title: Transport of microparticles in spatially and time-dependent flows
Creators: name: Laumann, Matthias
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Physik
subject: Mikrofluidik
subject: 530

description: This thesis deals with microfluidics and especially with the dynamics of micrometers sized particles like polymers, capsules, and red blood cells in flows of low Reynolds number. Thereby especially the transport of the particles perpendicular to the stream lines is studied, the so-called cross-streamline migration. A classical example is the migration of a soft particle to the center of a Poiseuille flow or the migration due to the repulsive interaction with a channel boundary. However, a migration can be caused by many further effects or can be controlled by external influences, what is studied in this thesis. These mechanisms of migration can be used in microfluidic devices to separate different kinds of particles, for example of healthy and cancer cells. Such microfluidic devices are promising tools in biotechnology, pharmacology, medical diagnostics and basic research. A possibility to generate a migration are curved stream lines, which are found for example in a channel with sinusoidally modulated boundaries, which is shown with a capsule and a red blood cell. These non-parallel stream lines in combination with the finite size of the particle lead to a cross-stream migration, what results in a second attractor. Because this effect depends on the properties of the particles, it can be used to analyze and separate particles like malignant (cancer) and healthy cells. Furthermore it is interesting to study the influence of a time-dependent flow on the migration. It is shown that even a linear shear flow can lead to a migration, if it is time-dependent and if the particle has an intrinsic asymmetry. If a time-dependent flow oscillates fast enough, then the inertia of an immersed particle is no longer negligible. An asymmetric deformable particle with a different density than the surrounding fluid may move on average in such a flow, even if the flow has a vanishing mean velocity. This thesis shows that the effect is strong enough to overcome gravity with the help of a Janus capsule and that one can achieve a net motion of a symmetric particle, e.g. a homogeneous capsule, by an asymmetric flow. Besides the influence of the flow on the migration, it is interesting how external forces affect the transport of particles. For this purpose the well-known migration of a soft particle in a Poiseuille flow is investigated in presence of an additional external force, for example a buoyancy force due to different densities of the fluid inside the capsule and of the surrounding fluid. Because of the deformation in the Poiseuille flow or in a linear shear flow the friction coefficient becomes anisotropic so that a force in flow direction leads also to a motion perpendicular to the flow. In a Poiseuille flow one finds a migration towards the center or the wall which leads in combination with the repulsive wall interaction to an attractor. The transport of particles in a fluid due to external forces allows furthermore to determine properties of the particle. In this way the magnetic moment of the magnetotactic bacterium Magnetospirillum gryphiswaldense was measured by moving it with the help of a known magnetic field through a quiescent fluid. With the models used here the friction coefficient of the spiral shaped bacteria was determined, what is needed for the measurement. With this kind of measurement it is possible to consider the shape of individual bacteria. The results may contribute to a deeper understanding of the Biosynthesis of the magnetosome and the resulting magnetic moments. In this work is also investigated how a gradient in the viscosity influences the migration of particles. Such a gradient can be generated directly by a temperature gradient because the viscosity of fluids strongly depends on the temperature. Additionally, one finds also in flows of shear thinning fluid through a channel a spatially dependent viscosity. In case of a deformable particle and a gradient perpendicular to a linear shear flow, on finds a migration to the lower viscosity and an attractor if the wall interaction is considered. In a Poiseuille flow one finds a second attractor close to the wall besides the one at the channel center. If particles are investigated that are smaller than those mentioned before, then the thermal noise can affect the transport in a flow. An example are here semi-flexible polymers or filaments, which display also a migration in a parabolic Poiseuille flow. Depending on the stiffness of the polymer, it migrates outwards or below a critical value to an attractor. In this thesis the effect of the noise is investigated. The noise increases this critical value. Furthermore the influence of the wall is studied. As appendix follows a discussion of simple bead models used to simulate soft particles in microflows (besides the Lattice Boltzmann Method). These bead models allow on the one hand efficient simulations and on the other hand they agree well with the literature and with other methods. In addition, they allow semi-analytical approximations which allow to roughly estimate the migration of a deformable particle in a flow and to determine the mechanisms that lead to a migration.