  The effect of a constant applied external force, induced for instance by an
electric or gravitational field, on the dispersion of Brownian particles in
periodic media with spatially varying diffusivity, and thus mobility, is
studied. We show that external forces can greatly enhance dispersion in the
direction of the applied force and also modify, to a lesser extent and in some
cases non-monotonically, dispersion perpendicular to the applied force. Our
results thus open up the intriguing possibility of modulating the dispersive
properties of heterogeneous media by using externally applied force fields.
These results are obtained via a Kubo formula which can be applied to any
periodic advection diffusion system in any spatial dimension.
