Active matter

Active matter consists of units that consume energy to generate persistent motion or forces. Their energy input drives collective behaviors that do not occur in systems at thermal equilibrium.

Self-propelled particle: A particle that converts energy into persistent motion through its own propulsion. It is the basic model for an active unit moving through a surrounding medium.
Statistical mechanics: The framework connecting microscopic states to macroscopic properties through probability and ensembles. Active matter borrows its statistical tools while often violating their equilibrium assumptions.
Bacterial turbulence: Chaotic-looking flow patterns generated by dense suspensions of swimming bacteria. It is a vivid experimental example of collective motion driven by living active units.
Equilibrium thermodynamics: The theory of systems whose macroscopic properties are determined by equilibrium states. Active matter continually dissipates energy and cannot generally be described by equilibrium state functions.
Active Brownian particle: A self-propelled particle whose direction changes through rotational diffusion. It captures how propulsion and random reorientation combine in a minimal active-matter model.
Nonequilibrium thermodynamics: The study of energy, entropy, and transport in systems away from thermodynamic equilibrium. Continuous energy consumption keeps active systems out of equilibrium.
Cytoskeleton: A dynamic network of protein filaments that gives cells structure and enables movement and transport. Motor proteins and filaments form biological active materials inside cells.
Passive matter: Matter whose components do not continuously consume energy to generate motion or force. It provides the direct comparison for identifying effects caused by activity.
Vicsek model: A model in which self-propelled particles align their directions with nearby neighbors amid noise. Local alignment in this model produces collective flocking from simple active rules.
Brownian motion: The irregular motion of particles caused by repeated collisions with surrounding molecules. Thermal fluctuations compete with the persistent propulsion characteristic of active units.

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