Cylindrical phase separation in colloidal suspensions.

When left overnight undisturbed in a covered beaker, suspensions of polystyrene microspheres were found to undergo a distinctive kind of macroscopically visible phase separation. Microspheres migrated radially, leaving a vertically oriented cylinder near the center of the beaker that was devoid of microspheres. Cylinder formation was preceded by formation of a microsphere-free plate at the suspension surface, which may be the precursor of the cylinder. The cylindrical phase separation was found to depend on illumination, which suggests that low-level photon energy from the laboratory environment is sufficient to drive this surprising pattern formation. So long as suspension parameters were set within certain ranges, the cylindrical pattern occurred regularly.