{"id":{"repo_id":"tu-berlin","oai_identifier":"oai:depositonce.tu-berlin.de:11303/17974"},"canonical_url":"https://search.dev.ndltd.org/etd/tu-berlin/oai:depositonce.tu-berlin.de:11303/17974","repository":{"repo_id":"tu-berlin","name":"Technische Universität Berlin","base_url":"https://api-depositonce.tu-berlin.de/server/oai/request"},"display":{"title":"Elongated microswimmers: influence of hydrodynamics","abstract":"Our nature is full of microscopic organisms such as algae, which are at the basis of the marine food chain and thus establish entire ecosystems, or bacteria, which are important for our nutrition and health, and also find a broad application in industrial processes. Thus, a broad research field is concerned with the many aspects of the biology, chemistry and physics of microorganisms. Physicists are particularly interested in the swimming motion of organisms such as algae, bacteria, spermatozoa and others. In collective motion, these microswimmers exhibit a diverse behavior such as the formation of swarms, vortices, convection rolls and plumes, and active turbulence. Therby, most microswimmers are elongated, and are otherwise characterized by different swimming mechanisms. Consequently, their dynamics arises from the interplay of different interactions, i.e., direct collisions and long-range flow fields. It is subject of current research to determine the role of the different interactions for the fascinating collective dynamics. In this context, we consider the squirmer rod, a model microswimmer that consists of several spherical squirmers, in order to create an elongated body shape. To realize the swimming mechanisms of bacteria and algae, i.e., pushers and pullers, we concentrate the surface slip-velocity field on the rear end or front side of the rod. This generates the force dipole, which these swimming mechanisms exert on the fluid. Using the simulation method of multi-particle collision dynamics (MPCD), we analyze the flow fields of the squirmer rods both in the bulk fluid as well as in the Hele-Shaw geometry, i.e., in narrow confinement between two parallel plates. Using hydrodynamic multipole expansions, we categorize the different multipole contributions created by neutral, and pusher squirmer rods in the bulk fluid as well as in the Hele-Shaw geometry. Thereby, we show how the confinement changes the radial decay of the flow fields of the force or source multipoles, and hence the characteristic flow fields compared to the bulk fluid. Further, we present a detailed study of the collective dynamics of neutral squirmer rods moving in the midplane of the Hele-Shaw geometry. From small to a large aspect ratio and density, we observe a disordered state, dynamic swarms, a single swarm, and a jammed cluster state and characterize them accordingly. We also investigate a wide range of aspect ratios and densities for pushers and pullers and provide corresponding state diagrams. The flow field of pushers destabilizes ordered structures and favors the disordered state at small densities and aspect ratios. As soon as geometric interactions become relevant for longer squirmer rods, we observe a turbulent state, as well as a dynamic cluster, while a single swarm and jammed clusters reappear at large aspect ratios. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers, which follow power laws with non-universal exponents. Pullers show a strong tendency to form swarms so that no disordered state occurs at the investigated densities. For larger swarms so that no disordered state occurs at the investigated densities. Another part of this work deals in more detail with the multi-particle collision dynamics (MPCD) method, which is widely used in soft matter physics to simulate fluid flows at micrometer scale. In general, in this model the fluid exhibits the equation of state of an ideal gas, making it highly compressible. This is in contrast to real fluids, which are incompressible for velocities well below the speed of sound. Therefore, we propose a modified collision rule that leads to a MPCD algorithm with a non-ideal equation of state and a significantly reduced