## Physik

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- Instabilität (6) (remove)

- Instabilities in layered liquids induced by external fields (2003)
- In this thesis, we have shown that the inclusion of a nematic degree of freedom in the macroscopic hydrodynamic description of smectic-A-like liquids leads to a number of interesting results. While the director and the layer normal are coupled such that they are parallel in equilibrium, in non-equilibrium situations, the director needs not be parallel to the smectic layer normal. This is in contrast to standard smectic-A hydrodynamics. Using irreversible thermodynamics and symmetry arguments, we derived a complete set of macroscopic hydrodynamic equations for the director variables, the layer displacement, the velocity field, and the moduli of the nematic and smectic order parameters. Recent experiments find that the parallel orientation of smectic-A- like liquids is destabilized by an applied shear. After destabilization, two typical scenarios are observed in a steady state situation: i) The layers are oriented perpendicular to the vorticity direction of the flow, i.e., they lie in the plane spanned by the velocity and the gradient direction (`perpendicular' orientation). ii) Closed multi-lamellar vesicles (`onions') form. A number of experiments indicate that the onset of this reorientation is controlled by the applied shear rate. In contrast to standard smectic-A hydrodynamics where shear in the parallel orientation has no effect on the layers, this destabilizing effect comes out naturally from our extended smectic-A hydrodynamics. The argumentation goes along the following lines. The shear field exerts a torque on the director that must be balanced by the coupling to the layer normal. In the limit of small angles, balancing these torques leads, in the steady state, to a shear-induced director tilt proportional to the shear rate. The preferred thickness of a smectic layer is directly connected to the projection of the averaged molecular axes on the layer normal, or, in terms of our model, the thickness is proportional to the projection of the director on the layer normal. If the director is tilted, this projection is shorter. This decrease of the projection is equivalent to an effective dilation, because the actual layer thickness is larger than the preferred layer thickness. Similar to the case of low molecular weight smectic-A liquid crystals under a dilative strain, this effective dilation leads to an undulation instability. To investigate the stability of the parallel alignment, we performed a linear and weakly non-linear analysis of the governing equations. The initial state is the above described spatially homogeneous director tilt with the smectic layers in the parallel orientation. The linear stability analysis showed an undulation instability which sets in above a critical tilt angle (or equivalently, a critical shear rate). This critical tilt angle turned out to depend strongly on the material parameters. For a typical low molecular weight thermotropic liquid crystal, we estimated the critical tilt angle to be on the order of a few degrees. The linear stability analysis also revealed that the nematic and smectic order is modulated close to the boundaries. Since the probability for the formation of defects is larger in regions with a decreased modulus of the order parameter, these variations in the modulus of the order parameter open the way for a destabilization of the layered structure. We note that a detailed investigation of this point is beyond the scope of the present work. Finally, we could exclude an oscillatory instability for all physically reasonable regions in parameter space. The weakly non-linear analysis shows that the bifurcation is supercritical for most physically relevant regions in the parameter space. A detailed comparison to an independent approach was undertaken in a collaboration with simulation physicists from the Max-Planck- Institute for Polymer Research in Mainz. In a molecular dynamics simulation, a model layered liquid consisting of chains of four particles (AABB) was considered. The interaction potential of particles not connected by springs is attractive for like particles and repulsive for particles of a different nature. The simulation demonstrated the two main predictions of our analytic theory: The director tilts in the flow direction and, above a critical shear rate, the layers show stationary undulations with a wave vector in the vorticity direction. Besides this good qualitative agreement, a reasonable quantitative agreement for the critical shear rate was found.

