Active matter refers to systems driven out of thermal equilibrium by the uptake and dissipation of energy directly at the level of the individual constituents, which then undergo systematic movement in a direction decided by their own internal state. This category of nonequilibrium systems was defined as the physical model of motile, metabolizing matter, but the definition has a wider application. In this thesis we work with monolayer of macro-scopic granular particles lying on a vibrated surface and show that it provides a faithful realisation of active matter. The vibration feeds energy into the tilting vertical motion of the particles, which transduces it into a horizontal movement via frictional contact with the base in a direction determined by its orientation in the plane. We show that the dynamics of the particles can be easily controlled by manipulating their geometrical shapes. In the second part of the thesis, not addressing active matter, we do experiments on a soft condensed mat-ter system of viscoelastic surfactant gel formed of an entangled network of wormlike micelles and shows shear-thinning and is therefore non-Newtonian. These systems have relaxation times of the order of seconds and we have studied their non-equilibrium response properties when driven out of equilibrium externally by the gravitational sedimentation of objects and rising air-bubbles. Chapter 1 gives a general introduction to the term active matter and emphasize particularly on how these systems are internally driven and work far away from the equilibrium. We then explain in detail how a system of granular particles lying on a vibrating surface acts as active matter. We later give a brief introduction to the field of soft condensed matter and discuss the viscoelastic properties of surfactant solutions and their phase behaviour. We end this chapter by giving a brief introduction to flocking and non-equilibrium fluctuation relations which act as prerequisite to the following chapters. In Chapter 2 we discuss the experimental techniques used by us. We will first describe the shapes and dimensions of the granular particles used in the experiments. Next we introduce the shaker set-up and describe the experimental cell in which the particles are confined and variation in cell’s boundary. We show the dynamics of the particles in a quasi one-dimensional channel and then in two-dimensions. We give a brief account of image analysis and tracking algorithms employed and other data analyses techniques. In Chapter 3, we study the non-equilibrium fluctuations of a self-propelled polar particle moving through a background of non-motile spherical beads in the context of the Gallavotti-Cohen Fluctuation Relation (GCFR), which generalizes the second law of thermodynamics by quantifying the relative probabilities of the instantaneous events of entropy consumption and production. We find a fluctuation relation for a non-thermodynamic quantity, the velocity component along the long axis of the particle. We calculate the Large Deviation Function (LDF) of the velocity fluctuations and find the first experimental evidence for its theoretically predicted slope singularity at zero. We also propose an independent way to estimate the mean phase-space contraction rate. In Chapter 4 we expand the analysis done in Chapter 3 and study the two-dimensional velocity vector of the particle in the context of Isometric Fluctuation Relation (IFR) which measures the relative probability of current fluctuations in different directions in space of dimension >1. We first show that the dynamics of the particle is not isotropic and present a minimal model for its dynamics as a biased random walker, driven by a noise with anisotropic strength and construct an Anisotropic IFR (AIFR). We then show that the velocity statistics of the polar particle agree with the AIFR. We also confirm that the GCFR can be obtained as a special case of AIFR when the velocity vectors point in opposite directions. We calculate the LDF of particle’s velocity vector and find an extended kink in the velocity plane. In Chapter 5 we study the flocking phenomenon of a collection of polar particles when moving through a background of non-motile beads. We show that in the presence of bead medium, polar particles can flock at much lower concentrations, in contrast to the Vicsek model which predicts flocking at high concentrations. We show that the moving rods lead to a bead flow which in turn helps them to communicate their orientations and velocities at much greater distances. We provide a phase diagram in the parameter space of concentrations of beads and polar particles and show power-law spatial correlations as we approach the phase boundary. We also discuss the numerical simulations and theoretical model presented which support the experiments results. In Chapter 6 we experimentally study the angle dependence of the trapping of collection of active granular rods in a chevron shaped geometry. We show the particles undergo a trapping-detrapping transition at θ = 1150. On the contrary, this angle value is θ = 700 for a single rod. We find a substantial decrease in rotational noise for a collection of particles inside a trap as compared to a single rod which explains the increased value of θ for the trapping-detrapping transition. We also show that polar active particles which tend to change their direction of motion do not show the trapping phenomenon. In Chapter 7 we conduct experiments on falling balls and rising air bubbles through a non-Newtonian solution of surfactant CTAT in water, which forms a viscoelastic wormlike micellar gel. We show that the motion of the ball undergoes a transition from a steady state to oscillatory as the diameter of the ball is increased. The oscillations in velocity of the ball are non-sinusoidal, consisting of high-frequency bursts occurring periodically at intervals long compared to the period within the bursts. We present a theoretical model based on a slow relaxation mechanism owing to structural instabilities in the constituent micelles of the viscoelastic gel. For the case of air bubbles, we show that an air bubble rising in the viscoelastic gel shows a discontinuous jump in the velocity beyond a critical volume followed by a drastic change in its shape from a teardrop to almost spherical. We also observe shape oscillations for bigger bubbles with the tail swapping in and out periodically.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18053号 / 理博第3931号 / 新制||理||1567(附属図書館) / 30911 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)講師 市川 正敏, 教授 山本 潤, 教授 佐々 真一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