compressibility. At the same time, our algorithm requires less computational resources compared to conventional MPCD algorithms. To further establish the algorithm, we provide analytic expressions for the equation of state and shear viscosity, which show a good agreement with simulations of the pressure in a fluid at rest, the shear viscosity in linear shear flow, and the velocity field of a Poiseuille flow. Using two exemplary squirmer rod systems, we further compare the results of the dynamics under the extended MPCD method to those with the established MPCD version with Andersen thermostat. Thereby, we investigate the dynamic swarm state and single swarm state, which create large pressure gradients due to the sum of the many individual squirmer-rod flow fields. For the single swarm state the extended MPCD fluid shows more homogeneous fluid density, and we make the interesting observation that dynamic swarms are more pronounced and exhibit a higher polar order for the extended MPCD method.","abstract_html":"Our nature is full of microscopic organisms such as algae, which are at the basis of the marine food chain and thus establish entire ecosystems, or bacteria, which are important for our nutrition and health, and also find a broad application in industrial processes. Thus, a broad research field is concerned with the many aspects of the biology, chemistry and physics of microorganisms. Physicists are particularly interested in the swimming motion of organisms such as algae, bacteria, spermatozoa and others. In collective motion, these microswimmers exhibit a diverse behavior such as the formation of swarms, vortices, convection rolls and plumes, and active turbulence. Therby, most microswimmers are elongated, and are otherwise characterized by different swimming mechanisms. Consequently, their dynamics arises from the interplay of different interactions, i.e., direct collisions and long-range flow fields. It is subject of current research to determine the role of the different interactions for the fascinating collective dynamics. In this context, we consider the squirmer rod, a model microswimmer that consists of several spherical squirmers, in order to create an elongated body shape. To realize the swimming mechanisms of bacteria and algae, i.e., pushers and pullers, we concentrate the surface slip-velocity field on the rear end or front side of the rod. This generates the force dipole, which these swimming mechanisms exert on the fluid. Using the simulation method of multi-particle collision dynamics (MPCD), we analyze the flow fields of the squirmer rods both in the bulk fluid as well as in the Hele-Shaw geometry, i.e., in narrow confinement between two parallel plates. Using hydrodynamic multipole expansions, we categorize the different multipole contributions created by neutral, and pusher squirmer rods in the bulk fluid as well as in the Hele-Shaw geometry. Thereby, we show how the confinement changes the radial decay of the flow fields of the force or source multipoles, and hence the characteristic flow fields compared to the bulk fluid. Further, we present a detailed study of the collective dynamics of neutral squirmer rods moving in the midplane of the Hele-Shaw geometry. From small to a large aspect ratio and density, we observe a disordered state, dynamic swarms, a single swarm, and a jammed cluster state and characterize them accordingly. We also investigate a wide range of aspect ratios and densities for pushers and pullers and provide corresponding state diagrams. The flow field of pushers destabilizes ordered structures and favors the disordered state at small densities and aspect ratios. As soon as geometric interactions become relevant for longer squirmer rods, we observe a turbulent state, as well as a dynamic cluster, while a single swarm and jammed clusters reappear at large aspect ratios. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers, which follow power laws with non-universal exponents. Pullers show a strong tendency to form swarms so that no disordered state occurs at the investigated densities. For larger swarms so that no disordered state occurs at the investigated densities. Another part of this work deals in more detail with the multi-particle collision dynamics (MPCD) method, which is widely used in soft matter physics to simulate fluid flows at micrometer scale. In general, in this model the fluid exhibits the equation of state of an ideal gas, making it highly compressible. This is in contrast to real fluids, which are incompressible for velocities well below the speed of sound. Therefore, we propose a modified collision rule that leads to a MPCD algorithm with a non-ideal equation of state and a significantly reduced compressibility. At the same time, our algorithm requires less computational resources compared to conventional MPCD algorithms. To further establish the algorithm, we provide analytic expressions for the equation of state and shear viscosity, which show a good agreement with simulations of the pressure in a fluid at rest, the shear viscosity in linear shear flow, and the velocity field of a Poiseuille flow. Using two exemplary squirmer rod systems, we further compare the results of the dynamics under the extended MPCD method to those with the established MPCD version with Andersen thermostat. Thereby, we investigate the dynamic swarm state and single swarm state, which create large pressure gradients due to the sum of the many individual squirmer-rod flow fields. For the single swarm state the extended MPCD fluid shows more homogeneous fluid density, and we make the interesting observation that dynamic swarms are more pronounced and exhibit a higher polar order for the extended MPCD method.","abstract_has_math":false,"creators":["Zantop, Arne Wolf"],"institution":null,"degree_name":null,"degree_level":null,"degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Stark, Holger"],"committee_chairs":[],"committee_members":[],"year":2023,"date_issued":"2023","date_published":"2023","updated_at":"2026-07-27T21:28:31Z","subjects":[],"languages":["en"],"rights":[],"rights_urls":["https://creativecommons.org/licenses/by/4.0/"],"identifier_entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://doi.org/10.14279/depositonce-16766"],"render_values":[{"text":"https://doi.org/10.14279/depositonce-16766","href":"https://doi.org/10.14279/depositonce-16766","code":true}]}]},"links":{"outbound_url":"https://depositonce.tu-berlin.de/handle/11303/17974","outbound_label":"Repository record","outbound_source":"dc:identifier.uri"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Stark, Holger"]},{"key":"dc:creator","label":"Author","values":["Zantop, Arne Wolf"]}]},{"id":"academic_context","label":"Academic Context","entries":[{"key":"dc:date.accessioned","label":"Dc Date Accessioned","values":["2023-01-11T12:53:04Z"]},{"key":"dc:date.available","label":"Dc Date Available","values":["2023-01-11T12:53:04Z"]},{"key":"dc:date.issued","label":"Date","values":["2023"]},{"key":"dc:type","label":"Dc Type","values":["Doctoral Thesis"]}]},{"id":"language_rights","label":"Language and Rights","entries":[{"key":"dc:language.iso","label":"Language (ISO)","values":["en"]},{"key":"dc:rights.uri","label":"Rights URI","values":["https://creativecommons.org/licenses/by/4.0/"]}]},{"id":"identifiers","label":"Identifiers","entries":[{"key":"dc:identifier.uri","label":"Identifier URI","values":["https://depositonce.tu-berlin.de/handle/11303/17974","https://doi.org/10.14279/depositonce-16766"]}]},{"id":"additional","label":"Additional Metadata","entries":[{"key":"dc:description.abstract","label":"Abstract","values":["Our nature is full of microscopic organisms such as algae, which are at the basis of the marine food chain and thus establish entire ecosystems, or bacteria, which are important for our nutrition and health, and also find a broad application in industrial processes. Thus, a broad research field is concerned with the many aspects of the biology, chemistry and physics of microorganisms. Physicists are particularly interested in the swimming motion of organisms such as algae, bacteria, spermatozoa and others. In collective motion, these microswimmers exhibit a diverse behavior such as the formation of