- Pattern Formation in Rotating Fluid Systems under the Influence of Magnetic Fields (2004)
- Patterns are observed in many different systems in nature. They are seen in the cloud streets, in sand ripples, in the morphology of plants and animals, on weather maps, in chemical reactions. In all these cases one deals with open, continuous dissipative systems which are driven out of equilibrium by an external stress. If this stress is larger than a certain threshold value, the symmetry of the temporally and spatially homogeneous ground state is spontaneously broken. The resulting patterns show then periodicity in space and/or in time. One of the best studied examples is the convection instability when a fluid layer is subjected to a temperature gradient. For instance, in a horizontal fluid layer heated from below and cooled from above a striped patterns of convection rolls develop. This scenario describes the famous Rayleigh- Benard convection (RBC), as a standard paradigm of pattern formation. Many concepts and mathematical tools to analyze the patterns have been developed and tested for this case. This thesis deals with two different pattern forming systems, namely a particular example of a convection instability and the case of a shear flow driven instability. In the first part of the thesis, a variation of the standard RBC is investigated. We consider the problem of convection induced by radial buoyancy in an electrically conducting fluid contained in a rotating (angular frequency, Omega) cylindrical annulus which is cooled at the inner surface and heated from outside. In addition, an azimuthal magnetic field (B) is applied for instance by an electrical current through the cylinder axis. The motivation of this study has come originally from the geophysical context. This setup is hoped to capture some important features of convection patterns in rotating stars and planets near the equatorial regions. The problem is also of considerable interest from a more general point of view in that it is concerned with formations of patterns in the presence of two competing directional effects, in this case rotation and the magnetic field. The second part of the thesis is devoted to the the pattern formation by a shear flow between two rotating and infinitely electrically conducting plates with a magnetic field perpendicular to the plates. This geometry is called the magnetic Ekman-Couette layer and has been a basic model for magnetic activities at the boundary of the Earth's liquid core or at the tachocline in the Sun below the convection zone for a few decades. To analyze the forementioned problems, various codes and computational tools had to be developed, for instance, we were able to describe complex spatio-temporal patterns by the direct simulations of the underlying hydrodynamic equations for our problems. The discussion of the physical details of the systems are postponed to the introductory sections of the corresponding parts of the thesis. In Chapter 1, a general formulation of the linear and nonlinear analysis, methods, which are applicable to both pattern forming systems in this work will be presented. The investigation of thermal convection in a plane layer which is a geometry equivalent to the cylindrical annulus will be discussed in Chapter 2. The next chapter (Chapter 3) covers both the linear and nonlinear analyses in the case of magnetic Ekman-Couette layer problem. Finally, in Chapter 4, we will present the general conclusions on both of the systems.

- Mechanics of living cells: nonlinear viscoelasticity of single fibroblasts and shape instabilities in axons (2006)
- Biomechanics is a field of major biological relevance. In spite of the vast complexity of biological matter, a number of generic features are found to hold in the mechanics of soft tissues throughout all of its length scales. A major goal in biomechanics is to reduce its general features to those of the cytoskeleton, the filamentous scaffold which provides cells with mechanical integrity, architecture and contractility. The first part of this report describes single-cell uniaxial stretching experiments performed on fibroblasts. When placed between fibronectin coated microplates, fibroblasts adopt a regular, symmetrical shape and generate forces. When a constant cell length is imposed, an increase with time of the pulling force can be observed. This active behaviour can be probed in more detail by superimposing small-amplitude