Preface

Active colloids represent a fascinating research frontier, where microscopic particles exhibit autonomous movement and self-organization. This unique behavior stems from their ability to convert various forms of energy into directed motion, leading to complex and dynamic systems that are far from equilibrium. The study of active colloids not only enhances our understanding of non-equilibrium phenomena, but also paves the way for groundbreaking applications in fields such as biomedicine, environmental science, and micro- and nanofabrication.

Although the term “active colloids” is most commonly used in the soft matter physics community, it is in fact a highly interdisciplinary topic. Researchers from diverse backgrounds – including physics, chemistry, materials science, biology, robotics, and engineering – are contributing to the rapidly evolving landscape of the field (and using different names such as micro- and nanomotors, colloidal motors, and microrobots). For example, materials scientists and engineers are intrigued by the possibility of designing new materials with tailored properties, either to enable new types of active colloids, or to build smart materials composed of active colloids. Biologists study active colloids to build minimal model systems for cellular processes and mimic biological systems, while roboticists are interested in harnessing their autonomous motion and collective behavior for medical and industrial applications. Soft matter physicists see a rich playground to study complex behaviors and emergent phenomena in non-equilibrium systems, while chemists are excited about a zoo of interesting processes that relate molecular behavior to macroscopic materials.

It is this multi- and interdisciplinarity that motivated this book. Despite the significant advancements and growing interest in this field, there is currently no comprehensive book available that consolidates this extensive knowledge. Our goal is to provide both an introduction for newcomers and a detailed reference for seasoned researchers, by bringing together materials and perspectives from various backgrounds into a single, cohesive resource. Typically, researchers must sift through scattered research papers and different books across multiple disciplines to gather the necessary information. Our book streamlines this process, making it a valuable reference for anyone involved in the study or application of active colloids, including students, academics, and industry professionals. This book can therefore be used as a textbook for classroom teaching, or simply as a general-purpose reference book.

The structure of Active Colloids: From Fundamentals to Frontiers is intended to provide a comprehensive and coherent exploration of active colloid research. The book is divided into four main sections, each designed to progressively build the reader’s understanding, from basic principles to advanced research frontiers. This organization ensures that readers, regardless of their prior knowledge, can follow a logical progression through the diverse and interdisciplinary aspects of active colloids.

The first section, “The Basics,” establishes the foundational knowledge necessary for understanding active colloids. It begins with the chapter “Introduction to Colloids”, authored by Sharma, Xiao, and Simmchen, that covers essential concepts in colloidal science, such as interparticle interactions, Brownian motion, diffusion, sedimentation, and electrokinetics. The second chapter, “Quantitative Microscopy of Active Colloids” by Cerbino and Giavazzi, introduces the key microscopy techniques as well as the quantitative and statistical analysis methods on the obtained optical images that are relevant to active colloids. The last chapter in this section, “Introduction to Active Colloids” by Sapre, Sen, and Ghosh, builds upon the first two chapters and takes the reader further, providing an overview of the entire research field of active colloids, including their types, propulsion mechanisms, collective behaviors, and key research frontiers.

Building upon the fundamentals presented in the first section, the second section of the book – “Various Types of Active Colloids”–introduces the different types of active colloids powered by a variety of mechanisms, including those powered by chemical reactions (by Chen et al.), magnetic fields (by Tierno), light (by Xu, Guan, and Guan), and local demixing (by Schmidt). This section also includes one chapter on self-propelled droplets (by Maass, Michelin, and Zarzar) and one on swimming microorganisms (by Jing and Liu). Each chapter in this section describes the underlying physicochemical principle of self-propulsion, and explores the unique properties and behaviors of the respective active colloids. By presenting a diverse range of active colloids, this section highlights the versatility and broad applicability of these fascinating systems. It is also helpful for readers looking for particular types of active colloids, synthetic or natural, for experiments in soft matter physics, materials, or bioengineering.

New experimental observations often require new theoretical insights and support from simulations. Since the early days of its development, the field of active colloids has witnessed and benefited from a strong collaboration between experimentalists, theoreticians, and simulation experts. Stimulating and rewarding as it is, however, this crosstalk between different disciplines requires basic knowledge of different areas of research, which often use different terminology and can be confusing. The third section of this book, “Theories and Modeling,” serves to mitigate this problem, by introducing the theoretical frameworks and modeling techniques that are essential for studying active colloids. Starting from foundational concepts in Stokes flow and self-phoresis (see the chapters from Montenegro-Johnson and Katsamba, as well as from Uspal), these chapters develop the mathematical description of these phenomena (e.g., Uspal on hydrodynamic singularities), and culminate in the introduction of advanced numerical methods (Montenegro-Johnson and Katsamba on the boundary element method) and simulation methods (the chapter from Qi, Hu, and Yang). These advanced methods are shown to be rooted in conceptual foundations. At the same time, practical considerations for performing simulations and numerical calculations are covered in detail. The reader will encounter coverage of such topics as low Reynolds number hydrodynamics, the modeling of collective behaviors, and common simulation methods, including Brownian dynamics, Stokesian dynamics, the lattice Boltzmann method, and multi-particle collision dynamics. In addition to introducing basic concepts and methodologies, the three chapters in this section serve two important goals: to promote understanding and facilitate communication between experimentalists and theorists, and to provide new perspectives on the common underlying physical mechanisms of experimental systems.

The fourth and final section, “Research Frontiers,” focuses on four cutting-edge research topics that continue to challenge and fuel the field of active colloids. Two chapters review the collective behaviors of active colloids, from experimental (by Cottin-Bizonne and Ybert) and theoretical/modeling (by Stark) perspectives, respectively. A second topic covered in this section, authored by Ketzetzi, Simmchen, and Isa, is about the individual and collective dynamics of active colloids in complex environments, such as near boundaries, in gradients, and in flows. The next chapter, authored by Sanchez and Katuri, introduces the applications of active colloids in self-assembly, materials design, sensing, therapeutics, and environmental remediation, and discusses which general strategies and approaches are best suited for different applications. Finally, the chapter by Volpe, Volpe, and Cichos explores the intersection of active matter and artificial intelligence (AI), an emerging and highly promising topic, by providing a brief overview of machine learning principles and their application in understanding and manipulating active matter.

All the authors contributing to this book are seasoned and well-renowned researchers from around the world, each an expert in their respective topic. They have provided comprehensive reviews written in an accessible and highly readable fashion, ensuring that complex concepts are understandable to a broad audience. Beyond covering fundamental knowledge and research frontiers, each chapter also presents illuminating and forward-looking research prospects and challenges. These insights are particularly valuable for newcomers to the field, offering them guidance and inspiration as they embark on their own research journeys in active colloids.

It is our sincere hope that by covering a wide range of topics and presenting insights from leading experts in the field, Active Colloids: From Fundamentals to Frontiers can be a comprehensive and accessible resource. Whether you are a student just beginning your journey in active matter research or an experienced scientist seeking the latest advances, this book will provide valuable knowledge and inspiration. This is not an advertisement – we, the editors, have personally learned a great deal from reading all these chapters, and we are sure that you will, too. Beyond personal growth, we hope that it will also encourage further exploration and collaboration in the vibrant and interdisciplinary field of active colloids.

We are indebted to the wonderful staff in the Royal Society of Chemistry Books Team, Merlin Fox, Amina Headley, Caroline Knapp, and those behind the scenes for trusting us with this book and making it happen. We are also very grateful to all the contributing authors who, in the midst of their busy teaching, research, and administrative work, have produced these informative, balanced, and scientifically sound book chapters.

Juliane Simmchen, William Uspal and Wei Wang