Artificial Brownian motors: Controlling transport on the nanoscale

In systems possessing spatial or dynamical symmetry breaking, Brownian motion combined with symmetric external input signals, deterministic or random, alike, can assist directed motion of particles at the submicron scales. In such cases, one speaks of "Brownian motors". In this review the constructive role of Brownian motion is exemplified for various one-dimensional setups, mostly inspired by the cell molecular machinery: working principles and characteristics of stylized devices are discussed to show how fluctuations, either thermal or extrinsic, can be used to control diffusive particle transport. Recent experimental demonstrations of this concept are reviewed with particular attention to transport in artificial nanopores and optical traps, where single particle currents have been first measured. Much emphasis is given to two- and three-dimensional devices containing many interacting particles of one or more species; for this class of artificial motors, noise rectification results also from the interplay of particle Brownian motion and geometric constraints. Recently, selective control and optimization of the transport of interacting colloidal particles and magnetic vortices have been successfully achieved, thus leading to the new generation of microfluidic and superconducting devices presented hereby. Another area with promising potential for realization of artificial Brownian motors are microfluidic or granular set-ups.....Comment: 57 pages, 39 figures; submitted to Reviews Modern Physics, revised versio

Zusammenspiel von Geometrie und Dynamik in mesoskopischen Modellsystemem

Diese Arbeit behandelt anhand verschiedenener mesoskopischer Modellsysteme das Zusammenspiel von Geometrie und Form eines Systems mit seinen Eigenschaften und seiner Dynamik. Im ersten Teil wird ein erweitertes strahlenoptisches Modell für dielektrische optische Mikrokavitäten untersucht. Strahlenoptik ist eine effiziente Methode, um die Fernfeldabstrahlung dieser Systeme vorherzusagen. Werden allerdings Systeme betrachtet, deren Abmessungen nur wenige Lichtwellenlängen betragen, können Korrekturen der geometrischen Optik notwendig werden, um Welleneffekte zu berücksichtigen. Diese Korrekturen sind die Goos-Hänchen-Verschiebung, eine seitliche Verschiebung des Strahls entlang der Grenzfläche, und der Effekt des Fresnel-Filterns, eine Korrektur des Winkels, die das Reflexions- und das Brechungsgesetz der Strahlenoptik und das Prinzip der Umkehrbarkeit des Strahlengangs bricht. Diese Strahlverschiebungen werden für ebene und gekrümmte Grenzflächen diskutiert, außerdem werden die Einflüsse verschiedener Parameter auf die Korrekturterme untersucht. Ein wichtiges Resultat ist, dass die Krümmung der Grenzfläche den Effekt des Fresnel-Filterns verstärkt, wohingegen sie die Goos-Hänchen-Verschiebung abschwächt. Anschließend wird das strahlenoptische Modell auf verschiedene Beispiele angewendet, nämlich Mikrokavitäten in der Form von deformierten Kreisscheiben, also Systeme mit gekrümmten Grenzflächen, und dreieckige Kavitäten, also Systeme mit ausschließlich ebenen Grenzflächen. Sowohl für Systeme mit gekrümmten als auch mit ebenen Grenzflächen kann es wichtig sein, die auf endlichen Wellenlängen beruhenden Korrekturen miteinzubeziehen, um eine gute Übereinstimmung zwischen der Strahlenbeschreibung und Ergebnissen aus Experimenten oder Wellensimulationen zu erhalten. Die Systeme können aber nicht nur durch ihre Grenzfläche charakterisiert werden, sondern auch dadurch, ob ihre klassische Dynamik chaotisch oder nicht-chaotisch ist. Für Systeme mit chaotischer Dynamik ist bekannt, dass die Fernfeldabstrahlung durch die instabile Mannigfaltigkeit des chaotischen Sattels bestimmt wird. Als Beispiele für nicht-chaotische Systeme werden deformierte Kreisscheiben mit kleinen Verformungen und Dreiecke betrachtet. Für diese Systeme wird erörtert, dass die Abstrahlung durch die Trajektorien mit den kleinsten, nichtverschwindenden Zerfallsraten bestimmt wird. Darüber hinaus kann es notwendig sein, Intensitätsverstärkung im Strahlenbild zu berücksichtigen, um verlässliche Ergebnisse für stark verlustbehaftete Lasersysteme zu erhalten. Im zweiten Teil werden graphenartige Systeme diskutiert. An diesen wird zuerst der Einfluss von einachsigen Verformungen in einem tight-binding-Modell