The 2019 Motile Active Matter Roadmap (1912.06710v1)

Abstract: Activity and autonomous motion are fundamental in living and engineering systems. This has stimulated the new field of active matter in recent years, which focuses on the physical aspects of propulsion mechanisms, and on motility-induced emergent collective behavior of a larger number of identical agents. The scale of agents ranges from nanomotors and microswimmers, to cells, fish, birds, and people. Inspired by biological microswimmers, various designs of autonomous synthetic nano- and micromachines have been proposed. Such machines provide the basis for multifunctional, highly responsive, intelligent (artificial) active materials, which exhibit emergent behavior and the ability to perform tasks in response to external stimuli. A major challenge for understanding and designing active matter is their inherent nonequilibrium nature due to persistent energy consumption, which invalidates equilibrium concepts such as free energy, detailed balance, and time-reversal symmetry. Unraveling, predicting, and controlling the behavior of active matter is a truly interdisciplinary endeavor at the interface of biology, chemistry, ecology, engineering, mathematics, and physics. The vast complexity of phenomena and mechanisms involved in the self-organization and dynamics of motile active matter comprises a major challenge. Hence, to advance, and eventually reach a comprehensive understanding, this important research area requires a concerted, synergetic approach of the various disciplines.

• The paper outlines a comprehensive framework for understanding motile active matter and its emergent collective behaviors.
• The paper employs theoretical models and computational methods, such as active Brownian particles and continuum theories, to analyze microswimmers.
• The paper highlights practical applications in drug delivery and material science while addressing challenges in controlling non-equilibrium systems.

Insights into Motile Active Matter: A Comprehensive Roadmap

The "2019. Motile Active Matter Roadmap," curated by a collaboration of scientists from diverse fields, provides a detailed exposition on the current state and future directions in the paper of active matter. This paper is a foundational document for researchers aiming to understand the complex phenomena associated with motile and dynamic systems ranging from microscale biological entities to engineered artificial materials.

The document systematically covers various dimensions of active matter, starting with the foundational principles that govern these systems. Active matter systems are characterized by their inherent non-equilibrium nature due to continuous energy consumption, which challenges conventional equilibrium concepts such as time-reversal symmetry and detailed balance.

Theoretical and Computational Approaches

The paper explores the theoretical foundations and computational methods essential for the analysis of active systems. Models like active Brownian particles and squirmers are discussed, which are pivotal in understanding the fundamental physical mechanisms across dry and wet active matter. These models are complemented by continuum theoretical approaches and dynamic density-functional theories, offering a robust framework for simulating the emergent collective behaviors observed in active systems.

Moreover, the document emphasizes the critical role of finite-element and discrete-element methods to simulate complex boundary conditions and interactions crucial for predicting the dynamics of active colloids and viscoelastic swimmer interactions.

Biological and Synthetic Microswimmers

Biological microswimmers such as sperm, bacteria, and algae inspire the development of synthetic nano- and micromachines. These machines are designed to mimic biological processes and provide a basis for creating highly responsive and multifunctional artificial materials.

The Roadmap discusses the propulsion mechanisms, ranging from cilia-beating and flagella rotation to more novel bio-inspired designs. It highlights the challenges in enhancing control over these systems, especially regarding their navigation strategies driven by chemotaxis, phototaxis, and recently expanded applications like swarm behaviors.

Collective Behaviors and Self-Organization

A haLLMark feature of this research field is the focus on collective behaviors emerging from simple interaction rules among particles in active systems. The paper systematically reviews phenomena such as motility-induced phase separation (MIPS), active turbulence, and the formation of complex patterns in different environmental conditions.

A striking insight is provided into how systems like flocks of birds, schools of fish, and bacterial colonies utilize simple local interactions to generate complex global patterns. Modeling such behaviors involves understanding the interplay between hydrodynamic interactions, chemical gradients, and particle self-propulsion, which collectively dictate the system's macroscopic outcomes.

Practical Implications and Future Directions

The implications of motile active matter research extend far beyond theoretical interest. Practically, they harbor potential in revolutionizing fields like drug delivery systems through micromachines, the development of novel materials with programmable properties, and enhancements in microfluidic devices.

Significant challenges remain in adequately describing and controlling these systems, especially regarding distribution and propulsion in complex environments like viscoelastic and non-Newtonian fluids or under varied external fields. Moreover, the integration of advanced machine learning techniques with experimental data promises to accelerate the development of predictive models and control algorithms for active systems.

Conclusion

The Roadmap represents a critical step toward a comprehensive understanding of active matter. It eloquently addresses the intricate dynamics emerging from microscale interactions and paves the way for novel applications across numerous fields. By fostering interdisciplinary collaboration, as evidenced by the diverse authorship, the paper sets a solid foundation for future research aimed at harnessing the potential of motile active systems in scientific and engineering domains.