oscillations at frequencies in the range 0.1--1 Hz. The response to the superimposed oscillations is then characterised by the viscoelastic moduli. These are seen to be a function of the average force acting on the cell. This master-relation holds for all cells. At low forces, both moduli are constant; beyond a crossover force, power-law stress stiffening is observed, where as a function of the average force both moduli go as a power-law with exponents in the range 1-1.8. The loss factor depends only weakly on the average force. Remarkably, the moduli are a function of the average force but are independent of the cell length. Therefore this mechanical behaviour is not strain stiffening; rather, it is an example of active, intrinsic stress stiffening. The precise way of sweeping force-space is seen to be irrelevant. The stiffening relation shows a striking similarity to rheological measurements performed on purified actin gels, in an unprecedented example of quantitative agreement between living and dead matter. This mechanical response originates in the semiflexible behaviour of biopolymers. The precise mechanism is however at present not fully understood. Here, a simple explanation is proposed. It is shown that stress stiffening in fibroblasts bears a strong resemblance to the nonlinear mechanics of Euler-Bernoulli beams, which also show a linear regime at low forces and a crossover to power-law stiffening. Systematic analysis of the response of fibroblasts to large amplitude deformations reveals a striking similarity to plasticity in metals. Fibroblasts can be described as showing kinematic (or directional) hardening, a hallmark of composite materials. The second part of this report addresses experiments performed on neurites. These comprise axons --the processes extended by neurons-- as well as PC12 neurites, a model system for axons. After a sudden increase in the external osmotic pressure, axons swell and a cylindrical-peristaltic shape transformation sets in. We interprete this transition as a Rayleigh-Plateau-like instability triggered by elastic membrane tension, similar to the pearling instability known in membrane tubes. Microtubuli disruption by nocodazol strongly increases the maximum amplitude of the instability, as well as slightly increases the wavenumber of the fastest mode, showing microtubuli to be the most important cytoskeletal component in stabilising neurites. After a hypoosmotic shock the neurite volume increases, reaches a maximum, and relaxes back close to its initial value. These experiments were performed at different temperatures and initial osmotic pressure differences. The relaxation time as a function of the temperature closely follows an Arrhenius dependence, suggesting the rate-limiting factor of the relaxation to be the movement of ions through channels. Similar experiments were also performed under drug-induced perturbation of actin, myosin and microtubuli. Cytoskeleton perturbation does not have any significant effect on volume relaxation, indicating that it takes place solely by changes in osmolarity, without a significant role for hydrostatic pressures. A clear effect of drugs is seen in the initial swelling phase, especially after microtubuli disruption by nocodazol. The rate and extent of swelling are significantly higher. Taking the effect of drugs on the evolution of neurite volume together with that on the pearling instability, we suggest that hydrostatic pressure is present in the initial swelling phase and determines the swelling rate. In conclusion, reproducible, quantitative experiments at the single-cell level have been developed which address biologically relevant phenomena. Following a time-honoured tradition in physics, both the cell-pulling experiments and the shape transformations in axons address highly symmetric systems, where the geometry does not preclude the understanding. First interpretations of the observed phenomena have been found, in terms of generic behaviours common to all objects under tension.

- The Rosensweig instability in isotropic magnetic gels (2008)
