Auswahl der wissenschaftlichen Literatur zum Thema „Magnetic microrheology“

We provide a statistical mechanics approach to study the linear microrheology of thermally equilibrated and homogeneous ferrofluids. Theexpressions for the elastic and loss moduli depend on the bulk microstructure of the magnetic fluid determined by the structure factor of thesuspension of magnetic particles. The comparison of the predicted microrheology with computer simulations confirms that as a function ofrelaxation frequency of thermal fluctuations of the particle concentration both theory and simulations have the same trends. At very shortfrequencies the viscous modulus relates to the translational and rotational self-diffusion coefficients of a ferro-particle.

We study active microrheology in 2D with Langevin simulations of tracer particles pulled through magnetic networks by a constant force. While non-magnetic tracers strongly deform the network in order to be able to move through, the magnetic tracers can do so by deforming the structure only slightly.

The protocol of microrheological measurements consists of recording the dynamics of the orientation of the rod when the magnetic field is applied at an angle to the rod and observing its relaxation after the field is switched off.

Here we present a new, compact magnetic tweezers design that enables precise application of a wide range of dynamic forces to soft materials without the need to raise or lower the magnet height above the sample. This is achieved through the controlled rotation of the permanent magnet array with respect to the fixed symmetry axis defined by a custom-built iron yoke. These design improvements increase the portability of the device and can be implemented within existing microscope setups without the need for extensive modification of the sample holders or light path. This device is particularly well-suited to active microrheology measurements using either creep analysis, in which a step force is applied to a micron-sized magnetic particle that is embedded in a complex fluid, or oscillatory microrheology, in which the particle is driven with a periodic waveform of controlled amplitude and frequency. In both cases, the motions of the particle are measured and analyzed to determine the local dynamic mechanical properties of the material.

Micrometer-sized magnetic wires are used to study the mechanical properties of human mucus collected after surgery. Our work shows that mucus has the property of a high viscosity gel characterized by large spatial viscoelastic heterogeneities.

We report the development of a microrheology technique that incorporates a magnetic-field-induced simulator on total internal reflection microscopy (TIRM) to probe the near-surface dynamics and viscoelastic behaviors of soft matter like polymer solution/gels and colloidal dispersions.

