Observation of a phase transition in the sedimentation velocity of hard spheres

Reduced sedimentation velocities are reported for suspensions of nearly hard PMMA (polymethylmethacrylate) spheres as a function of volume fraction. The absolute sedimentation velocities are sufficiently slow compared to crystal-growth rates so that phase separation is achieved during the sedimentation process. As a result the analytic behavior of the measured sedimentation velocity changes as a function of volume fraction at the freezing and melting points. This transition serves as a definitive marker for comparison with theoretical predictions of sedimentation velocities for hard-sphere liquids and crystals.