des hexagonalen Gitters untersucht. Einachsige Stauchung des Gitters führt zu einem Phasenübergang und zur Ausbildung von Randzuständen senkrecht zur Verzerrungsrichtung. Diese Randzustände sind unabhängig von der genauen Terminierung des Gitters. Als zweites wird ein Strahlenmodell eingeführt, das eine Beschreibung von Graphen-Bauelementen ermöglichen könnte, die genauso effizient ist wie die Strahlenbeschreibung von optischen Systemen.In this work, we study the interplay of the system geometry and the boundary shape with the properties and dynamics of different mesoscopic model systems. In the first part, we investigate extended ray-optical models for dielectric optical microcavities. Ray optics is an efficient tool to predict the far-field emission of these systems. When considering systems on lengths scales on the order of few light wavelengths, however, corrections to geometrical optics can be necessary to account for wave effects. These corrections to ray optics are the Goos-Hänchen shift, a lateral shift along the interface, and the Fresnel filtering effect, an angular shift, which violates the ray-optical laws of reflection and refraction and the principle of ray-path reversibility. We discuss these beam shift effects at planar and curved boundaries and study the influence of different parameters on the correction terms. An important finding is that the boundary curvature enhances the Fresnel filtering effect, whereas, it reduces the Goos-Hänchen shift. We apply the ray-optical model to deformed microdisks, systems with curved boundaries, and to triangular cavities, systems with only planar boundaries. Correctly including finite-wavelength effects can be important to establish good agreement between the ray description and results of experiments or wave simulations for both, systems with curved and with planar boundaries. The systems can not only be classified by their boundary shape but also by their classical ray dynamics, whether it is chaotic or non-chaotic. For systems with chaotic dynamics it is well known that the unstable manifold of the chaotic saddle determines the far-field emission. For non-chaotic systems - we consider deformed disks with small deformations and triangles - we discuss that the trajectories with the smallest nonzero decay rates determine the emission pattern. Further, we include intensity amplification in the ray-optical model and find it necessary to obtain reliable results in highly lossy lasing systems. In the second part, we discuss graphene-like systems. For these systems, we study the influence of uniaxial strain in a tight-binding model on the honeycomb lattice. Uniaxial compression leads to a phase transition and to the formation of edge states perpendicular to the strain direction independent of the edge termination. Further, we introduce a ray model as a possibility to describe graphene devices as efficiently as the ray-optical model allows for dielectric optical microcavities

Flowing matter

This open access book, published in the Soft and Biological Matter series, presents an introduction to selected research topics in the broad field of flowing matter, including the dynamics of fluids with a complex internal structure -from nematic fluids to soft glasses- as well as active matter and turbulent phenomena.Flowing matter is a subject at the crossroads between physics, mathematics, chemistry, engineering, biology and earth sciences, and relies on a multidisciplinary approach to describe the emergence of the macroscopic behaviours in a system from the coordinated dynamics of its microscopic constituents.Depending on the microscopic interactions, an assembly of molecules or of mesoscopic particles can flow like a simple Newtonian fluid, deform elastically like a solid or behave in a complex manner. When the internal constituents are active, as for biological entities, one generally observes complex large-scale collective motions. Phenomenology is further complicated by the invariable tendency of fluids to display chaos at the large scales or when stirred strongly enough. This volume presents several research topics that address these phenomena encompassing the traditional micro-, meso-, and macro-scales descriptions, and contributes to our understanding of the fundamentals of flowing matter.This book is the legacy of the COST Action MP1305 “Flowing Matter”