La matière active est un domaine en pleine expansion au cours de ces dernières années. Elle est constituée d'entités capables d'utiliser une source d'énergie pour produire un travail local comme de l'auto-propulsion. Cette matière, hors équilibre, possède des propriétés fascinantes comme l'auto-organisation telle qu'observée dans une nuée d'oiseaux. Cependant, cette matière ne se limite pas au vivant et des système actifs abiotiques ont été développés. En particulier, au cours de cette thèse, nous utilisons des microparticules auto-propulsées. Nos objectifs sont de comprendre comment elles s'organisent en présence de gravité et au contact d'une paroi. Notre système est constitué de colloïdes Janus Au/Pt capables de s'auto-propulser en présence d'eau oxygénée par des mécanismes phorétiques. Les colloïdes étant plus denses que l'eau, ils forment une monocouche au fond du récipient. Si ce fond est légèrement incliné, nous observons une sédimentation 2D. Pour les systèmes colloïdaux à l'équilibre, le profil de sédimentation renferme l'équation d'état du système. Pour les systèmes actifs, une équation d'état n'existe pas dans le cas général mais on peut toutefois définir des grandeurs thermodynamiques analogues. J'ai mesuré le profil de sédimentation de mon système actif et je l'ai confronté à des modèles développés pour des particules brownienne actives en milieu « sec » (ABPs). J'ai pu ainsi montrer que le rôle du fluide porteur ne peut être négligé. Dans une deuxième partie, nous nous sommes intéressés aux propriétés de mouillage de ce système. La matière active est connue pour présenter des propriétés de mouillage effectives mais aucune étude expérimentale avec un système analogue au notre ne s'est focalisée sur le phénomène de mouillage d'une paroi plongée à la verticale dans un sédiment. Nous montrons qu'il s'y forme une couche d'adhésion accompagnée d'une remontée de la densité à la paroi. Pour mieux comprendre les phénomènes observés, nous les avons confrontés à un modèle numérique d'ABPs pour lequel nous pouvons faire varier les interactions entre les particules et la paroi. En jouant sur l'adhésion et l'alignement à la paroi, on est capable de reproduire les résultats expérimentaux. En effet, l'implémentation de ces interactions à la paroi permet, dans une certaine mesure, de prendre en compte numériquement le fluide porteur et donc les interactions hydrodynamique et phorétique de nos colloïdes avec la paroi. On montre ainsi que ces interactions exacerbe grandement la polarisation de la vitesse de propulsion des particules à la paroi qui est en grande partie à l'origine de la remontée de densité. En effet, il a été démontré qu'en régime stationnaire et dilué, les particules loin de la paroi sont capables de se polariser à l'encontre de la gravité. Nous montrons que cette polarisation est amplifiée par un alignement sur une paroi verticale. De plus, l'ajout d'une attraction supplémentaire permet de piéger plus fortement les particules le long de la paroi qui vont alors remonter plus haut que ne le feraient des ABPs sans interactions avec la paroi. Au fur à mesure de leur ascension, les particules vont « s'évaporer » et chuter loin de la paroi conduisant à des mouvements globaux dans le système. La paroi agit comme un moteur de la circulation qui met en mouvement les particules dans le système de façon collective à une échelle bien plus grande que celle de la particule. Enfin, dans la perspective de caractériser la microrhéologie de la matière active, nous présentons également dans cette thèse l'ensemble des avancées sur la conception d'un nouveau microrhéomètre magnétique ainsi que les travaux sur la stabilisation des colloïdes sur des surfaces de verre dans l'objectif de concevoir des cellules d'imagerie sur mesure
Active matter is a rapidly expanding field in recent years. It consists of entities able to use an energy source to produce local work such as self-propulsion. Such matter, by being out of equilibrium, has fascinating properties such as self-organization as seen in a flock of birds. However, active matter is not limited to biological systems. Active abiotic systems have also been developed. Indeed, during this thesis, we study a system made of self-propelled microparticles. Our objectives are to understand how they organize in the presence of gravity and in contact with a wall. Our system is made of Janus Au/Pt colloids that can self-propel in the presence of hydrogen peroxide by phoretic mechanisms. The colloids being denser than water, they form a monolayer on the bottom of their container. Provided a small tilting angle, we can observed 2D sedimentation. For colloidal systems at equilibrium, the sedimentation profile contains the equation of state of the system. For active systems, an equation of state does not exist in the general case, but analogous thermodynamic quantities can be defined. I measured the sedimentation profile of my active system and compared it to models developed for active Brownian particles in a "dry" environment (ABPs). I showed that the role of the background fluid cannot be neglected. In a second part, we studied the wetting properties of our system. Active mater is known to have effective wetting properties, yet no experimental study with a system analogous to ours has focused on the wetting phenomenon of a wall vertically immersed in a sediment. We show that an adhesion layer is formed with the density rising at the wall. To better understand the observed phenomena, we have confronted them with a numerical model of ABPs for which we can vary the interactions between the particles and the wall. By playing on the adhesion and the alignment with the wall, we are able to reproduce the experimental results. Indeed, the implementation of these interactions at the wall enables, to a certain extent, to take into account numerically the background fluid and thus the hydrodynamic and phoretic interactions that our colloids have with the wall. We thus show that these interactions greatly exacerbates the polarization of the propulsion velocity of the particles at the wall which is largely responsible for the density rise. Indeed, it is known that in the dilute and stationary regime, particles far from the wall are able to polarize against gravity. This polarization is amplified by an alignement with a vertical wall. Furthermore, the addition of an additional attraction allows particles to be more strongly trapped at the wall, and rise higher than ABPs without wall interactions would. As they rise, the particles will "evaporate" and fall away from the wall leading to global fluxes in the system. The wall acts as a pump that sets the particles in motion in the system collectively at a much larger scale than the particle. Finally, because we want to investigate the microrheology on active matter, we also present in this thesis all the updates on the design of a new magnetic microrheometer as well as the work on the stabilization of colloids on glass surfaces with the objective of designing custom imaging cells

