Experiments were conducted on concentrated suspensions of neutrally buoyant particles which exhibit negative dielectrophoresis. We found that, due to interparticle electrical interactions, such suspensions undergo a phase separation when subjected to a high-gradient ac field $(\sim \mathrm{k}\mathrm{V}/\mathrm{m}\mathrm{m})$ and form a propagating distinct front between the regions enriched with and depleted of particles. A generalization of our theory for the thermodynamics of the field-induced phase transitions in suspensions of polarized particles [Phys. Rev. E 52, 1669 (1995); 54, 5428 (1996); 60, 3015 (1999)] is proposed for the front propagation, and its predictions are shown to be consistent with the experiments even though the model contains no fitting parameters. The combination of field-induced dielectrophoresis and phase transition provides a method for strongly concentrating particles in prespecified regions of dielectrophoretic devices.

• Received 26 August 2003

DOI:https://doi.org/10.1103/PhysRevE.69.021402