Structures possessing spatial asymmetry should act as pumps in the presence of dissipation alone, without the need for macroscopic forces or temperature differences to drive vectorial motion. It has been shown theoretically that particles subjected to an asymmetric periodic potential can display net directional motion even if the space-averaged force is zero. Here we demonstrate such behaviour experimentally. We have studied the behaviour of colloidal particles suspended in solution and exposed to a sawtooth dielectric potential which is turned on and off periodically. The particles exhibit net motion with a velocity that depends on their size, suggesting applications in separation processes for objects in the size range 0.1-5 microns--a range that includes biological structures such as viruses, cells and chromosomes. We furthermore point out the analogy between our device and motor protein assemblies.

Interdigitated microelectrodes have been used to investigate the passive dielectrophoretic levitation of latex beads as a function of the frequency and voltage of the applied electrical signal, the suspending medium conductivity, bead size and characteristic dimensions of the electrodes. The variations of the electric field strength E and of the factor ∇E 2 as functions of the height above the electrode plane were determined by computer-aided modelling. As predicted from a simple theory based on the balancing of the gravitational settling force with the negative dielectrophoretic force, the height of levitation was found to be independent of the bead size and at 1 MHz, at which the levitation was at its largest, to be only weakly dependent on the conductivity of the suspending medium. The frequency dependence of the levitation height was found to be in close agreement with theory, based on the known dielectric properties of the beads and on previously determined electrode polarization effects. Apart from using this method to investigate the dielectric properties of particles, a particularly important application is envisaged to be that of particle separation, based on differences in dielectric properties, using dielectrophoretic levitation in combination with field-flow fractionation techniques.

Dielectrophoresis (DEP) -the movement of particles in nonuniform electric fields -can be used in combination with Field Flow Fractionation (FFF) to separate particles with differing dielectric properties. An introduction is given to the technique of DEP-FFF and its application to the separation of cells and other particles. The separation of yeast cells using the subtechniques of steric and hyperlayer DEP-FFF is demonstrated. It is shown that the hyperlayer-DEP-FFF techniques have a number of advantages, including an improved separation efficiency and reduced adhesion to chamber walls. The hyperlayer-DEP-FFF separation technique is also independent of particle size and allows the use of higher medium conductivities than for conventional DEP methods. 2857

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