This thesis focuses on experimental studies on collective motion using swimming bacteria as model active-matter systems. It offers comprehensive reviews of state-of-the-art theories and experiments on collective motion from the viewpoint of nonequilibrium statistical physics. The author presents his experimental studies on two major classes of collective motion that had been well studied theoretically. Firstly, swimming filamentous bacteria in a thin fluid layer are shown to exhibit true, long-range orientational order and anomalously strong giant density fluctuations, which are considered universal and landmark signatures of collective motion by many numerical and theoretical works but have never been observed in real systems. Secondly, chaotic bacterial turbulence in a three-dimensional dense suspension without any long-range order as described in the first half is demonstrated to be capable of achieving antiferromagnetic vortex order by imposing a small number of constraints with appropriate periodicity. The experimental results presented significantly advance our fundamental understanding of order and fluctuations in collective motion of motile elements and their future applications.

This thesis describes lyotropic chromonic liquid crystals (LCLCs) with exotic elastic and viscous properties. The first part of the thesis presents a thorough analysis of the elastic and viscous properties of LCLCs as functions of concentration, temperature and ionic contents, while the second part explores an active nematic system: living liquid crystals, which represent a combination of LCLC and living bacteria. LCLCs are an emerging class of liquid crystals that have shown profound connections to biological systems in two aspects. First, the assembly process of the chromonic aggregates is essentially the same as DNA oligomers and other super-molecular assemblies of biological origin. LCLCs thus provide an excellent model system for studying physical properties such as the elasticity and viscosity of these supramolecular assemblies. Second, LCLCs are biocompatible, thus serving as a unique anisotropic matrix to interface with living systems such as bacteria. This thesis deepens our understanding of both aspects. The noncovalent nature of chromonic aggregation produces the unique viscoelasticity to be found in LCLCs, which differs dramatically from that of traditional LCs. Anisotropic interactions between LCLCs and bacteria lead to fascinating phenomena such as the deformation of LCLCs with a characteristic wavelength determined by the elasticity of the LCLCs and the activity of the bacteria, orientationally controlled trajectories of bacteria and visualization of 24 nm flagella motion.

The physical principles of swimming and flying in animals are intriguingly different from those of ships and airplanes. The study of animal locomotion therefore holds a special place not only at the frontiers of pure fluid dynamics research, but also in the applied field of biomimetics, which aims to emulate salient aspects of the performance and function of living organisms. For example, fluid dynamic loads are so significant for swimming fish that they are expected to have developed efficient flow control procedures through the evolutionary process of adaptation by natural selection, which might in turn be applied to the design of robotic swimmers. And yet, sharply contrasting views as to the energetic efficiency of oscillatory propulsion – especially for marine animals – demand a careful assessment of the forces and energy expended at realistic Reynolds numbers. For this and many other research questions, an experimental approach is often the most appropriate methodology. This holds as much for flying animals as it does for swimming ones, and similar experimental challenges apply – studying tethered as opposed to free locomotion, or studying the flow around robotic models as opposed to real animals. This book provides a wide-ranging snapshot of the state-of-the-art in experimental research on the physics of swimming and flying animals. The resulting picture reflects not only upon the questions that are of interest in current pure and applied research, but also upon the experimental techniques that are available to answer them.

This volume developed from a Workshop on Natural Locomotion in Fluids and on Surfaces: Swimming, Flying, and Sliding which was held at the Institute for Mathematics and its Applications (IMA) at the University of Minnesota, from June 1-5, 2010. The subject matter ranged widely from observational data to theoretical mechanics, and reflected the broad scope of the workshop. In both the prepared presentations and in the informal discussions, the workshop engaged exchanges across disciplines and invited a lively interaction between modelers and observers. The articles in this volume were invited and fully refereed. They provide a representative if necessarily incomplete account of the field of natural locomotion during a period of rapid growth and expansion. The papers presented at the workshop, and the contributions to the present volume, can be roughly divided into those pertaining to swimming on the scale of marine organisms, swimming of microorganisms at low Reynolds numbers, animal flight, and sliding and other related examples of locomotion.

A pedagogical review of the mathematical modelling in fluid dynamics necessary to understand the motility of most microorganisms on Earth.

Recently, there has been an increasing focus on various biological and physical systems known as "active matter". Examples of such systems range from individual units, such as motile cells or artificial self-propelled particles, to large systems of interacting active particles or individuals. The emergence of large-scale collective motion, as exhibited by flocks of birds or bacterial colonies, is just one prominent and fascinating example of self-organization in active matter systems. In this work, we discuss different individual-based models of active matter using the concept of active Brownian motion. The first part of this work explores the dynamical behavior of single active particles with a particular emphasis on the impact of so-called active fluctuations. The second part extends the scope of this study to interacting active Brownian particles and their collective behavior. First, a systematic derivation of kinetic equations for active Brownian particles with velocity alignment is presented. Further on, motivated by recent biological observations, a new type of "escape-pursuit" model of collective motion is introduced and successfully employed in modeling collective locust behavior.

Covers several research fields dealing with transport. This work covers three main topics including road traffic, granular matter, and biological transport. It considers different points of views including modelling, simulations, experiments, and phenomenological observations.

This book is a systematic introduction to a new and exciting field of patterns in granular matter. Granular materials are collections of discrete macroscopic solid grains with a typical size large enough that thermal fluctuations are negligible. Despite this seeming simplicity, properties of granular materials are different from conventional solids, liquids and gases due to the dissipative and highly nonlinear nature of forces among grains. The last decade has seen an explosion of interest to nonequilibrium phenomena in granular matter among physicists, both on the experimental and theoretical side. Among these phenomena, one of the most interesting is the ability of granular matter upon mechanical excitation to form highly ordered patterns such as ripples, avalanches, or bands of segregated materials. This book presents a comprehensive review of experiments and novel theoretical concepts needed to understand the mechanisms of pattern formation in granular materials. This book is written for experienced physicists interested in this new rapidly developing field, as well as young researchers and graduate students entering this field. We hope that both experimentalists and theorists already working in the field will find it useful.

This book presents a comprehensive review of various aspects of the novel and rapidly developing field of active matter, which encompasses a wide variety of self-organized self-driven energy-consuming media or agents. Most naturally occurring examples are of biological origin, spanning all scales from intracellular structures to swimming and crawling cells and microorganisms, to living tissues, bacterial colonies and flocks of birds. But the field also encompasses artificial systems, from colloids to soft robots. Intrinsically out of equilibrium and free of constraints of time-reversal symmetry, such systems display a range of surprising and unusual behaviors.​ In this book, the author emphasizes connections between fluid-mechanical, material, biological and technological aspects of active matter. He employs a minimum of mathematical tools, ensuring that the presentation is accessible to a wider scientific community. Richly illustrated, it gives the reader a clear picture of this fascinating field, its diverse phenomena and its open questions.