Nonequilibrium systems, as physical systems, are characterized by the absence of thermodynamic equilibrium, typically involving persistent fluxes of matter, energy, or momentum, and non-vanishing thermodynamic forces such as gradients of temperature, chemical potential, or velocity. Their macroscopic behavior cannot be fully described by equilibrium statistical mechanics, requiring frameworks like nonequilibrium thermodynamics, kinetic theory, and stochastic processes. These systems may exhibit time-dependent evolution, dissipation, entropy production, and emergent structures (e.g., patterns or self-organization) sustained by external driving. Nonequilibrium conditions are fundamental in transport phenomena, reaction–diffusion processes, driven granular media, active matter, and many open systems exchanging energy or particles with their environment.
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