Micro-devices are developed for uses in targeted drug delivery and microscale manipulation. Here we numerically and analytically study two promising devices in early stages of development. Firstly, we study Armoured Microbubbles (AMBs) which can self-propel as artificial microswimmers or facilitate microfluidic mixing in a channel when held stationary on a wall. Secondly, we study an artificial cilium, which due to its unique design, when placed in an array, easily produces a metachronal wave for fluid transportation. The Armoured Microbubble was designed by our experimental collaborators (group of Philippe Marmottant, University Grenoble Alpes) and consists of a partial hollow sphere, inside which a bubble is caught. Under ultrasound the bubble oscillates, generating a streaming flow in the surrounding fluid and producing a net force. Motivated by the AMB but considering initially a general setup, using matched asymptotic expansions we calculate the streaming flow around a spherical body undergoing arbitrary, but known, small-amplitude surface shape oscillations. We then specialise back to the AMB and consider its excitation under ultrasound, using a potential flow model with mixed boundary conditions, to identify the resonant frequencies and mode shapes, including the dependence of the resonance on the AMB shape parameters. Returning to our general streaming model, we applied the mixed boundary conditions directly to this model, calculating the streaming around the AMB, in good agreement with experiments. Using hydrodynamic images and linear superposition, this model was extended to incorporate one wall, and AMB compounds. We then study the streaming flows generated by arrays of AMBs in confined channels, by modelling each AMB as its leading order behaviour (with corrections where required) and superposing the individual flow fields of all the AMBs. We identified the importance of two confining walls on the streaming flow around the array, and compared these flows to experiments in five cases. Motivated by this setup, we theoretically considered the extension of a two fluid interface passing through an AMB array to quickly identify good AMB arrays for mixing. We then studied the second artificial micro-device: an artificial cilium. Tsumori et. al. produced a cilium of PDMS containing aligned ferromagnetic filings, which beat under a rotating magnetic field. We modelled a similar cilium but assumed paramagnetic filings, using a force model balancing elastic, magnetic and hydrodynamic forces identifying the cilium beat pattern. This agreed with our equilibrium model and asymptotic analysis. We then successfully identified that the cilium applies the most force to the surrounding fluid at an intermediate value of the two dimensionless numbers quantifying the dynamics.

This thesis describes the use of a micropipette deflection technique to measure the viscous forces experienced by the millimeter sized undulatory swimmer and model organism C. elegans. Using a specialized pipette, we are able to simultaneously measure both the lateral and propulsive forces acting on the worm. We find that the measured force curves are well described by Resistive Force Theory, which is a low Reynolds number hydrodynamic model. This work constitutes the first justification of its applicability at Reynolds numbers of this magnitude (roughly 0.1). Through our comparison with Resistive Force Theory, we extract the worm's drag coefficients, which are in agreement with an existing theoretical prediction. Through a simple scaling argument, we obtain a relationship between the size of the worm and the typical viscous forces, which is in good agreement with our data. We also present a study aimed at measuring how the hydrodynamic forces on the worm change in proximity to solid boundaries. Using micropipette deflection, forces are measured at controlled distances from a single planar boundary and midway in between two parallel boundaries. We find the viscous forces and drag coefficients to increase significantly as the worm approaches a boundary. We find a constant value for the ratio of normal to tangential drag coefficients at all distances from a single boundary, but measure it to increase significantly as the worm is confined between two boundaries. In addition, the worm is seen to undergo a continuous gait modulation, primarily characterized by a decreased swimming amplitude, as it is subject to larger drag forces in confinement. Finally, the interactions between two worms swimming nearby one another are probed. Worms are held adjacent to one another using micropipettes, and are found to tangle with each other, rather than interact hydrodynamically. We develop simple models that well capture the onset and probability of tangles as a function of the separation distance between the worms. / Thesis / Master of Science (MSc)

