Active fluids are a fascinating class of materials composed of self-propelled units, such as living cells, synthetic micro-swimmers, or motor proteins, which consume free energy locally to produce mechanical work. Unlike passive fluids, active fluids are inherently nonequilibrium owing to the continuous free energy input at the microscopic level. This nonequilibrium nature leads to unique flow behaviors and emergent phenomena, such as spontaneous formation/transition of flow patterns, large-scale collective motion, and unusual rheological properties. The presence of internal active stresses also introduces novel concepts and challenges, such as understanding how active stresses couple with fluid flow and how instabilities and turbulence arise in active fluid systems. Moreover, the study of active fluids intersects with living matter and soft matter physics. Understanding the nonequilibrium mechanics of active fluids is an important step towards elucidating the physics of living matter. The advance will inspire new directions for soft matter physics and revolutionize materials science, enabling the design of life-like autonomous materials with the ability of environmental sensing and mechanical adaptability.

In recent decades, extensive efforts have been devoted to developing theoretical frameworks, experimental techniques and numerical models for understanding the nonequilibrium mechanics of active fluids. Most of the efforts were built upon the framework of "active matter theory" that describes the statistical mechanics of self-propelled agents interacting sterically and hydrodynamically. A plethora of emergent phenomena in active fluids have been successfully explained or predicted in the active matter framework, but there remain many challenging issues. The proposed Symposium aims to bring together researchers in fluid mechanics, statistical physics, biological physics, and materials science to discuss most recent development in the field of active fluids, including but not limited to the following topics: active turbulence, spatiotemporal self-organization, flow-structure interaction, topological excitation, odd viscoelasticity, nonreciprocity, and applications of machine learning. The Symposium will also discuss directions beyond the classical active matter framework, such as: How do mechanochemical coupling and bioenergetics control the behaviors of living active fluids? Can phase transitions in active fluids be understood with nonequilibrium thermodynamics? What is the design principle of active flow networks for desired mechanical functionalities? We envisage that the Symposium will boost interdisciplinary research and collaboration in the field of active fluids.