  In charged colloidal suspensions, the competition between square-well
attraction and long-range Yukawa repulsion leads to various stable domains and
Wigner supercrystals. Using a continuum model and symmetry arguments, a phase
diagram of spheres, cylinders, and lamellae is obtained as a function of two
control parameters, the volume fraction and the ratio between the surface
tension and repulsion. Above a critical value of the ratio, the microphase
cannot be supported by the Yukawa repulsion and macroscopic phase separation
occurs. This finding quantitatively explains the lack of pattern formation in
simple liquids because of the small hard sphere diameter in comparison with the
size of macromolecules. The phase diagram also predicts microphase separation
at zero value of the ratio, suggesting the possibility of self-assembly in
repulsive systems.
