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Viscoelastic properties of ferrofluid - Study of normal stresses differences Master Thesis Hana Šustková Abstract: Since 1960, a liquid becoming strongly magnetized in the presence of a mag- netic field, called ferrofluid, is known. This colloidal liquid made of ferro- or ferrimagnetic nanoparticles, with diameter in range 10-20 nm, suspended in a carrier fluid. This work focuses on the study of the viscoelastic properties of a selected ferrofluid - the steady-state and dynamic properties of the fluid with comparison to a standard fluid (basis fluid, etc.) and a MR fluid. Rheological measurements are carried out using a commercial rheometer. A magnetic cell for this rheometer is designed and built. In this magnetic cell the experiments are realized, the whole system is calibrated and the experiments performed. The ferrofluid APG513a, MR-fluid and reference fluid were used. Study of normal stresses differences (N1,N2) in liquids is carried out. Ac- cording to derived theoretical model, the course of N1 and N2 should be strongly dependent on the field - this was studied directly by measuring of the normal forces or indirectly applying the Laun's rule. A good accordance of theory and experiment was found and an anisotropic behaviour of ferrofluid was studied. The shear-thinning ability of the ferrofluid in normal direction...

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.

Nesta tese apresentamos um estudo do comportamento magnético de sistemas de nanopartículas por meio de medidas experimentais e simulações Monte Carlo. Estudamos o papel das interações entre partículas experimentalmente a baixas temperaturas em amostras de ferrofluidos comerciais por meio de curvas ZFC-FC, delta m e diagramas FORC. Observamos nas curvas ZFC-FC o fenômeno de super-resfriamento e transições de fase do estado sólido para o líquido em ferrofluidos. Para amostras de cristais líquidos dopados com nanopartículas magnéticas, observamos a transição entre as fases isotrópica e nemática. Detectamos em amostras de ferrofluidos e em soluções micelares dopadas com nanopartículas um aumento da viscosidade na presença de um campo magnético aplicado, o chamado efeito magnetoviscoso, que surge devido às interações entre partículas. Nas simulações Monte Carlo, vimos que a temperatura crítica (Tc) diminui com o tamanho das partículas, e que esse comportamento pode ser descrito por uma lei de escala. As simulações também mostraram que uma camada morta na superfície das nanopartículas provoca uma pequena diminuição na temperatura crítica, o que não ocorre quando adicionamos uma camada dura, que pode aumentar significativamente Tc. Para simulações de um sistema de nanopartículas interagentes, demos especial atenção a interpretar de que forma as interações magnetizantes e desmagnetizantes se manifestam em diagramas FORC para um conjunto de nanopartículas com distribuição de tamanhos. Observamos que uma interação desmagnetizante está associada a um deslocamento do pico do diagrama FORC para campos locais de interação Hb positivos e que a presença de uma interação magnetizante pode deslocar esse pico para campos Hc , relacionados à distribuição de coercividades do sistema, maiores. / In this thesis we present a study of the behavior of a system of magnetic nanoparticles by means of experimental measurements and Monte Carlo simulations. We experimentally study the role of the interactions between particles at low temperatures in commercial samples of ferrofluids through ZFC-FC, delta m curves, and FORC diagrams. We observed the phenomenon of supercooling and phase transitions from solid to liquid states in the ZFC-FC curves of ferrofluids. For the samples of liquid crystal doped with magnetic nanoparticles, we saw the transition between the isotropic and nematic phases. We detected in the samples of ferrofluids and in micellar solutions doped with nanoparticles an increase of the viscosity in the presence of an applied magnetic field, the so-called magnetoviscous effect, which arises due to interactions between particles. In the Monte Carlo simulations, we found that the critical temperature (Tc) decreases with particle size, a behavior that is described well by a scaling law. The simulations also showed that a dead layer on the surface of the nanoparticles causes a slight decrease in the critical temperature value, what does not occur when we add a hard layer, which increases Tc significantly. For simulations of a system of interacting nanoparticles, we paid special attention to interpret how the magnetizing and demagnetizing interactions manifest themselves in FORC diagrams for a set of nanoparticles with size distribution. We observed that demagnetizing interactions is associated with a displacement of the peak of the FORC diagram to positive values of the local field interaction Hb , and that the presence of a magnetizing interaction can shift this peak to larges values of the Hc field, related to the distribution of coercivities.

