  Diverse processes -- e.g., environmental pollution, groundwater remediation,
oil recovery, filtration, and drug delivery -- involve the transport of
colloidal particles in porous media. Using confocal microscopy, we directly
visualize this process in situ and thereby identify the fundamental mechanisms
by which particles are distributed throughout a medium. At high injection
pressures, hydrodynamic stresses cause particles to be continually deposited on
and eroded from the solid matrix -- notably, forcing them to be distributed
throughout the entire medium. By contrast, at low injection pressures, the
relative influence of erosion is suppressed, causing particles to localize near
the inlet of the medium. Unexpectedly, these macroscopic distribution behaviors
depend on imposed pressure in similar ways for particles of different charges,
although the pore-scale distribution of deposition is sensitive to particle
charge. These results reveal how the multiscale interactions between fluid,
particles, and the solid matrix control how colloids are distributed in a
porous medium.
