place d'outil permettant la caractérisation des ferrofluides sont également présentées. Les efforts de caractérisation se sont concentrés sur la rhéologie, au regard du champ magnétique, du cisaillement et de la température, ainsi que sur le comportement magnétique du ferrofluide à différentes températures. / The Electrical Engineering in general and power electronics, in particular, plays an increasingly important role in embedded systems. The reliability of electronic systems strongly depends on the management of their temperature. Cooling systems today are heavy, bulky, and consumers of energy, which is in disagreement with embedded systems. It is therefore necessary to look for new systems, more reliable, lighter and use less energy. The subject of this thesis focuses on the use of ferrofluids, magnetic colloidal suspensions at low Curie temperature, the magnetic properties vary strongly with temperature between ambient and one hundred degrees Celsius, for use as coolant in cooling systems. The magnetic properties strongly dependent on the temperature of such fluid allow the actuation of the latter by the action of a magnetic field coupled at a temperature gradient so that all solid parts are stationary. The cooling system is no longer subject to the wear of the pump for the circulation of the coolant. The system is thus globaly more reliable and less energy consuming. The energy for moving the ferrofluid being extracted directly losses components. The behavior of ferrofluids is too little known today to design and optimize a pump magneto static. A major effort of modeling and characterization should be conducted. This manuscript presents a practical study verifying the principle of hydrostatic pressure created by magnetothermal coupling. A modeling of the distribution of local forces by moving the ferrofluid and the development tool for the characterization of the ferrofluids are also presented. Characterization efforts focused on rheology, under the magnetic field, shear and temperature, as well as on the magnetic behavior of the ferrofluid at different temperatures.

Estruturas líquidas cristalinas estão presentes em materiais compostos por moléculas com anisotropia de forma e são caracterizadas por uma ordem orientacional de longo alcance. Na mesofase nemática as moléculas alongadas tendem a se alinhar paralelas entre si, a direção média de orientação define é caracterizada por um vetor unitário, chamado de diretor n. Os cristais líquidos liotrópicos são obtidos usualmente a partir da dispersão de moléculas anfílicas em água. Essas moléculas anfílicas, por terem uma parte polar e uma parte apolar, tendem a formar agregados anisotrópicos, com a parte polar da molécula na superfície. Esse tipo de estrutura esta presente em todos os seres vivos e o exemplo mais clássico é a membrana da célula. A existência de uma superfície de contorno ou uma fronteira modifica as propriedades de um material na vizinhança dessa fronteira. Isso é particularmente verdadeiro para os cristais líquidos, que são muitos sensíveis as condições de contorno, mesmos quando estas não são tão intensas. De fato essa propriedade e bastante útil para a fabricação de dispositivos eletro-ó pticos. Um dos efeitos de superfície mais evidente é a mudança no ordenamento das moléculas devido a quebra de simetria na superfície. Nas fases nemáticas esse efeito microscópico resulta na formação de uma camada superficial com uma ordem posicional que se estende ao longo de uma certa distância no volume. 0 outro efeito resulta da origem a mudanças significativas no estado orientacional das moléculas no volume devido a presença de forças superficiais. Esse efeito é conhecido como ancoramento. Na ausência de campos extemos ou condições de contorno qualquer orientação é igualmente provável. Neste trabalho serão apresentados os resultados de investigação das propriedades de cristais líquidos liotrópicos na interface com um substrata. Estudamos o comportamento dinâmico da camada superficial quando o campo magnético é aplicado e induz uma re­ orientação do diretor. Nesse estudo são utilizadas amostras de cristal líquidos liotrópicos. / Liquid crystalline structures are found in materials made from molecules which are anisotropic in shape; such material are characterized mainly by a long range orientational order. In the nematic phase, the rodlike molecules tend to align parallel to each other. The average orientation defines a unit vector, called director n. Lyotropic liquid crystals are usually obtained by the dispersion of amphiphilic molecules in water. Due the fact that these molecules present a polar head and a non-polar tail, they tend to form aggregates with the polar part at the aggregate surface. This type of struc­ ture is present in all living being; the most classical example is the cell membrane. The existence of a boundary surface affects the properties of a material close to this boundary. This particularly true for liquid crystals, which are very sensitive to boundary conditions, even when weak. Indeed, this property is quite useful in the fabrication of electro-optic devices. One of the most obvious surface effect is the change in the molecules organization due to a breaking of the symmetry at the boundary surface. In the nematic phases, this microscopic effect results in the formation of a surface la yer with a positional order that extends through the bulk up to a certain distance from the surface. The other effect of the surface results in a change in the orientational state of the molecules in the volume due to the sur face forces. This macroscopic effect is known as anchoring. In the absence of any external field or boundary conditions, all molecular orientation is equally probable. In this work, we will present some results of the investigation of the properties of lyotropic liquid crystals at the interface with a solid substrate. We have studied the dy­ namic of the surface layer when a applied magnetic field induces a reorientation of the nematic director. In this study different systems were used: lyotropic liquid crystals in the nematic phase, ferronematics (nematic phase doped with ferrofluid) and filled nematics (nematic phase doped with silica nano-spheres). By means of transmittance measurements.