Advances in molecular-simulation methods allow for ever larger systems of particles to be studied and on longer timescales. Calculations are reaching such a scale that they can be used to address a vast range of key questions across chemistry, physics, and engineering. In this work, molecular dynamics and Monte Carlo simulations are employed to address two key areas: the structure and dynamics of simple aqueous ionic salt solutions at high concentrations; and the structure, dynamics, and phase behaviour of dipolar fluids (such as colloidal ferrofluids). The first part of the work begins with a study of the structure and dynamics in metastable, supersaturated, aqueous solutions of potassium chloride, and the possible relevance of these to the phenomenon of non-photochemical laser-induced nucleation (NPLIN). It is thought that the potassium and chloride ions form long-lived, amorphous clusters that may, under the influence of nanosecond laser pulses, undergo structural reorganisation to form post-critical crystal nuclei. It is found that spontaneous nucleation does not occur on the simulation timescale, but that amorphous clusters do form with cluster lifetimes comparable to those of the shortest laser pulses that can be used in NPLIN ( 100 picoseconds). Next, an alternative scenario for NPLIN involving rapid laser heating of impurity particles is examined by simulating heated carbon nanoparticles in saturated aqueous solutions of sodium chloride. The concentration at which an aqueous sodium chloride solution first crystallises on the simulation timescale is determined. A spherical carbon impurity is then added to a system with concentration close to, but lower than, the concentration at which crystallisation occurs on the simulation timescale. The effects that adding, and heating, this impurity has on the structure of this near-crystallising system are then observed. The second part of the work discusses model dipolar fluids, of direct relevance to colloidal ferrofluids (suspensions of magnetised nanoparticles in simple carrier liquids). The two-body, dipole-dipole interaction is long-ranged and anisotropic, and it is computationally expensive to handle in molecular simulations. Here a new method is proposed that relies on a formal mapping between the partition function of a dipolar fluid and that of a hypothetical fluid with many-body, short-ranged, isotropic interactions. Only the leading-order two-body interactions (akin to the van der Waals attraction) and three-body interactions (corresponding to the Axilrod-Teller potential) are retained. It is shown that this simple model is sufficient to reproduce the characteristic particle chaining and the associated disappearance of the vapour-liquid phase transition of dipolar fluids. Finally, the dynamical response of ferrofluids to oscillating magnetic fields (the dynamic magnetic susceptibility [DMS]) is studied. The DMS of ferrofluids, predicted by a new theory that takes into account the leading-order effects of dipole-dipole interactions, are critically compared to those found using Brownian-dynamics simulations of monodisperse systems of dipolar particles. This new theory is found to provide more accurate predictions of the DMS than previous theories, with the DMS predicted to a high degree of accuracy for systems with dipolar coupling strength in the experimentally achievable region.

Nesta tese apresentamos um estudo do comportamento magnético de sistemas de nanopartículas por meio de medidas experimentais e simulações Monte Carlo. Estudamos o papel das interações entre partículas experimentalmente a baixas temperaturas em amostras de ferrofluidos comerciais por meio de curvas ZFC-FC, delta m e diagramas FORC. Observamos nas curvas ZFC-FC o fenômeno de super-resfriamento e transições de fase do estado sólido para o líquido em ferrofluidos. Para amostras de cristais líquidos dopados com nanopartículas magnéticas, observamos a transição entre as fases isotrópica e nemática. Detectamos em amostras de ferrofluidos e em soluções micelares dopadas com nanopartículas um aumento da viscosidade na presença de um campo magnético aplicado, o chamado efeito magnetoviscoso, que surge devido às interações entre partículas. Nas simulações Monte Carlo, vimos que a temperatura crítica (Tc) diminui com o tamanho das partículas, e que esse comportamento pode ser descrito por uma lei de escala. As simulações também mostraram que uma camada morta na superfície das nanopartículas provoca uma pequena diminuição na temperatura crítica, o que não ocorre quando adicionamos uma camada dura, que pode aumentar significativamente Tc. Para simulações de um sistema de nanopartículas interagentes, demos especial atenção a interpretar de que forma as interações magnetizantes e desmagnetizantes se manifestam em diagramas FORC para um conjunto de nanopartículas com distribuição de tamanhos. Observamos que uma interação desmagnetizante está associada a um deslocamento do pico do diagrama FORC para campos locais de interação Hb positivos e que a presença de uma interação magnetizante pode deslocar esse pico para campos Hc , relacionados à distribuição de coercividades do sistema, maiores. / In this thesis we present a study of the behavior of a system of magnetic nanoparticles by means of experimental measurements and Monte Carlo simulations. We experimentally study the role of the interactions between particles at low temperatures in commercial samples of ferrofluids through ZFC-FC, delta m curves, and FORC diagrams. We observed the phenomenon of supercooling and phase transitions from solid to liquid states in the ZFC-FC curves of ferrofluids. For the samples of liquid crystal doped with magnetic nanoparticles, we saw the transition between the isotropic and nematic phases. We detected in the samples of ferrofluids and in micellar solutions doped with nanoparticles an increase of the viscosity in the presence of an applied magnetic field, the so-called magnetoviscous effect, which arises due to interactions between particles. In the Monte Carlo simulations, we found that the critical temperature (Tc) decreases with particle size, a behavior that is described well by a scaling law. The simulations also showed that a dead layer on the surface of the nanoparticles causes a slight decrease in the critical temperature value, what does not occur when we add a hard layer, which increases Tc significantly. For simulations of a system of interacting nanoparticles, we paid special attention to interpret how the magnetizing and demagnetizing interactions manifest themselves in FORC diagrams for a set of nanoparticles with size distribution. We observed that demagnetizing interactions is associated with a displacement of the peak of the FORC diagram to positive values of the local field interaction Hb , and that the presence of a magnetizing interaction can shift this peak to larges values of the Hc field, related to the distribution of coercivities.

