Flow and structure in nonequilibrium Brownian many-body systems

D. de las Heras, and M. Schmidt
Phys. Rev. Lett., 125, 018001, (2020)     DOI: 10.1103/PhysRevLett.125.018001
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We present a fundamental classification of forces relevant in nonequilibrium structure formation under collective flow in Brownian many-body systems. The internal one-body force field is systematically split into contributions relevant for the spatial structure and for the coupled motion. We demonstrate that both contributions can be obtained straightforwardly in computer simulations, and present a power functional theory that describes all types of forces quantitatively. Our conclusions and methods are relevant for flow in inertial systems, such as molecular liquids and granular media.

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Flow and structural forces

Understanding and predicting nonequilibrium phenomena is a challenge in Statistical Physics, relevant in several research fields from quantum-mechanical to biophysical systems. We provide fundamental steps forward in that direction for systems undergoing Brownian dynamics, such as e.g. colloidal and active systems. Four different types of force fields determine the time evolution of a Brownian system: (i) the diffusive term due to the random Brownian motion, (ii) the external force, (iii) the friction force due to the solvent-particle interaction, and (iv) the internal force field due to the interparticle interactions. The internal force field is the only nontrivial contribution that needs to be approximated in any theoretical treatment. Here we rationalize and classify the internal force field and identify four fundamentally different types of internal forces that naturally split into forces that act on the flow and forces that act on the structure of the system. We provide a method to obtain all forces in computer simulations and also construct a theory that reproduces the observed behaviour. The internal force field plays also a major role in other dynamical systems. The implications of our work go therefore beyond Brownian dynamics and are relevant to e.g. molecular and granular systems.

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