swarms, vortices, convection rolls and plumes, and active turbulence. Therby, most microswimmers are elongated, and are otherwise characterized by different swimming mechanisms. Consequently, their dynamics arises from the interplay of different interactions, i.e., direct collisions and long-range flow fields. It is subject of current research to determine the role of the different interactions for the fascinating collective dynamics. In this context, we consider the squirmer rod, a model microswimmer that consists of several spherical squirmers, in order to create an elongated body shape. To realize the swimming mechanisms of bacteria and algae, i.e., pushers and pullers, we concentrate the surface slip-velocity field on the rear end or front side of the rod. This generates the force dipole, which these swimming mechanisms exert on the fluid. Using the simulation method of multi-particle collision dynamics (MPCD), we analyze the flow fields of the squirmer rods both in the bulk fluid as well as in the Hele-Shaw geometry, i.e., in narrow confinement between two parallel plates. Using hydrodynamic multipole expansions, we categorize the different multipole contributions created by neutral, and pusher squirmer rods in the bulk fluid as well as in the Hele-Shaw geometry. Thereby, we show how the confinement changes the radial decay of the flow fields of the force or source multipoles, and hence the characteristic flow fields compared to the bulk fluid. Further, we present a detailed study of the collective dynamics of neutral squirmer rods moving in the midplane of the Hele-Shaw geometry. From small to a large aspect ratio and density, we observe a disordered state, dynamic swarms, a single swarm, and a jammed cluster state and characterize them accordingly. We also investigate a wide range of aspect ratios and densities for pushers and pullers and provide corresponding state diagrams. The flow field of pushers destabilizes ordered structures and favors the disordered state at small densities and aspect ratios. As soon as geometric interactions become relevant for longer squirmer rods, we observe a turbulent state, as well as a dynamic cluster, while a single swarm and jammed clusters reappear at large aspect ratios. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers, which follow power laws with non-universal exponents. Pullers show a strong tendency to form swarms so that no disordered state occurs at the investigated densities. For larger swarms so that no disordered state occurs at the investigated densities. Another part of this work deals in more detail with the multi-particle collision dynamics (MPCD) method, which is widely used in soft matter physics to simulate fluid flows at micrometer scale. In general, in this model the fluid exhibits the equation of state of an ideal gas, making it highly compressible. This is in contrast to real fluids, which are incompressible for velocities well below the speed of sound. Therefore, we propose a modified collision rule that leads to a MPCD algorithm with a non-ideal equation of state and a significantly reduced compressibility. At the same time, our algorithm requires less computational resources compared to conventional MPCD algorithms. To further establish the algorithm, we provide analytic expressions for the equation of state and shear viscosity, which show a good agreement with simulations of the pressure in a fluid at rest, the shear viscosity in linear shear flow, and the velocity field of a Poiseuille flow. Using two exemplary squirmer rod systems, we further compare the results of the dynamics under the extended MPCD method to those with the established MPCD version with Andersen thermostat. Thereby, we investigate the dynamic swarm state and single swarm state, which create large pressure gradients due to the sum of the many individual squirmer-rod flow fields. For the single swarm state the extended MPCD fluid shows more homogeneous fluid density, and we make the interesting observation that dynamic swarms are more pronounced and exhibit a higher polar order for the extended MPCD method.","Unsere Natur ist voll