  Applying an electric field to an aqueous colloidal dispersion establishes a
complex interplay of forces among the highly mobile simple ions, the more
highly charged but less mobile colloidal spheres, and the surrounding water.
This interplay can induce a wide variety of visually striking dynamical
instabilities, even when the applied field is constant. This Article reports on
the highly organized patterns that emerge when electrohydrodynamic forces
compete with gravity in thin layers of charge-stabilized colloidal spheres
subjected to low voltages between parallel plate electrodes. Depending on the
conditions, these spheres can form into levitating clusters with morphologies
ranging from tumbling clouds, to toroidal vortex rings, to writhing labyrinths.
