Lars Onsager was a Norwegian-born American physical chemist and a prominent figure in the field of statistical mechanics and thermodynamics. He is best known for his work on irreversible processes and for formulating the Onsager reciprocal relations, which describe how various thermodynamic fluxes and forces are related in systems that are not in equilibrium.
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Lars Onsager formulated reciprocal relations connecting cross-coupled transport coefficients near thermodynamic equilibrium. The Onsager reciprocal relations link microscopic reversibility with symmetry properties of macroscopic linear response, subject to the relevant time-reversal and field conventions.[ref]
Reciprocal Relations in Irreversible Processes. I. by Lars Onsager. author list and the described study’s abstract or introductory account