Le sujet de cette thèse s'inscrit dans le cadre de l'étude de la matière active. Ces systèmes sont composés de "particules actives" capables de s'organiser spontanément (transition de phase), et de manière autonome (sans application d'un champs extérieur), créant ainsi des dynamiques complexes comme les transition de phase dynamiques, synchronisation, instabilités etc...De nombreuses études tendent à montrer le rôle important des interactions entre particules active dans l'émergence de ces dynamiques. Nous avons abordé ces questions à travers l'étude d'une suspension de micro-algues Chlamydomonas reinhardtii. Il s'agit d'un système modèle de micro-nageur couramment utilisé pour l'étude des suspensions actives. Nous avons notamment étudié un phénomène de migration spontanée de la suspension, permettant de concentrer des micro-algues grâce à une source de lumière. Puis nous avons étudié le processus de dispersion d'un amas concentré de micro-algues. Nous avons notamment mis en évidence le rôle des interactions hydrodynamiques entre micro-algues dans cette dynamique de dispersion. / The subject of this thesis is part of the study of the active matter. These systems are composed of "active particles" capable of organizing themselves spontaneously (phase transition), and autonomously (without application of an external field), thus creating complex dynamics such as dynamical phase transition, synchronization, instabilities etc ...Numerous studies tend to show the important role of interactions between active particles in the emergence of these dynamics. We have addressed these issues through the study of a suspension of microalgae Chlamydomonas reinhardtii. It is a model system of micro-swimmer commonly used for the study of active suspensions. We studied in particular a phenomenon of spontaneous migration of the suspension, allowing to concentrate micro-algae thanks to a light source. We then studied the dispersal process of a concentrated bloom of microalgae. In particular, we have highlighted the role of hydrodynamic interactions between micro-algae in this dispersion dynamics.

No description available.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24646号 / 工博第5152号 / 新制||工||1984(附属図書館) / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 山本 量一, 教授 外輪 健一郎, 教授 松坂 修二 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

<div>In many natural and industrial processes, bubbles are exposed to surface-active contaminants (surfactants) that may cover the whole or part of the bubble interface. A partial coverage of the bubble interface results in a spontaneous self-propulsion mechanism, which is yet poorly understood.</div><div>The main goal of this study is to enhance the understanding of the flow and interfacial mechanisms underlying the self-propulsion of small surfactant contaminated bubbles. The focus is on characterizing the self-propulsion regimes generated by the presence of surface-active species, and the influence of surfactant activity and surface coverage on the active bubble motion. </div><div>The study was developed by simultaneously solving the full system of partial differential equations governing the free-surface flow physics and the surfactant transport on the deforming bubble interface using multi-scale numerical simulation. </div><div>Results show in microscopic detail how surface tension gradients (Marangoni stresses) induced by the uneven interfacial coverage produce spontaneous hydrodynamics flows (Marangoni flows) on the surrounding liquid, leading to bubble motion. Results also establish the influence of both surfactant activity and interfacial coverage on total displacement and average bubble velocity at the macroscale. </div><div>Findings from this research improve the fundamental understanding of the free-surface dynamics of self-propulsion and the associated transport of surface-active species, which are critical to important natural and technological processes, ranging from the Marangoni propulsion of microorganisms to the active motion of bubbles and droplets in microfluidic devices. Overall, the findings advance our understanding of active matter behavior; that is, the behavior of material systems with members able to transduce surface energy and mass transport into active movement.</div>