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.

Les ferrofluides sont des suspensions colloïdales de nanoparticules magnétiques dispersées dans un liquide porteur. La possibilité de moduler les propriétés des ferrofluides in situ en appliquant un champ magnétique externe leur procure un fort potentiel d’étude, à la fois d’un point de vue fondamental ou pour des applications industrielles variées. En particulier, les nanospinels de ferrite ferrimagnétiques MFe2O4 (M = Fe2+, Co2+, Mn2+…) sont largement étudiés pour leurs propriétés électriques et magnétiques. Plus spécifiquement, une forte énergie d’anisotropie de ces matériaux à l’échelle nanométrique est requise pour des applications dans le stockage de l’information ou l’hyperthermie pour lesquels ils sont considérés. Une connaissance fine des mécanismes régissant ces propriétés d’anisotropies magnétiques est ainsi primordiale pour la création de nouveaux objets aux propriétés magnétiques contrôlées à l’échelle nanométrique. L’originalité de notre approche consiste à utiliser une technique fine du magnétisme, le dichroïsme magnétique circulaire des rayons X (XMCD) à l’étude des anisotropies et couplages magnétiques des nanospinels composants les ferrofluides. Au cours de cette thèse, nous nous sommes intéressés à différentes stratégies possibles pour induire une forte énergie d’anisotropie aux nanospinels de ferrite par l’utilisation de cobalt. Des nanoparticules de tailles et compositions variées ont été obtenues par différentes voies de synthèse, et nous démontrons que l’anisotropie magnétique de ces systèmes est fortement gouvernée par la symétrie de site du Co2+ en structure spinel qui peut être directement corrélé au processus de synthèse utilisé. Nous nous sommes aussi intéressés à l’ordre et au couplage magnétique de ferrite spinels structurés en coeur-coquille, dont le cœur et la coquille sont réalisés à partir de matériaux aux propriétés magnétiques intrinsèques différentes. Nous montrons ainsi que pour des nanospinels MnFe2O4@CoFe2O4, la très fine coquille formée de CoFe2O4 impose une forte anisotropie magnétique au cœur doux de MnFe2O4. Enfin, nous nous sommes intéressés à une troisième classe de ferrofluide à base de nanospinels, les ferrofluides binaires, constitué d’un mélange physique de ferrofluides aux propriétés magnétiques intrinsèques différentes. Pour de tels systèmes, il est essentiel de préserver le liquide porteur du ferrofluide pour ne pas dénaturer les interactions entre particules existantes. L’un des objectifs de cette thèse fut donc d’étendre la technique du XMCD à l’étude d’échantillons de ferrofluides in situ, dans leur phase liquide ou gelée. Nous avons débuté la conception d’une cellule liquide compatible avec les rayons X mous et un environnement ultra-vide sur la ligne de lumière DEIMOS (SOLEIL) qui est toujours en développement... / Ferrofluids are colloidal suspensions of magnetic nanoparticles dispersed in a carrier liquid. The intimate interaction between the magnetic nanoparticles and the liquid provides a unique system, from both fundamental and industrial application point of views, whose flow and properties can be precisely controlled using an external magnetic field. Magnetic nanoparticles of spinel ferrites MFe2O4 (M = Fe2+, Co2+, Mn2+…) are of particular scientific interest and have been extensively studied for their electrical and magnetic properties. Spinel ferrites find potential applications, notably in storage devices, for computers, or hyperthermia, for cancer treatment, where high magnetic anisotropy energies are required at the nanoscale. However, deeper knowledges of the fine mechanisms playing a significant role on the magnetic anisotropies existing in the nanospinels are necessary to help the creation of rationalized materials with controlled magnetic anisotropies for the requirement of the system. In this thesis, we have used X-ray Magnetic Circular Dichroism (XMCD) as an original approach for probing the magnetic anisotropies and magnetic couplings of nanospinels obtained in ferrofluids. The nanoparticles are iron bearing spinels for which cobalt ions have been introduced in the spinel structure of the nanoparticles as a true makers of magnetic anisotropy. First, magnetic nanospinels have been synthesized by tuning their size and composition and using different synthesis processes. XMCD investigations revealed that the coercive field of the nanospinels is governed by the concentration of Co2+ ions sitting in octahedral sites of the spinel structure, and this can be directly linked to some synthesis parameters. Then, we have investigated core@shell nanoparticles, which can be synthesized with an appropriate choice of magnetic anisotropies for the core and the shell in order to tailor optimal magnetic properties. In the case of MnFe2O4@CoFe2O4, our findings reveal that the very thin CoFe2O4 shell imposes a strong magnetic anisotropy to the otherwise very soft MnFe2O4 core. The other class of ferrofluids that has been investigated during this thesis are binary ferrofluids that are constituted of two different types of magnetic nanoparticles. For such systems, the carrier liquid must be preserved to understand the magnetic interactions in the ferrofluid as they are. Another motivation of this thesis was thus to extend XMCD to the in situ investigation of the nanospinels dispersed in ferrofluids. We have been started a liquid cell development in the DEIMOS beamline at SOLEIL. The setup is still in progress and is aimed at being compatible with soft X-Rays short penetration depth and ultra-high vacuum environment. Hard X-ray photon-in/photon-out spectroscopy coupled to XMCD (1s2p RIXS-MCD) can be a very valuable alternative to soft X-ray XMCD at K-edge of 3d elements when liquid cell sample environment is required. The instrumental development of a liquid cell used with 1s2p RIXS-MCD spectroscopy allowed us to investigate the nanoparticles directly in the ferrofluids revealing interparticles magnetic couplings in binary ferrofluids.

