We have reported a large family of previously unrecognized
self-organized colloidal patterns that form in constant vertical
electric fields at biases just above the decomposition voltage of
water. Our simple electrolysis cell gives rise to a complicated coupled
system of hydrodynamic flows and ionic fluxes
that are shepherded by the very colloidal particles they transport.
We have discussed the electrohydrodynamic forces on a single particle
in this system, and suggested how many-body coupling might give
rise to the various dynamical structures we have observed.
At low voltages, colloidal spheres in a specific range of charges,
densities and mobilities are found to
form various microscopic quasi-steady-state clusters through the competition of gravity and
electrohydrodynamic levitating forces.
These patterns falls into two categories: clusters levitated into the bulk
and clusters localized near the lower electrode.
Their structures and evolutions are different,
but all share some general features, such as persistent circulation
and coarsening with increasing voltage.
The simple behavior of a single particle in the field
demonstrates that these microscopic clusters are formed cooperatively.
Patterns' dependence on such control parameters as the composition
and number density of particles, cell thickness 
,
salt concentration and applied voltage range have been discussed.
This work was supported by the Donors of the Petroleum Research Fund of the American Chemical Society.