Dynamics of Active Matter

The past decade has witnessed an explosion of interest in active matter, i.e., materials systems whose constituent particles consume energy and generate mechanical motion or work. Paradigmatic examples of active particles include catalytically active Janus colloids, which consume molecular “fuel” in order to swim through solution; flagellated and ciliated biological microswimmers, such as e. coli and opalina; and the nanoscale machinery of the cell, such as the kinesin motor protein. The intrinisically nonequilibrium character of active systems can lead to novel emergent structures and dynamics. For instance, individual active particles can bind to, and move along, confining solid surfaces through hydrodynamic interactions; two or more active colloids can self-organize into “active molecules” through non-reciprocal interactions; and collections of many particles can form dynamic “living clusters” or exhibit mesoscale turbulence.

This initial decade of work established some basic conceptual frameworks for understanding active matter. For instance, the hydrodynamic or continuum approach to colloidal phoresis has been successfully adapted to modeling the motion of catalytically active Janus colloids in unbounded and confined solutions. Motility-induced phase separation has been systematically characterized in simulations of active Brownian particles. Additionally, hydrodynamic and kinetic theories that treat an active suspension as a fluid have captured aspects of swarming, flocking, and mesoscale turbulence. Current research is seeking, inter alia, to extend these core approaches and results to more complex and multiphysical scenarios. Accordingly, potential topics for this Special Issue include:

• Tactic response of active particles to ambient fields (e.g., chemical, hydrodynamic, optical, and acoustic fields). Recent work has explored how the microscopic properties of a Janus colloid (e.g., shape and surface chemistry) are related to macroscopic transport coefficients (e.g., drift velocity and diffusivity) in ambient fields. Similarly, the relationhip between the microscopic run-and-tumble dynamics of micro-organisms and population-level chemotactic and rheotactic behavior has acquired new importance in light of recent particle tracking experiments.
• Rheology of active fluids. Experiments and theoretical modeling have revealed novel rheological phenomena in suspensions of active particles, including superfluidity and spontaneous macroscopic flow. Potential directions for further research include collective phenomena (i.e., going beyond the dilute limit) and the incorporation of more complex interactions into microscopic dynamics (i.e., going beyond point-particle and squirmer models.)
• Pattern formation, phase separation, and turbulence arising from the interplay of motility and activity-induced interactions (e.g., chemical signaling and hydrodynamic interactions). The general significance of hydrodynamic interactions for phase separation is still a hotly debated question.
• Self-organization of dissipative materials and active machines.

• Swimming in complex media, including porous media and viscoelastic and anisotropic fluids.

• Statistical mechanics of active matter, including entropy production, fluctuation theorems, and violations of detailed balance.

Prof. William E. Uspal
Guest Editor

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