Nonequilibrium thermodynamics

Nonequilibrium thermodynamics studies systems with macroscopic gradients or flows, where irreversible processes transport matter, energy, or momentum and generate entropy.

Entropy production: The rate at which irreversible processes generate entropy within a system. It quantifies the thermodynamic cost of transport and other irreversible changes.
Thermodynamic equilibrium: A state in which macroscopic variables no longer change and no net thermodynamic driving gradients remain. Equilibrium supplies the reference state from which nonequilibrium departures are measured.
Heat conduction: The transfer of thermal energy through matter driven by temperature differences. Fourier's law is a canonical flux-force relation in irreversible thermodynamics.
Ludwig Boltzmann: A nineteenth-century physicist who connected thermodynamic entropy with statistical properties of matter. His statistical account of entropy helped frame the microscopic origins of irreversibility.
Fluctuation theorem: A relation describing the probabilities of positive and negative entropy production in fluctuating systems. It quantifies irreversibility beyond average near-equilibrium behavior.
Onsager reciprocal relations: Relations equating paired cross-coefficients that couple thermodynamic fluxes to driving forces near equilibrium. They predict reciprocal coupling between processes such as heat and electric-current transport.
Local thermodynamic equilibrium: The approximation that small regions of a nonequilibrium system have well-defined equilibrium state variables. It allows temperature and chemical potential to be assigned within slowly varying systems.
Diffusion: The net movement of particles from regions of higher concentration toward lower concentration. Chemical-potential gradients describe the driving force behind diffusive transport.
Lars Onsager: A physical chemist whose reciprocity relations established key results in near-equilibrium irreversible thermodynamics. His work gave a general symmetry principle for coupled transport.
Dissipative structure: An organized pattern maintained in a system by flows of energy or matter and continual dissipation. Such structures raise questions about order emerging far from equilibrium.

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