von mikroskopisch kleinen Organismen wie Algen, die an der Basis der marinen Nahrungskette stehen und damit ganze Ökosysteme begründen, oder Bakterien, die für unsere Ernährung und Gesundheit wichtig sind und auch in industriellen Prozessen eine breite Anwendung finden. Daher befasst sich ein breites Forschungsfeld mit den vielfältigen Aspekten der Biologie, Chemie und Physik von Mikroorganismen. Physiker:innen interessieren sich dabei besonders für die Schwimmbewegungen von Organismen wie Algen, Bakterien, Spermatozoen und Anderen. In kollektiver Bewegung zeigen diese Mikroschwimmer ein vielfältiges Verhalten, wie die Bildung von Schwärmen, Wirbeln, Konvektionsrollen und Plumes sowie aktive Turbulenz. Dabei sind die meisten Mikroschwimmer länglich, und zeichnen sich sonst durch verschiedene Schwimmmechanismen aus. Ihre Dynamik entsteht folglich durch das Zusammenspiel der verschiedenen Wechselwirkungen also durch direkte Zusammenstöße und weitreichenden Flussfelder. Die Rolle der verschiedenen Wechselwirkungen für die faszinierende kollektive Dynamik zu bestimmen, ist Gegenstand aktueller Forschung. In diesem Kontext betrachten wir den Squirmer-Rod, ein Mikroschwimmermodell, das wir aus mehreren kugelförmigen Squirmern zusammensetzen, um eine längliche Körperform zu erzeugen. Um die Schwimmmechanismen von Bakterien und Algen zu realisieren, d.h. Pusher und Puller, konzentrieren wir das Oberflächengeschwindigkeitsfeld auf die Rückseite oder die Vorderseite des Rods und können so den Kraft-Dipol dieser Schwimmmechanismen generieren. Mit Hilfe der Simulationsmethode der Vielteilchenstoßdynamik (MPCD) analysieren wir die Strömungsfelder dieser Squirmer-Rods sowohl in der freien Flüssigkeit, sowie in der Hele-Shaw-Geometrie, d.h. eingeschlossen zwischen zwei dichten parallelen Platten. Mittels der hydrodynamischen Multipolentwicklung kategorisieren wir die unterschiedlichen Anteile der Flussfelder von neutralen und Pusher Squirmer-Rods in der freien Flüssigkeit, sowie eingeschlossen zwischen zwei Platten. Dabei zeigen wir wie der Einschluss in der Hele-Shaw-Geometrie die radiale Reichweite der Flussfelder der Kraft- oder Quellmultipole, und somit der charakteristischen Flussfelder, im Vergleich zur freien Flüssigkeit verändert. Weiter präsentieren wir eine ausführliche Studie der kollektiven Dynamik neutraler Squirmer-Rods, die sich in der Mittelebene einer Hele-Shaw-Geometrie bewegen. Von einem kleinen bis zu einem großen Längenverhältnis und einer großen Dichte beobachten wir einen ungeordneten Zustand, dynamische Schwärme, einen einzelnen Schwarm, sowie ein blockiertes Cluster und charakterisieren diese entsprechend. Auch für Pusher und Puller untersuchen wir eine breite Spanne von Längenverhältnissen und Dichten und liefern entsprechende Zustandsdiagramme. Das Strömungsfeld von Pushern destabilisiert dabei geordnete Strukturen und begünstigt den ungeordneten Zustand bei kleinen Dichten und Längenverhältnissen. Sobald geometrische Wechselwirkungen bei längeren Squirmer-Rods relevant werden, beobachten wir einen turbulenten Zustand, sowie ein dynamisches Cluster, während bei großen Längenverhältnissen wieder ein einzelner Schwarm und blockierte Cluster auftreten. Die spektrale Leistungsdichte des turbulenten Zustands zeigt dabei zwei unterschiedliche Energiekaskaden bei kleinen und großen Wellenzahlen die Potenzgesetzen mit nicht-universellen Exponenten folgen. Puller hingegen zeigen eine starke Tendenz zur Bildung von dynamischen Schwärmen, sodass bei den dabei untersuchten Dichten gar keine ungeordneten Zustände auftreten. Bei größerem Längenverhältnis tritt wieder ein einzelner Schwarm oder ein blockiertes Cluster auf. Ein weiterer Teil dieser Arbeit befasst sich eingehender mit der Methode der Vielteilchenstoßdynamik (MPCD), welche in der Physik der weichen Materie häufig verwendet wird, um Hydrodynamik im Mikrometerbereich zu simulieren. In der Regel weist in diesem Modell die Flüssigkeit die Zustandsgleichung eines idealen Gases auf, wodurch sie stark kompressibel ist. Dies steht jedoch im Widerspruch zu realen Flüssigkeiten, welche für Geschwindigkeiten weit unterhalb