Equilibrium statistical mechanics has a remarkable property: the steady state probability distribution can be calculated by a procedure independent of the detailed dynamics of the system under consideration. The partition function contains the complete thermodynamics of the system. The calculation of the partition function itself might be a daunting task and one might need to resort to approximate methods in practice. But there is no problem in principle on how to do the statistical mechanics of a system that is at thermal equilibrium. Nonequilibrium statistical mechanics is a completely different story. There is no general formalism, even in principle, the application of which is guaranteed to yield the probability distribution, even for stationary states, without explicit consideration of the dynamics of the system. Instead, there are several methods of wide applicability drawn from experience which work for particular classes of systems. Frequently, one writes down phenomenological equations of motion based on general principles of conservation and symmetry and attempts to extract the dynamical response and correlations. The motivation for studying nonequilibrium systems is the very simple fact that they are ubiquitous in nature and exhibit very rich, diverse and often counter-intuitive phenomenon. We ourselves are an example of a very complex nonequilibrium system. This thesis examines three problems which illustrate the generic features of a typical driven system maintained out of thermal equilibrium. The first chapter provides a very brief discussion of nonequilibrium systems. We outline the tools that are commonly employed in the theoretical description of driven systems, and discuss the response of physical systems to applied perturbations. Chapter two considers a very simple model for a single self-propelled particle with an internal asymmetry, and nonequilibrium energy input in the form of Gaussianwhite noise. Our model connects three key nonequilibrium quantities – drift velocity, mean internal force and position-velocity correlations. We examine this model in detail and solve it using perturbative, numerical and exact methods. We begin chapter three with a brief introduction to the sedimentation of particle-fluid suspensions. Some peculiarities of low Reynolds number hydrodynamics are discussed with particular emphasis on the sedimentation of colloidal particles in a viscous fluid. We then introduce the problem of velocity fluctuations in steady sedi-mentation. The relevance of the current study to an earlier model and improvements made in the present work are then discussed. A physical understanding of our model and the conclusions that result from its analysis are an attempt to resolve the old problem of divergent velocity fluctuations in steadily sedimentating suspensions. The fourth chapter is a study to probe the nature of the fluctuations in a driven suspension of point-particles. Fluctuation relations that characterise large-deviations are a current topic of intense study. We show in this chapter that the random dynamics of suspended particles in a driven suspension occasionally move against the driving force, and that the probability of such rare events obeys a steady state fluctuation relation. In the final chapter, we summarise the models studied and point out the common features that they display. We conclude by pointing out some ways in which the problems discussed in this thesis can be extended upon in the future.

The main subject of this thesis is to examine thermo-osmotic flows, which occur on interfaces of non-uniform temperature. Such thermo-osmotic flows are purely non-thermal equilibrium phenomena. Along the non-isothermal interface, specific interaction of a liquid and its solutes with a boundary vary in strength across the interface, according to the local temperature. This boundary can be a solid, a membrane or a phase boundary. The flow is thereby continuously pumping fluid across the interface in direction of the local temperature gradient, resulting in an extended flow pattern in the bulk due to mass conservation. In a system containing particles and heat sources in a liquid under spatial confinement, the thermo-osmotic flow may drive particles in a directed manner, or can lead to collective phenomena. To approach this broad topic of (self-)thermophoresis and collective motion of active particles and quantify the role of the thermo-osmotic flow upon the latter effects, different experiments have been performed: The first experiments aim to quantify the thermo-osmotic flow at a non-isothermal liquid/solid interface for two fundamentally different substrate properties. Further, the bulk flow was investigated for two different systems. The form and spatial extension of this bulk flow pattern depends sensitively on the form of the container and the interface, as well as on the thermo-osmotic flow. The first system is a liquid film confined between two planar glass cover slips. The second case is a Janus particle immobilized on one of the glass slips. In the first case, the non-uniform temperature profile is generated by optical heating of a nanometer sized gold colloid, and in the second case, the heat source is the Janus particle. The bulk flow pattern consists, for the second case, of the flow pattern created by the glass cover slips and the one created by the Janus particle. The following experiments are focusing on the dynamics of mobile self-thermophoretic Janus particles. In particular, their dynamics and the contributions of the thermo-osmotic flow to the interaction of multiple active particles are investigated. To investigate those particles under controlled conditions and examine their interactions at low concentrations for an effectively unlimited amount of time, a real-time feedback algorithm was co-developed to gain control of the motion of multiple active particles simultaneously, called ”photon nudging”. With the help of this method, first experiments have been performed to quantify the dynamics of a Janus particle located close to a heat source.