- Die vorliegende Arbeit befasst sich mit der nichtlinearen theoretischen Analyse der Rosensweig Instabilität in isotropen magnetischen Gelen. Die Rosensweig Instabilität beschreibt den Übergang einer zunächst flachen Oberfläche zwischen einer magnetischen Flüssigkeit zu einer hexagonal geordneten Stacheloberfläche, sobald ein senkrecht zur flachen Oberfläche angelegtes homogenes Magnetfeld einen bestimmten kritischen Wert überschreitet. Startet man den Vernetzungsprozess in einer Mischung aus Polymeren, Vernetzungsreagenzien und einem Ferrofluid, so erhält man ein isotropes Ferrogel, ein elastisches Medium, welches zusätzlich superparamagnetisches Verhalten aufweist. Theoretisch lässt sich zeigen, dass auch die Oberfläche dieser Medien in einem angelegten Magnetfeld instabil wird, wobei die typische Wellenlänge im Vergleich zu gewöhnlichen Ferrofluiden unverändert bleibt, während die kritische Magnetfeldstärke mit wachsendem elastischen Schermodul steigt. Besondere Aufmerksamkeit kommt in der Diskussion dem stationären Charakter der Rosensweig Instabilität zu. Dieser ist, wie sich herausstellt, als ein Grenzprozess zu interpretieren, bei welchem die Dynamik der charakteristischen Mode mit Annäherung an die Schwelle immer stärker verlangsamt wird und schließlich zu einem statischen Oberflächenmuster führt. Der Grund für dieses Grenzverhalten ist in der deformierbaren Oberfläche und im Besonderen in der daraus resultierenden kinematischen Randbedingung zu sehen. Unter Anwendung der Energiemethode nach Gailitis, wird die Oberflächenenergiedichte bezüglich regulärer Streifen, Quadrate und Hexagone minimiert. Es zeigt sich, dass am Einsatz der Instabilität Hexagone das energetisch favorisierte Oberflächenmuster sind. Für hohe Magnetfeldstärken hingegen bilden Quadrate die bevorzugte Anordnung. Die Energiemethode hat jedoch bedeutende Nachteile, die als Motivation für eine schwach nichtlineare Analyse der fundamentalen hydrodynamischen Gleichungen und der Herleitung einer Amplitudengleichung dienen. Ganz besondere Beachtung verdient dabei die Bestimmung des adjungierten Systems für die Rosensweig Instabilität. Dieses ist zur Befriedigung der Fredholmschen Alternative, die wiederum die Amplitudengleichungen liefert, von zentraler Bedeutung. Zur Herleitung der adjungierten Gleichungen und der dazugehörigen Randbedingungen wird die Erkenntnis aus der Diskussion der linearen Instabilität, dass das System als dynamisch zu betrachten und der statische Grenzfall erst am Ende zu vollziehen ist, benutzt. Des weiteren stellt es sich als wichtig heraus, die Gleichungen zunächst für ein kompressibles Medium zu adjungieren und ebenfalls erst am Ende die Näherung für inkompressible Medien zu bestimmen. Das adjungierte System wird ebenfalls für die Marangoni Instabilität bestimmt. Dort induzieren Temperaturfluktuationen an der Oberfläche eines Fluids Fluktuationen der Oberflächenspannung, die wiederum Konvektion hervorrufen. Mit Hilfe der Lösungen des adjungierten Systems lassen sich nun die Lösbarkeitsbedingungen in der zweiten und dritten Störungsordnung erfüllen und man erhält letztlich die Amplitudengleichung. Im Rahmen unser Näherungen entkoppeln die hydrodynamischen Volumengleichungen von denen des Magnetfeldes. Allerdings müssen die Lösungen auch noch den Randbedingungen genügen und im Besonderen ist die normale Randbedingungen in den höheren Ordnungen nicht trivial erfüllt. Vielmehr liefert sie noch eine zusätzliche Bedingung zur Fredholmschen Alternative. In der Arbeit wird zum ersten Mal der quadratische Koeffizient aus den fundamentalen hydrodynamischen Gleichungen abgeleitet. Dieser garantiert zum einen die Existenz von Hexagonen, zum anderen das Auftreten einer transkritischen Bifurkation. Beides sind experimentell bestätigte Eigenschaften der Rosensweig Instabilität. Zum anderen enthält die Amplitudengleichung für Ferrogele eine zweifache Zeitableitung. Die linearisierte Amplitudengleichung nimmt im Fall der Ferrogele die Gestalt eines gedämpften harmonischen Oszillators an. Im Fall der Rosensweig Instabilität in Ferroflüssigkeiten, deren zugehörige Amplitudengleichung ebenfalls bestimmt wird, tritt diese zweifache Zeitableitung nicht auf. Die Rosensweig Instabilität ist im Rahmen unserer Näherungen rein oberflächengetriebenen. Das motiviert die Frage, inwieweit dünne magnetische Filme oder Membranen instabil werden können. Diese Frage wird in dieser Arbeit ebenfalls diskutiert. Beschränkt man sich in einer linearen Stabilitätsanalyse auf den symmetrischen Fall, das heißt der isotrope Ferrogelfilm ist auf beiden Seiten vom gleichen Medium umgeben, so findet man, dass der Film linear nicht instabil werden kann. Eine Instabilität zeigt sich nur im Fall von anisotropen magnetischen Gelen oder im Fall eines magnetischen Kontrastes zwischen den beiden umgebenden Medien.