der Schallgeschwindigkeit inkompressibel sind. Wir schlagen daher eine modifizierte Kollisionsregel vor, die zu einem MPCD-Algorithmus mit einer nicht idealen Zustandsgleichung und einer deutlich reduzierten Kompressibilität führt. Gleichzeitig benötigt unser Algorithmus im Vergleich zu herkömmlichen MPCD-Algorithmen weniger Rechenressourcen. Um dem Algorithmus ein theoretisches Fundament zu geben, liefern wir analytische Ausdrücke für die Zustandsgleichung und die Scherviskosität, die eine gute Übereinstimmung mit Simulationen des Drucks in einem ruhenden Fluid, der Scherviskosität bei linearer Scherströmung und dem Geschwindigkeitsfeld einer Poiseuille-Strömung zeigen. Anhand zweier exemplarischer Squirmer-Rod-Systeme vergleichen wir außerdem die Ergebnisse der Dynamik der Squirmer-Rods unter der erweiterten MPCD-Methode mit denen der etablierten MPCD-Version mit Andersen-Thermostat. Dabei betrachten wir die Zustände der dynamischen Schwärme und des einzelnen Schwarms, die durch die Summe der vielen Flussfelder einzelner Squirmer-Rods große Druckgradienten erzeugen. Für den Zustand des einzelnen Schwarms zeigt die erweiterte MPCD-Methode eine homogenere Fluiddichte, und wir machen die interessante Beobachtung, dass die dynamischen Schwärme bei der erweiterten MPCD-Methode stärker ausgeprägt sind und eine höhere polare Ordnung aufweisen für die erweiterte MPCD-Methode."]},{"key":"dc:title","label":"Title","values":["Elongated microswimmers: influence of hydrodynamics"]}]}],"canonical_facts":{"dc:contributor.advisor":["Stark, Holger"],"dc:creator":["Zantop, Arne Wolf"],"dc:date.accessioned":["2023-01-11T12:53:04Z"],"dc:date.available":["2023-01-11T12:53:04Z"],"dc:date.issued":["2023"],"dc:description.abstract":["Our nature is full of microscopic organisms such as algae, which are at the basis of the marine food chain and thus establish entire ecosystems, or bacteria, which are important for our nutrition and health, and also find a broad application in industrial processes. Thus, a broad research field is concerned with the many aspects of the biology, chemistry and physics of microorganisms. Physicists are particularly interested in the swimming motion of organisms such as algae, bacteria, spermatozoa and others. In collective motion, these microswimmers exhibit a diverse behavior such as the formation of swarms, vortices, convection rolls and plumes, and active turbulence. Therby, most microswimmers are elongated, and are otherwise characterized by different swimming mechanisms. Consequently, their dynamics arises from the interplay of different interactions, i.e., direct collisions and long-range flow fields. It is subject of current research to determine the role of the different interactions for the fascinating collective dynamics. In this context, we consider the squirmer rod, a model microswimmer that consists of several spherical squirmers, in order to create an elongated body shape. To realize the swimming mechanisms of bacteria and algae, i.e., pushers and pullers, we concentrate the surface slip-velocity field on the rear end or front side of the rod. This generates the force dipole, which these swimming mechanisms exert on the fluid. Using the simulation method of multi-particle collision dynamics (MPCD), we analyze the flow fields of the squirmer rods both in the bulk fluid as well as in the Hele-Shaw geometry, i.e., in narrow confinement between two parallel plates. Using hydrodynamic multipole expansions, we categorize the different multipole contributions created by neutral, and pusher squirmer rods in the bulk fluid as well as in the Hele-Shaw geometry. Thereby, we show how the confinement changes the radial decay of the flow fields of the force or source multipoles, and hence the characteristic flow fields compared to the bulk fluid. Further, we present a detailed study of the collective dynamics of neutral squirmer rods moving in the midplane of the Hele-Shaw geometry. From small to a large aspect ratio and density, we observe a disordered state, dynamic swarms, a single swarm, and a jammed cluster state and characterize them accordingly. We also