Suspensions of the aerobic bacteria {\it Bacilus subtilis} develop patterns and flows from the interplay of motility, chemotaxis and buoyancy.In sessile drops, such bioconvectively driven flows carry plumes down the slanted meniscus and concentrate cells at the drop edge, while in pendant drops such self-concentration occurs at the bottom.These dynamics are explained quantitatively by a mathematical model consisting of oxygen diffusion and consumption, chemotaxis, and viscous fluid dynamics.Concentrated regions in both geometries comprise nearly close-packed populations, forming the collective ``Zooming BioNematic'' (ZBN) phase.This state exhibits large-scale orientational coherence, analogous to the molecular alignment of nematic liquid crystals, coupled with remarkable spatial and temporal correlations of velocity and vorticity, as measured by both novel and standard applications of particle imaging velocimetry.To probe mechanisms leading to this phase, response of individual cells to steric stress was explored, finding that they can reverse swimming direction at spatial constrictions without turning the cell body.The consequences of this propensity to flip the flagella are quantified, showing that "forwards" and "backwards" motion are dynamically and morphologically indistinguishable.Finally, experiments and mathematical modeling show that complex flows driven by previously unknown bipolar flagellar arrangements are induced when {\it B. subtilis} are confined in a thin layer of fluid, between asymmetric boundaries.The resulting driven flow circulates around the cell body ranging over several cell diameters, in contrast to the more localized flows surrounding free swimmers.This discovery extends our knowledge of the dynamic geometry of bacteria and their flagella, and reveals new mechanisms for motility-associated molecular transport and inter-cellular communication.

abstract: Locomotion of microorganisms is commonly observed in nature. Although microorganism locomotion is commonly attributed to mechanical deformation of solid appendages, in 1956 Nobel Laureate Peter Mitchell proposed that an asymmetric ion flux on a bacterium's surface could generate electric fields that drive locomotion via self-electrophoresis. Recent advances in nanofabrication have enabled the engineering of synthetic analogues, bimetallic colloidal particles, that swim due to asymmetric ion flux originally proposed by Mitchell. Bimetallic colloidal particles swim through aqueous solutions by converting chemical fuel to fluid motion through asymmetric electrochemical reactions. This dissertation presents novel bimetallic motor fabrication strategies, motor functionality, and a study of the motor collective behavior in chemical concentration gradients. Brownian dynamics simulations and experiments show that the motors exhibit chemokinesis, a motile response to chemical gradients that results in net migration and concentration of particles. Chemokinesis is typically observed in living organisms and distinct from chemotaxis in that there is no particle directional sensing. The synthetic motor chemokinesis observed in this work is due to variation in the motor's velocity and effective diffusivity as a function of the fuel and salt concentration. Static concentration fields are generated in microfluidic devices fabricated with porous walls. The development of nanoscale particles that swim autonomously and collectively in chemical concentration gradients can be leveraged for a wide range of applications such as directed drug delivery, self-healing materials, and environmental remediation. / Dissertation/Thesis / Ph.D. Mechanical Engineering 2011

Main subject of this diploma thesis is design proposal of self-propelled forage harvester which uses tracks for moving. This is associated with a construction changes while technical, estetical and ergonomical requirements remain the same or will be improved.

