Active matter is composed of objects that convert energy from their surroundings into directed motion. Examples can be found at various length scales and involve biofilament-motor protein suspensions, swimming microorganisms, self-propelled colloids, drops, and bubbles, crawling eukaryotic cells, and flying birds, to name a few. These systems often operate in complex environments, such as soils, the ocean, or the human body, which are characterized by geometrical disorder, chemical fields, and (non-Newtonian) hydrodynamic flows. These environmental heterogeneities affect both their individual swim gait and transport behavior as well as collective phenomena, such as motility-induced phase separation and tissue formation. Major advances in experimental methods, ranging from the development of novel microscopy tools to the design of tailored microfluidic devices, together with continuously evolving data analysis methods make possible the study of active materials in controlled, complex s...
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