- Nonlinear macroscopic description of liquid crystalline elastomers in external fields (2008)
- We concentrate on a continuum characterization of the macroscopic behavior of side-chain liquid single crystal elastomers (SCLSCEs). These materials consist of chemically crosslinked polymer backbones, to which mesogenic units are attached as side-groups. Due to specific routes of synthesis SCLSCEs feature a monodomain of the liquid crystalline order in the ground state. Their macroscopic coupling of liquid crystalline order and elastic mechanical deformations makes them unique. In particular, we investigate the macroscopic behavior of cholesteric and nematic SCLSCEs in external electric and mechanical fields. We characterize the respective liquid crystalline state using the director field and describe the current state of mechanical distortion by a displacement field. The specific coupling between these two components is taken into account explicitly by additional macroscopic variables. These are the relative rotations between the director orientation and the polymer network. Using this kind of description, we first study the influence of an external electric field on the state of a cholesteric SCLSCE. For this purpose, the field direction is chosen to be parallel to the helical axis of the cholesteric mesogen alignment. Director reorientation and mechanical distortions are analyzed to linear order. In the case of low electric field amplitudes, we find an effect that has been termed rotatoelectric. Here, with increasing electric field amplitude, the director arrangement rotates around the helical axis, relative to the polymer network. This effect is specific for cholesteric SCLSCEs. We discuss several aspects important for an experimental observation of this effect. Next, we investigate the dielectric instabilities of a cholesteric SCLSCE in a Frederiks splay geometry at higher electric field amplitudes. On the one hand, we find a scenario that corresponds to the Frederiks transition in conventional low molecular weight liquid crystals. Here, the director reorientation arises in a way that is spatially homogeneous in the directions perpendicular to the cholesteric helical axis. On the other hand, however, we also find a qualitatively different instability. The latter is characterized by spatial undulations of the director reorientation, which occur in at least one direction perpendicular to the cholesteric helical axis. We recover the same results in the case of an external magnetic field. Besides, we discuss elastic mechanical compressions or dilations of a cholesteric SCLSCE in the directions parallel and perpendicular to the cholesteric helical axis. Here, small amplitudes of deformation lead to a distortion of the cholesteric helical structure. In the simplest case, we obtain an elongation or compression of the cholesteric helix along its axis. Furthermore, we propose ways to experimentally access so-far unknown values of the material parameters involved. We proceed by developing a model to characterize the nonlinear macroscopic behavior of the materials. For this purpose, we identify two coupled preferred directions in nematic and cholesteric SCLSCEs. One of them is imprinted into the polymer network during the process of synthesis to align the mesogens in a liquid crystalline monodomain. On the other hand, the actual average mesogen orientation may deviate from this imprinted direction and is described by the director field. We derive expressions characterizing nonlinear relative rotations between these two coupled preferred directions and we include them as macroscopic variables into our description. Using our model, we first investigate the shear deformation of a nematic SCLSCE. If the shear plane contains the director, the latter will be reoriented due to the mechanical deformation. In addition, however, we find as a nonlinear effect that the director reorientation acts back onto the elastic mechanical distortion of the material. This leads to compressive and dilative strain deformations. Finally, we study the specific stress-strain behavior of nematic SCLSCEs. It has been found for nematic SCLSCEs stretched perpendicularly to the initial director orientation that their director reorients towards the stretching direction. This reorientation of the director sets in above a critical threshold strain. In the strain regime where the director reorientation occurs, the slope of the corresponding stress-strain curve is significantly decreased. We demonstrate that our model describes this nonlinear behavior. Furthermore, we compare the predictions of our model with experimental data. As a result, we find that nonlinear relative rotations play the central role in the macroscopic characterization of the behavior of the materials. However, we also conclude that the macroscopic stress-strain behavior can be qualitatively influenced by those contributions to the elastic response that are not connected to the director reorientation and relative rotations.