Superconductor

This book contains a collection of works intended to study theoretical and experimental aspects of superconductivity. Here you will find interesting reports on low-Tc superconductors (materials with Tc 30 K). Certainly this book will be useful to encourage further experimental and theoretical researches in superconducting materials

Active Emulsions: Physicochemical Hydrodynamics and Collective Behavior

Active matter is a collection of constituent elements that constantly consume energy, convert it to mechanical work, and interact with their counterparts. These materials operate out of equilibrium and exhibit fascinating collective dynamics such as spontaneous pattern formation. Self-organization of bio-polymers within a cell, collective migration of bacteria in search of nutrition, and the bird flocks are paragons of active living matter and the primary source of our knowledge on it. To understand the overarching physical principles of active matter, it is desirable to build artificial systems that are capable of imitating living active matter while ruling out the biological complexities. The goal of this thesis is to study active micro-droplets as a paradigm for biomimetic artificial active particles, using fundamental principles of fluid dynamics and statistical physics. The Marangoni-driven motility in these droplets is reminiscent of the locomotion of some protozoal organisms, known as squirmers. The main scientific objectives of this research are to (i) investigate the potential biomimetic features of active droplets including compartmentalization, adaptability (e.g. multi-gait motility), and information processing (signaling and sensing) and (ii) study the implications of those features in the collective dynamics of active emulsions governed by hydrodynamic and autochemotactic interactions. These objectives are addressed experimentally using microfluidics and microscopy, integrated with quantitative image analysis. The quantitative experimental results are then compared with the predictions from theory or simulations. The findings of this thesis are presented in five chapters. First, we address the challenge of compartmentalizing active droplets. We use microfluidics to generate liquid shells (double emulsions). We propose and successfully prove the use of a nematic liquid crystal oil to stabilize the liquid shells, which are otherwise susceptible to break up during motility. We investigate the propulsion dynamics and use that insight to put forward routes to control shell motion via topology, chemical signaling, and topography. In the second results chapter, we establish the bimodal dynamics of chaotic motility in active droplets; a regime that emerges as a response to the increase of viscosity in the swimming medium. To establish the physical mechanism of this dynamical transition, we developed a novel technique to simultaneously visualize the hydrodynamic and chemical fields around the droplet. The results are rationalized by quantitative comparison to established advection-diffusion models. We further observe that the droplets undergo self-avoiding random walks as a result of interaction with the self-generated products of their activity, secreted in the environment. The third results chapter presents a review of the dynamics of chemotactic droplets in complex environments, highlighting the effects of self-generated chemical interactions on the droplet dynamics. In the fourth results chapter, we investigate how active droplets sense and react to the chemical gradients generated by their counterparts--- a behavior known as autochemotaxis. Then, we study the collective dynamics governed by these autochemotactic interactions, in two and three dimensions. For the first time, we report the observation of ‘history caging’, where swimmers are temporarily trapped in an evolving network of repulsive chemical trails. The caging results in a plateau in the mean squared displacement profiles as observed for dense colloidal systems near the glass transition. In the last results chapter, we investigate the collective dynamics in active emulsions, governed by hydrodynamic interactions. We report the emergence of spontaneously rotating clusters. We show that the rotational dynamics originates from a novel symmetry breaking mechanism for single isotropic droplets. By extending our understanding to the collective scale, we show how the stability and dynamics of the clusters can be controlled by droplet activity and cluster size. The experimental advancements and the findings presented in this thesis lay the groundwork for future investigations of emergent dynamics in active emulsions as a model system for active matter. In the outlook section, we present some of the new questions that have developed in the course of this research work and discuss a perspective on the future directions of the research on active droplets.2022-01-1