Micron scale robots going inside our body and curing various ailments is a technolog¬ical dream that easily captures our imagination. However, with the advent of novel nanofabrication and nanocharacterization tools there has been a surge in the research in this field over the last decade. In order to achieve locomotion (swim) at these small length scales, special strategies need to be adopted, that is able to overcome the large viscous damping that these microbots have to face while moving in the various bod¬ily fluids. Thus researchers have looked into the swimming strategies found in nature like that of bacteria like E.coli found in our gut or spermatozoa in the reproductive mucus. Biomimetic swimmers that replicate the motion of these small microorganisms hold tremendous promise in a host of biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. In one such method of swimming at very low Reynolds numbers, a micron scale helix has been fabricated and rendered magnetic by putting a magnetic material on it. Small rotating magnetic fields could be used then to rotate the helix, which translated as a result of the intrinsic translation rotation coupling in a helix. The present work focussed on the development of such a system of nanopropellers, a few microns in length, the characterization of its dynamics and velocity fluctuations originating from thermal noise. The work has also showed a possible application of the nanopropellers in microrheology where it could be used as a new tool to measure the rheological characteristics of a complex heterogeneous environment with very high spatial and temporal resolutions. A generalized study of the dynamics of these propellers under a rotating field, has showed the existence of a variety of different dynamical configurations. Rigid body dynamics simulations have been carried out to understand the behaviour. Significant amount of insight has been gained by solving the equations of motion of the object analytically and it has helped to obtain a complete understanding, along with providing closed form expressions of the various characteristics frequencies and parameters that has defined the motion. A study of the velocity fluctuations of these chiral nanopropellers has been carried out, where the nearby wall of the microfluidic cell was found to have a dominant effect on the fluctuations. The wall has been found to enhance the average level of fluctuations apart from bringing in significant non Gaussian effects. The experimentally obtained fluctuations has been corroborated by a simulation in which a time evolution study of the governing 3D Langevin dynamics equations has been done. A closer look at the various sources of velocity fluctuations and a causality study thereof has brought out a minimum length scale below which helical propulsion has become impractical to achieve because of the increased effect of the orientational fluctuations of the propeller at those small length scales. An interesting bistable dynamics of the propeller has been observed under certain experimental conditions, in which the propeller randomly switched between the different dynamical states. This defied common sense because of the inherent deterministic nature of the governing Stokes equation. Rigid body dynamics simulations and stability analysis has shown the existence of time scales in which two different dynamical states of the propeller have become stable. Thus the intrinsic dynamics of the system has been found to be the reason behind the bistable behaviour, randomness being brought about by the thermal fluctuations present in the system. Finally, in a novel application of the propellers, they have been demonstrated as a tool for microrheological mapping in a complex fluidic environment. The studies done in this work have helped to develop this method of active microrheology in which the measurement times are orders of magnitude smaller than its existing counterparts.

Micron scale robots going inside our body and curing various ailments is a technolog¬ical dream that easily captures our imagination. However, with the advent of novel nanofabrication and nanocharacterization tools there has been a surge in the research in this field over the last decade. In order to achieve locomotion (swim) at these small length scales, special strategies need to be adopted, that is able to overcome the large viscous damping that these microbots have to face while moving in the various bod¬ily fluids. Thus researchers have looked into the swimming strategies found in nature like that of bacteria like E.coli found in our gut or spermatozoa in the reproductive mucus. Biomimetic swimmers that replicate the motion of these small microorganisms hold tremendous promise in a host of biomedical applications like targeted drug delivery, microsurgery, biochemical sensing and disease diagnosis. In one such method of swimming at very low Reynolds numbers, a micron scale helix has been fabricated and rendered magnetic by putting a magnetic material on it. Small rotating magnetic fields could be used then to rotate the helix, which translated as a result of the intrinsic translation rotation coupling in a helix. The present work focussed on the development of such a system of nanopropellers, a few microns in length, the characterization of its dynamics and velocity fluctuations originating from thermal noise. The work has also showed a possible application of the nanopropellers in microrheology where it could be used as a new tool to measure the rheological characteristics of a complex heterogeneous environment with very high spatial and temporal resolutions. A generalized study of the dynamics of these propellers under a rotating field, has showed the existence of a variety of different dynamical configurations. Rigid body dynamics simulations have been carried out to understand the behaviour. Significant amount of insight has been gained by solving the equations of motion of the object analytically and it has helped to obtain a complete understanding, along with providing closed form expressions of the various characteristics frequencies and parameters that has defined the motion. A study of the velocity fluctuations of these chiral nanopropellers has been carried out, where the nearby wall of the microfluidic cell was found to have a dominant effect on the fluctuations. The wall has been found to enhance the average level of fluctuations apart from bringing in significant non Gaussian effects. The experimentally obtained fluctuations has been corroborated by a simulation in which a time evolution study of the governing 3D Langevin dynamics equations has been done. A closer look at the various sources of velocity fluctuations and a causality study thereof has brought out a minimum length scale below which helical propulsion has become impractical to achieve because of the increased effect of the orientational fluctuations of the propeller at those small length scales. An interesting bistable dynamics of the propeller has been observed under certain experimental conditions, in which the propeller randomly switched between the different dynamical states. This defied common sense because of the inherent deterministic nature of the governing Stokes equation. Rigid body dynamics simulations and stability analysis has shown the existence of time scales in which two different dynamical states of the propeller have become stable. Thus the intrinsic dynamics of the system has been found to be the reason behind the bistable behaviour, randomness being brought about by the thermal fluctuations present in the system. Finally, in a novel application of the propellers, they have been demonstrated as a tool for microrheological mapping in a complex fluidic environment. The studies done in this work have helped to develop this method of active microrheology in which the measurement times are orders of magnitude smaller than its existing counterparts.

