  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 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.