Les dispersions de nanoparticules magnétiques (NPs) dans les solvants polaires sont utilisées dans de nombreuses applications dans des domaines variés, du biomédical à l'environnement ou à l'énergie. Aussi appelés ferrofluides (FFs), ces systèmes sont des dispersions de ferrites spinelle magnétiques pouvant être stabilisées par des répulsions électrostatiques. Cela nécessite une bonne compréhension de l'interface NPs/solvant porteur, qui contrôle les interactions entre NPs, la nanostructure et de nombreuses autres propriétés. Nous étudions ici en milieu aqueux la réactivité électrochimique de particules c¿ur/couronne de type MFe2O4@ Fe2O3 (M = Fe,Co,Mn,Cu,Zn), espèces électroactives non conventionnelles. La voltammétrie à signaux carrés et la coulométrie à potentiel contrôlé permettent d'étudier la coquille de maghémite ( Fe2O3), dont le rôle est la protection de l'oxyde mixte du c¿ur en milieu acide. D'autre part, un nouveau procédé d'élaboration de dispersions dans les solvants polaires, testé dans l'eau, est appliqué au diméthylsulfoxide (DMSO). A partir du point de charge nulle des NPs, un ajout connu d'acide ou de base permet de contrôler la charge des NPs, la nature des contreions et la quantité d'électrolyte libre. Des dispersions stabilisées par des répulsions électrostatiques sont obtenues dans le DMSO. La diffusion de rayons X aux petits angles et la diffusion dynamique de la lumière sont utilisées pour comprendre la nanostructure et quantifier les interactions entre particules. De forts effets spécifiques liés aux ions sont mis en évidence ainsi que le rôle de l'interface solide liquide, en particulier sur les propriétés de thermodiffusion. / Dispersions of magnetic nanoparticles (NPs) in polar solvents have been inspiring many applications, to cite a few, biomedical, industrial and thermoelectrical ones. Also called ferrofluids (FFs), they are usually colloidal dispersions of magnetic spinel ferrite NPs, which can be stabilized thanks to electrostatic repulsion. A good understanding of the interface between NPs and the carrier solvent is thus a key point, which governs the interparticle interactions, the nanostructure and many other applicative properties. We study here the electrochemical reactivity of core-shell ferrite MFe2O4@ Fe2O3 (M=Fe,Co,Mn,Cu,Zn) NPs in aqueous medium. Square-wave voltammetry and potential controlled coulometry techniques are used on these non-conventional electroactive systems in order to evidence the shell of maghemite ( Fe2O3), the main function of which is to ensure the thermodynamical stability of NPs in acidic medium. We also present a new process for the elaboration of maghemite based FF in polar solvents, tested in water and applied to dimethyl sulfoxide (DMSO). Departing from the point of zero charge, the NPs are charged in a controlled way by adding acid or base, which enables us to better control the charge and the counter-ions nature, as well as the amount of free electrolyte in the dispersion. Stable dispersions are obtained thanks to electrostatic repulsion, also in DMSO. Small Angle X-ray scattering and Dynamic Light Scattering are used to understand the nanostructure and quantify the interparticle interactions. Specific ionic effects are evidenced as well as the strong influence of the solid/liquid interface on the migration of the NPs in a thermal gradient.