Magnetism is a convenient force for actively pulling colloidal particles in a material. Many materials of interest in a microrheology experiment have a negligible magnetic susceptibility, and so embedded magnetic particles can be subject to relatively strong forces by fields imposed from outside of the sample. These are usually generated by electromagnets, but can also include the use of permanent magnets, or a combination of both. Such “magnetic tweezers” are used as sensitive force probes, capable of generating forces ranging from femtonewtons to nanonewtons. Magnetic forces and magnetic materials are reviewed and magnetic tweezer designs discussed. Linear and non-linear measurements using magnetic tweezers are presented, including studies yield stress and shear thinning. The operating regime of magnetic tweezer microrheology is presented, which enables microrheology experiments to access stiffer materials.

We present a comprehensive overview of microrheology, emphasizing the underlying theory, practical aspects of its implementation, and current applications to rheological studies in academic and industrial laboratories. Key methods and techniques are examined, including important considerations to be made with respect to the materials most amenable to microrheological characterization and pitfalls to avoid in measurements and analysis. The fundamental principles of all microrheology experiments are presented, including the nature of colloidal probes and their movement in fluids, soft solids, and viscoelastic materials. Microrheology is divided into two general areas, depending on whether the probe is driven into motion by thermal forces (passive), or by an external force (active). We present the theory and practice of passive microrheology, including an in-depth examination of the Generalized Stokes-Einstein Relation (GSER). We carefully treat the assumptions that must be made for these techniques to work, and what happens when the underlying assumptions are violated. Experimental methods covered in detail include particle tracking microrheology, tracer particle microrheology using dynamic light scattering and diffusing wave spectroscopy, and laser tracking microrheology. Second, we discuss the theory and practice of active microrheology, focusing specifically on the potential and limitations of extending microrheology to measurements of non-linear rheological properties, like yielding and shear-thinning. Practical aspects of magnetic and optical tweezer measurements are preseted. Finally, we highlight important applications of microrheology, including measurements of gelation, degradation, high-throughput rheology, protein solution viscosities, and polymer dynamics.

Active microrheology uses external forces (most typically magnetic or optical) to force microrheological probes into motion. These techniques short-circuit the Einstein component of passive microrheology. Active microrheology provides an additional handle to probe material properties, and has been used both to extend the range of materials amenable to microrheological analysis, and to examine material properties that are inaccessible to passive microrheology. Three main topics are presented: the use of active microrheology to extend the range of passive microrheology, while maintaining many of the advantages (small sample size, wide frequency range, etc.); its use to complement passive microrheology in active systems, which convert chemical fuel to mechanical work, in order to elucidate the power provided by molecular motors, for instance; and its application (and potential limitations) to investigate the non-linear response properties of materials, including shear thinning and yielding.

An overview of soft matter behavior in a rotating magnetic field is given and the basic phenomena of single ferromagnetic and paramagnetic particles are described: synchronous and asynchronous regimes, structural instability leading to the precessional regime and others. Their applications in microrheology are discussed. As a particular example of an active magnetic system, magnetotactic bacteria are considered and several important phenomena, such as complex trajectories, synchronization, diffusion due to the internal noise are described. As an important application of these descriptions, hydrodynamics with spin is considered and the conditions for the transformation from microscopic rotational motion of the particles to the macroscopic motion of their suspension are described. Finally, exciting perspectives for further development of the field, such as hydrodynamics of systems with odd viscosity, are briefly discussed.