This thesis investigates several phenomena that are associated with (self-)thermophoretic Janus particles with hemispheres made from different materials serving as a paradigm for active propul- sion on the microscale. (i) The dynamics of a single Janus sphere in the external temperature field created by an immobilized heat source is studied. I show that the particle’s angular velocity is solely determined by the temperature profile on the equator between the Janus particle’s hemispheres and their phoretic mobility contrast. (ii) The distinct polarization-density patterns observed for active-particle suspensions in activity landscapes are addressed. The results of my approximate theoretical model agree well with exact numerical and measurement data for a thermophoretic microswimmer, and can serve as a template for more complex applications. The essential physics behind the formal results is robustly captured and elucidated by a schematic two-species “run- and-tumble” model. (iii) I investigate coarse-grained models of suspended self-thermo- phoretic microswimmers. Starting from atomistic molecular dynamics simulations, the coarse-grained de- scription of the fluid in terms of a local molecular temperature field is verified, and effective nonequilibrium temperatures characterizing the particle’s so called hot Brownian motion are mea- sured from simulations. They are theoretically shown to remain relevant for any further spatial coarse-graining towards a hydrodynamic description of the entire suspension as a homogeneous complex fluid. / In dieser Arbeit untersuche ich mehrere Phänomene, die im Zusammenhang mit (selbst-)thermo- phoretischen Janusteilchen auftreten. Diese Teilchen bestehen aus zwei Halbkugeln mit unter- schiedlichen Materialeigenschaften und dienen in dieser Arbeit als Musterbeispiel für aktive Fort- bewegung auf der Mikroskala. (i) Die Dynamik eines einzelnen Janusteilchens im externen Temper- aturfeld einer ortsfesten Heizquelle wird untersucht. Es wird gezeigt, dass die Winkelgeschwindigkeit des Teilchens ausschließlich durch das Temperaturprofil am Äquator zwischen den Hemisphären des Janusteilchens und dem Unterschied ihrer phoretischen Mobilitäten bestimmt wird. (ii) Ich befasse mich mit den charakteristischen Polarisations- und Dichteprofilen, die für aktive Teilchen in Aktivitätslandschaften beobachtet werden. Die Ergebnisse meines approximativen theoretis- chen Modells stimmen gut mit exakten numerischen Lösungen und Messdaten für einen ther- mophoretischen Mikroschwimmer überein und können als Vorlage für komplexere Anwendungen dienen. Die wesentliche Physik hinter den formalen Ergebnissen wird durch ein schematisches Zwei-Spezies-“Run-and-Tumble”-Modell erfasst und erklärt. (iii) Ich untersuche Coarse-Graining- Modelle von suspendierten selbst-thermophoretischen Mikroschwimmern. Ausgehend von atom- istischen molekulardynamischen Simulationen wird die grobkörnige (coarse-grained) Beschreibung des Fluids in Form eines lokalen molekularen Temperaturfeldes verifiziert. Anschließend berechne ich effektive Nichtgleichgewichtstemperaturen, die die sogenannte heiße Brownsche Bewegung der Teilchen charakterisieren, und vergleiche diese mit Simulationsdaten. Es wird gezeigt, dass diese effektiven Temperaturen für jede weitere räumliche Vergröberung hin zu einer hydrodynamischen Beschreibung der gesamten Suspension als homogenes komplexes Fluid relevant bleiben.

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Animal groups often exhibit highly coordinated collective motion in a variety of situations. For example, bird flocks, schools of fish, a flock of sheep being herded by a dog and highly efficient traffic on an ant trail. Although these phenomena can be observed every day all over the world our knowledge of what rules the individual's in such groups use is very limited. Questions of this type has been studied using so called self-propelled particle (SPP) models, most of which assume that collective motion arises from individuals aligning with their neighbors. Here we introduce and analyze a SPP-model based on attraction alone. We find that it produces all the typical groups seen in alignment-based models and some novel ones. In particular, a group that exhibits collective motion coupled with non-trivial internal dynamics. Groups that have this property are rarely seen in SPP-models and we show that even when a repulsion term is added to the attraction only model such groups are still present. These findings suggest that an interplay between attraction and repulsion may be the main driving force in real flocks and that the alignment rule may be superfluous. We then proceed to model two different experiments using the SPP-model approach. The first is a shepherding algorithm constructed primarily to model experiments where a sheepdog is herding a flock of sheep. We find that in addition to modeling the specific experimental situation well the algorithm has some properties which may make it useful in more general shepherding situations. The second is a traffic model for leaf-cutting ants bridges. Based on earlier experiments a set of traffic rules for ants on a very narrow bridge had been suggested. We show that these are sufficient to produce the observed traffic dynamics on the narrow bridge. And that when extended to a wider bridge by replacing 'Stop' with 'Turn' the new rules are sufficient to produce several key characteristics of the dynamics on the wide bridge, in particular three-lane formation.