An Electric Bottle for Colloids

We discuss a method for controlling volume fraction using dielectrophoretic equilibrium. In an equilibrium colloidal suspension, the external chemical potential variation by a non-uniform electric field is balanced by a non-uniform concentration to produce a uniform total chemical potential. If the equation of state is known, the concentration can be determined for a given electric field. On the other hand, an unknown equation of state can be determined by measuring the concentration profile in a known electric field. Dipole-dipole interactions are expected to be important, but the emergence of such effects can be detected experimentally, in particular by the dependence on the magnitude of the electric field. Experiments are performed on one-micron polymethylmethacrylate (PMMA) hard spheres suspended in a density-matching solvent. A parallel plate capacitor is used to create a non-uniform electric field. Measurements using confocal microscopy agree well with calculations based only on the hard sphere equation of state and measured particle, solvent, and electrode properties.