In this work, photocatalytic bismuth vanadate microparticles of different morphologies were investigated as artificial single.component microswimmers. In the first chapter, the motion mechanism as well as influence factors on the motion behavior such as solution pH and solution conductivity, hydrogen peroxide fuel concentration and decomposition kinetics and the surface charge of the particles were studied in detail. Furthermore, fluid flow profiles around the particles were determined. In the following chapter, interactions between the microswimmers were studied and exploited to create active assemblies that can be used to integrate multiple functionalities in one assembly. Eventually, alternative propulsion mechanisms besides hydrogen peroxide fuel decomposition were studied. The presented approaches include the catalysis of an organic oxidation and photoreduction of noble metals onto the particles, which proved to increase their catalytic activity and enabled propulsion of the modified microswimmers in pure water.:Acknowledgments III List of Abbreviations V 1. Introduction 1 2. Fundamentals of Photocatalysis and Active Matter 5 2.1. Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.1. Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.2. Processes in Semiconductor Photocatalysis . . . . . . . . . . . . . . . . 5 2.1.3. Properties of Bismuth Vanadate . . . . . . . . . . . . . . . . . . . . . . 10 2.1.4. Photocatalytic H2O2 decomposition . . . . . . . . . . . . . . . . . . . . 12 2.2. Active Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2.1. Motion at the Microscale . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2.2. Mechanisms of Catalytic Active Motion . . . . . . . . . . . . . . . . . . 18 2.2.3. Light-Driven Active Motion . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.2.4. Origin of Asymmetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 3. Aim and Motivation 27 4. Results and Discussion 29 4.1. Microparticle Synthesis und Characterization . . . . . . . . . . . . . . . . . . . 29 4.1.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.1.2. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 4.1.3. Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4.1.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 4.2. Motion Studies and Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.2.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.2.2. Characterization of Active Motion . . . . . . . . . . . . . . . . . . . . . 44 4.2.3. Influence of Experimental Conditions . . . . . . . . . . . . . . . . . . . . 54 4.2.4. Adjustment of Motion Mode by pH . . . . . . . . . . . . . . . . . . . . . 67 4.2.5. Flow Fields Around Single Crystalline Microparticles . . . . . . . . . . . 73 4.2.6. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 4.3. Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.3.2. Interactions Between BiVO4 Microswimmers . . . . . . . . . . . . . . . 83 4.3.3. Interactions between Spheroidal Microswimmers . . . . . . . . . . . . . 84 4.3.4. Surface Modification of Spheroidal Microswimmers . . . . . . . . . . . . 88 4.3.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 4.4. Towards Alternative Propulsion Reactions . . . . . . . . . . . . . . . . . . . . . 93 4.4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 4.4.2. Oxidation of Dibenzylamine . . . . . . . . . . . . . . . . . . . . . . . . . 94 4.4.3. Photodeposition of Metals . . . . . . . . . . . . . . . . . . . . . . . . . . 97 4.4.4. Towards Propulsion in Pure Water . . . . . . . . . . . . . . . . . . . . . 98 4.4.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 5. Summary and Final Remarks 103 6. Experimental Details 113 6.1. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 6.1.1. Microparticle Synthesis and Characterization . . . . . . . . . . . . . . . 113 6.1.2. Motion Studies and Mechanism . . . . . . . . . . . . . . . . . . . . . . . 114 6.1.3. Interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 6.1.4. Towards Alternative Propulsion Reactions . . . . . . . . . . . . . . . . . 117 6.2. Apparatus and Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 6.2.1. Scanning Electron Microscopy (SEM) . . . . . . . . . . . . . . . . . . . 117 6.2.2. Transmission Electron microscopy (TEM) . . . . . . . . . . . . . . . . . 117 6.2.3. Nitrogen physisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.4. Gas Chromatography (GC) . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.5. Powder X-ray Crystallography (XRD) . . . . . . . . . . . . . . . . . . . 118 6.2.6. Absorption Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.7. Fluorescence Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 118 6.2.8. Zeta Potential Measurements . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2.9. Fluorescence Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 6.2.10. Light Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119 A. Appendix 123 Bibliography 139 List of Publications 149 Erklärung 151

A prevalent morphology in the microscopic world of artificial microswimmers, bacteria and viruses is that of a helix. The intriguingly different physics at play at the small scale level make it necessary for bacteria to employ swimming strategies different from our everyday experience, such as the rotation of a helical filament. Bio-inspired microswimmers that mimic bacterial locomotion achieve propulsion at the microscale level using magnetically actuated, rotating helical filaments. A promising application of these artificial microswimmers is in non-invasive medicine, for drug delivery to tumours or microsurgery. Two crucial features need to be addressed in the design of microswimmers. First, the ability to selectively control large ensembles and second, the adaptivity to move through complex conduit geometries, such as the constrictions and curves of the tortuous tumour microvasculature. In this dissertation, a mechanics-based selective control mechanism for magnetic microswimmers is proposed, and a model and simulation of an elastic helix passing through a constricted microchannel are developed. Thereafter, a theoretical framework is developed for the propulsion by stiff elastic filaments in viscous fluids. In order to address this fluid-structure problem, a pertubative, asymptotic, elastohydrodynamic approach is used to characterise the deformation that arises from and in turn affects the motion. This framework is applied to the helical filaments of bacteria and magnetically actuated microswimmers. The dissertation then turns to the sub-bacterial scale of bacteriophage viruses, 'phages' for short, that infect bacteria by ejecting their genetic material and replicating inside their host. The valuable insight that phages can offer in our fight against pathogenic bacteria and the possibility of phage therapy as an alternative to antibiotics, are of paramount importance to tackle antibiotics resistance. In contrast to typical phages, flagellotropic phages first attach to bacterial flagella, and have the striking ability to reach the cell body for infection, despite their lack of independent motion. The last part of the dissertation develops the first theoretical model for the nut-and-bolt mechanism (proposed by Berg and Anderson in 1973). A nut being rotated will move along a bolt. Similarly, a phage wraps itself around a flagellum possessing helical grooves, and exploits the rotation of the flagellum in order to passively travel along and towards the cell body, according to this mechanism. The predictions from the model agree with experimental observations with respect to directionality, speed and the requirements for succesful translocation.