Structural Nonequilibrium Forces in Driven Colloidal Systems

N. C. X. Stuhlm├╝ller, T. Eckert, D. de las Heras, and M. Schmidt
Phys. Rev. Lett., 121, 098002, (2018)     DOI: 10.1103/PhysRevLett.121.098002
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We identify a structural one-body force field that sustains spatial inhomogeneities in nonequilibrium overdamped Brownian many-body systems. The structural force is perpendicular to the local flow direction, it is free of viscous dissipation, it is microscopically resolved in both space and time, and it can stabilize density gradients. From the time evolution in the exact (Smoluchowski) low-density limit, Brownian dynamics simulations, and a novel power functional approximation, we obtain a quantitative understanding of viscous and structural forces, including memory and shear migration.

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Applicable for a broad range of physical systems, we present and characterize a novel type of non-equilibrium force, which we refer to as the structural force. This contribution to the total microscopic force balance is perpendicular to the flow direction in the system and therefore it does not dissipate energy, even though it only occurs in out-of-equilibrium situations. The structural force is not of hydrodynamic nature, since it originates only from the interparticle interactions.
We demonstrate, using computer simulations, that the structural force is able to sustain density gradients. Therefore it constitutes a candidate for an essential mechanism of nonequilibrium structure formation. We anticipate that the structural force is the key to explaining many exciting and poorly understood out-of-equilibrium phenomena, including shear migration, shear banding, lane formation in opposite driven colloids, as well as phase separation in active colloidal systems.
Our results are also relevant for experimental work as the structural force is in principle accessible by means of e.g. confocal microscopy and laser tweezer setups.

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