Мы исследуем термодинамические свойства магнитных жидкостей с небольшими и умеренными значениями параметра диполь-дипольного взаимодействия в отсутствии внешнего магнитного поля. Магнитные жидкости моделируются монодисперсной системой дипольных твердых сфер. Построена теория, основанная на обратном кумулянтном преобразовании вириального разложения свободной энергии Гельмгольца в ряд по плотности. Методом компьютерного моделирования Mayer Sampling получены данные для пятого вириального коэффициента. По данным компьютерного моделирования получены аналитические выражения для четвертого и пятого вириальных коэффициентов. В работе были определены аналитические выражения для свободной энергии, давления, химического потенциала и коэффициента градиентной броуновской диффузии. Построенная теория хорошо согласуется с данными компьютерного моделирования в широкой области концентраций феррочастиц (φ≤4) и интенсивности межчастичного диполь-дипольного взаимодействия (λ≤4). / We investigate thermodynamic properties of ferrofluids with low and intermediate dipolar coupling constant. The model of dipolar hard spheres is observed. In this work a new theory is constructed based on the transformation of the virial expansion for the Helmholtz free energy into a logarithmic form. Analytical expressions of the forth and the fifth virial coefficients are obtained on the basis of numerical results of Mayer-sampling calculation. In this work analytical expressions for free energy, chemical potential, pressure and gradient diffusion coefficient were obtained. The comparison between theory and computer simulation shows good agreement for dipolar coupling constant λ≤4 and for particle volume fraction φ≤4.

Confronted with the global demographic changes and the increasing pressure on modern healthcare system, there has been a surge of developing new technology platforms in the past decades. Droplet microfluidics is a prominent example of such technology platforms, which offers an efficient format for massively parallelized screening of a large number of samples and holds great promise to boost the throughput and reduce the costs of modern biomedical activities. Despite recent achievements, the realization of a compact and generic screening system which is suited for resource-limited settings and point-of-care applications remains elusive. To address the above challenges, the dissertation focuses on the development of a compact multifunctional droplet micro-magnetofluidic system by exploring the advantages of magnetic in-flow detection principles. The methodologies behind a novel technique for biomedical applications, namely, magnetic in-flow cytometry have been put forth, which encompass magnetic indexing schemes, quantitative multiparametric analytics and magnetically-activated sorting. A magnetic indexing scheme is introduced and intrinsic to the magnetofluidic system. Two parameters characteristic of the magnetic signal when detecting magnetically functionalized objects, i.e. signal amplitude and peak width, providing information which is necessary to perform quantitative analysis in the spirit of optical cytometry has been proposed and realized. Magnetically-activated sorting is demonstrated to actively select individual droplets or to purify a population of droplets of interest. Together with the magnetic indexing scheme and multiparametric analytic technique, this functionality synergistically enables controlled synthesis, quality administration and screening of encoded magnetic microcarriers, which is crucial for the practical realization of magnetic suspension arrays technologies. Furthermore, to satisfy the needs of cost-efficient fabrication and high-volume delivery, an approach to fabricate magnetofluidic devices on flexible foils is demonstrated. The resultant device retains high performance of its rigid counterpart and exhibits excellent mechanical properties, which promises long-term stability in practical applications.