investigate a wide range of aspect ratios and densities for pushers and pullers and provide corresponding state diagrams. The flow field of pushers destabilizes ordered structures and favors the disordered state at small densities and aspect ratios. As soon as geometric interactions become relevant for longer squirmer rods, we observe a turbulent state, as well as a dynamic cluster, while a single swarm and jammed clusters reappear at large aspect ratios. The power spectrum of the turbulent state shows two distinct energy cascades at small and large wave numbers, which follow power laws with non-universal exponents. Pullers show a strong tendency to form swarms so that no disordered state occurs at the investigated densities. For larger swarms so that no disordered state occurs at the investigated densities. Another part of this work deals in more detail with the multi-particle collision dynamics (MPCD) method, which is widely used in soft matter physics to simulate fluid flows at micrometer scale. In general, in this model the fluid exhibits the equation of state of an ideal gas, making it highly compressible. This is in contrast to real fluids, which are incompressible for velocities well below the speed of sound. Therefore, we propose a modified collision rule that leads to a MPCD algorithm with a non-ideal equation of state and a significantly reduced compressibility. At the same time, our algorithm requires less computational resources compared to conventional MPCD algorithms. To further establish the algorithm, we provide analytic expressions for the equation of state and shear viscosity, which show a good agreement with simulations of the pressure in a fluid at rest, the shear viscosity in linear shear flow, and the velocity field of a Poiseuille flow. Using two exemplary squirmer rod systems, we further compare the results of the dynamics under the extended MPCD method to those with the established MPCD version with Andersen thermostat. Thereby, we investigate the dynamic swarm state and single swarm state, which create large pressure gradients due to the sum of the many individual squirmer-rod flow fields. For the single swarm state the extended MPCD fluid shows more homogeneous fluid density, and we make the interesting observation that dynamic swarms are more pronounced and exhibit a higher polar order for the extended MPCD method.","Unsere Natur ist voll von mikroskopisch kleinen Organismen wie Algen, die an der Basis der marinen Nahrungskette stehen und damit ganze Ökosysteme begründen, oder Bakterien, die für unsere Ernährung und Gesundheit wichtig sind und auch in industriellen Prozessen eine breite Anwendung finden. Daher befasst sich ein breites Forschungsfeld mit den vielfältigen Aspekten der Biologie, Chemie und Physik von Mikroorganismen. Physiker:innen interessieren sich dabei besonders für die Schwimmbewegungen von Organismen wie Algen, Bakterien, Spermatozoen und Anderen. In kollektiver Bewegung zeigen diese Mikroschwimmer ein vielfältiges Verhalten, wie die Bildung von Schwärmen, Wirbeln, Konvektionsrollen und Plumes sowie aktive Turbulenz. Dabei sind die meisten Mikroschwimmer länglich, und zeichnen sich sonst durch verschiedene Schwimmmechanismen aus. Ihre Dynamik entsteht folglich durch das Zusammenspiel der verschiedenen Wechselwirkungen also durch direkte Zusammenstöße und weitreichenden Flussfelder. Die Rolle der verschiedenen Wechselwirkungen für die faszinierende kollektive Dynamik zu bestimmen, ist Gegenstand aktueller Forschung. In diesem Kontext betrachten wir den Squirmer-Rod, ein Mikroschwimmermodell, das wir aus mehreren kugelförmigen Squirmern zusammensetzen, um eine längliche Körperform zu erzeugen. Um die Schwimmmechanismen von Bakterien und Algen zu realisieren, d.h. Pusher und Puller, konzentrieren wir das Oberflächengeschwindigkeitsfeld auf die Rückseite oder die Vorderseite des Rods und können so den Kraft-Dipol dieser Schwimmmechanismen generieren. Mit Hilfe der Simulationsmethode der Vielteilchenstoßdynamik (MPCD) analysieren wir die Strömungsfelder dieser Squirmer-Rods sowohl in der freien Flüssigkeit, sowie in der