- Surface Deformations of Magnetic Continua in Homogeneous Fields (2009)
- In this thesis, experiments with magnetic liquids and gels are presented. Ferrofluids are synthetically created suspensions of magnetic nanoparticles in a carrier liquid. By adding a gelator, such a ferrofluid can be turned into a ferrogel. The magnetic properties of these substances are similar to a usual paramagnet with the important difference, that the susceptibility of the former is higher by a factor of 10^3 to 10^6. By the application of a homogeneous field, a transformation of the shape of a magnetic sample can be induced. In this thesis, four experiments on the surface deformation in homogeneous magnetic fields are presented. Two geometric configurations are considered: a horizontally extended flat layer with a free surface as well as a spherical sample. In both cases, the application of a homogeneous magnetic field leads to changes of the shape of the free boundary. In the case of the spherical geometry, the sample is deformed into a prolate ellipsoid under the action of the field, the so called magnetodeformational effect. In case of the extended flat layer, an abrupt shape transition into a patterned state takes place, the normal field or Rosensweig instability. In contrast to the smooth deformation of the sphere, this is an instability, which breaks the translational symmetry, and the transition occurs at a certain threshold value of the magnetic induction. Each of the four experiments in this thesis is briefly summarized in the following paragraphs. Part I of the thesis considers ferrofluids. In chapter 2, the ideal geometry of an infinitely extended flat layer is intentionally reduced to a cylinder such that only a single spike in the centre exists, and the solution space becomes rotationally symmetric. This makes the problem very feasible for experimental methods and numerical simulations. Two measurement techniques are applied and compared to each other, namely an X-ray technique, where the surface deformation is extracted from radioscopic images, and a laser technique, which focuses a laser spot onto the surface. The experiments and the simulations, the latter performed in close cooperation with a group in Athens, show a convincing agreement within a few percent. It remains an open question, whether the result can be deduced in analytic form, however. In chapter 3, a highly viscous ferrofluid is utilized to study the nonlinear dynamics of the normal field instability at very low Reynolds numbers. The linear growth rate for the growth and decay of the pattern at small amplitudes is extracted from the measurements and compared with an existing theoretical model. In addition, the measurement technique provides the reconstruction of a fully nonlinear amplitude equation, which is qualitatively compared to model equations. These nonlinear amplitude equations can only describe the dynamics of the growth in the immediate vicinity of the critical point so far. For a quantitative comparison, there is a need for a model with an extended range of validity. Additionally, localized patterns are observed which arise spontaneously in the neighbourhood of the unstable solution branch, which have previously been observed with the help of an external disturbance Part II of the thesis deals with thermoreversible ferrogels. Chapter 4 studies the magnetodeformational effect. A ferrogel sphere is exposed to homogeneous magnetic field. When the field is applied suddenly, the sphere not only elongates in the direction of the field, but also vibrates about the new equilibrium. On a longer time scale, the deformation continuously increases due to the viscoelastic properties of the gel. Both phenomena can well be described by a harmonic oscillator model, where the spring constant changes with time. From the deformation parallel and perpendicular to the applied field, Poisson´s ratio can be calculated, which turns out to be close to the limit of incompressibility. The absolute values of the deformation are compared to recent theoretical models. The resulting deviation of about 10% is attributed to the viscoelastic properties of the ferrogel, which are not taken into account in the static models. In chapter 5, the normal field instability is realized for the first time with a ferrogel. A flat layer of a thermoreversible ferrogel is exposed to a homogeneous magnetic field at different temperatures, where the gel is viscoelastic. This is a consequence of the need for a very soft material, such that the growth of the pattern is not completely suppressed by the elastic forces. The magnetic field is periodically modulated in time, and the amplitude of the instability is measured, which is modulated with the same frequency. The comparison with rheological measurements reveals a scaling of the modulated amplitude with the complex viscosity of the ferrogel. A comparison with the theoretical model for a ferrogel is difficult due to the viscoelasticity of the gel.