Hele-Shaw-Geometrie, d.h. eingeschlossen zwischen zwei dichten parallelen Platten. Mittels der hydrodynamischen Multipolentwicklung kategorisieren wir die unterschiedlichen Anteile der Flussfelder von neutralen und Pusher Squirmer-Rods in der freien Flüssigkeit, sowie eingeschlossen zwischen zwei Platten. Dabei zeigen wir wie der Einschluss in der Hele-Shaw-Geometrie die radiale Reichweite der Flussfelder der Kraft- oder Quellmultipole, und somit der charakteristischen Flussfelder, im Vergleich zur freien Flüssigkeit verändert. Weiter präsentieren wir eine ausführliche Studie der kollektiven Dynamik neutraler Squirmer-Rods, die sich in der Mittelebene einer Hele-Shaw-Geometrie bewegen. Von einem kleinen bis zu einem großen Längenverhältnis und einer großen Dichte beobachten wir einen ungeordneten Zustand, dynamische Schwärme, einen einzelnen Schwarm, sowie ein blockiertes Cluster und charakterisieren diese entsprechend. Auch für Pusher und Puller untersuchen wir eine breite Spanne von Längenverhältnissen und Dichten und liefern entsprechende Zustandsdiagramme. Das Strömungsfeld von Pushern destabilisiert dabei geordnete Strukturen und begünstigt den ungeordneten Zustand bei kleinen Dichten und Längenverhältnissen. Sobald geometrische Wechselwirkungen bei längeren Squirmer-Rods relevant werden, beobachten wir einen turbulenten Zustand, sowie ein dynamisches Cluster, während bei großen Längenverhältnissen wieder ein einzelner Schwarm und blockierte Cluster auftreten. Die spektrale Leistungsdichte des turbulenten Zustands zeigt dabei zwei unterschiedliche Energiekaskaden bei kleinen und großen Wellenzahlen die Potenzgesetzen mit nicht-universellen Exponenten folgen. Puller hingegen zeigen eine starke Tendenz zur Bildung von dynamischen Schwärmen, sodass bei den dabei untersuchten Dichten gar keine ungeordneten Zustände auftreten. Bei größerem Längenverhältnis tritt wieder ein einzelner Schwarm oder ein blockiertes Cluster auf. Ein weiterer Teil dieser Arbeit befasst sich eingehender mit der Methode der Vielteilchenstoßdynamik (MPCD), welche in der Physik der weichen Materie häufig verwendet wird, um Hydrodynamik im Mikrometerbereich zu simulieren. In der Regel weist in diesem Modell die Flüssigkeit die Zustandsgleichung eines idealen Gases auf, wodurch sie stark kompressibel ist. Dies steht jedoch im Widerspruch zu realen Flüssigkeiten, welche für Geschwindigkeiten weit unterhalb der Schallgeschwindigkeit inkompressibel sind. Wir schlagen daher eine modifizierte Kollisionsregel vor, die zu einem MPCD-Algorithmus mit einer nicht idealen Zustandsgleichung und einer deutlich reduzierten Kompressibilität führt. Gleichzeitig benötigt unser Algorithmus im Vergleich zu herkömmlichen MPCD-Algorithmen weniger Rechenressourcen. Um dem Algorithmus ein theoretisches Fundament zu geben, liefern wir analytische Ausdrücke für die Zustandsgleichung und die Scherviskosität, die eine gute Übereinstimmung mit Simulationen des Drucks in einem ruhenden Fluid, der Scherviskosität bei linearer Scherströmung und dem Geschwindigkeitsfeld einer Poiseuille-Strömung zeigen. Anhand zweier exemplarischer Squirmer-Rod-Systeme vergleichen wir außerdem die Ergebnisse der Dynamik der Squirmer-Rods unter der erweiterten MPCD-Methode mit denen der etablierten MPCD-Version mit Andersen-Thermostat. Dabei betrachten wir die Zustände der dynamischen Schwärme und des einzelnen Schwarms, die durch die Summe der vielen Flussfelder einzelner Squirmer-Rods große Druckgradienten erzeugen. Für den Zustand des einzelnen Schwarms zeigt die erweiterte MPCD-Methode eine homogenere Fluiddichte, und wir machen die interessante Beobachtung, dass die dynamischen Schwärme bei der erweiterten MPCD-Methode stärker ausgeprägt sind und eine höhere polare Ordnung aufweisen für die erweiterte MPCD-Methode."],"dc:identifier.uri":["https://depositonce.tu-berlin.de/handle/11303/17974","https://doi.org/10.14279/depositonce-16766"],"dc:language.iso":["en"],"dc:rights.uri":["https://creativecommons.org/licenses/by/4.0/"],"dc:title":["Elongated microswimmers: influence of hydrodynamics"],"dc:type":["Doctoral Thesis"]},"updated_at":"2026-07-27T21:28:31Z"}