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The potential energy surfaces generated by microelectrodes of polynomial and interdigitated castellated geometry have been calculated for particles experiencing both positive and negative dielectrophoretic forces. The resulting forms of particle collection at these electrodes are governed by the locations of the potential energy wells, and the theoretical predictions of the modes of collection for particles experiencing positive and negative dielectrophoretic forces are verified using mixtures of viable and non-viable yeast cells, as well as of bacteria and blood cells. An important result for the interdigitated electrodes is the finding that particles trapped in potential energy wells under the action of negative dielectrophoresis can be more easily removed from the electrode structure (e.g. by fluid flow or gravitational forces) than those trapped under positive dielectrophoresis. This was verified for mixtures of bacteria and blood cells, viable and non-viable yeast cells.

The effective electrical conductivity values for a variety of Gram-negative and -positive bacteria have been determined in the frequency range 10 kHz to 100 kHz. This information enabled experimental conditions to be selected where micro-organisms of different species can be separated using dielectrophoresis -the movement of particles induced by non-uniform AC electric fields. Mixtures of micro-organisms of different species were separated locally on a microscope slide using micro-electrodes of polynomial geometry, and their physical isolation into two separate suspensions was accomplished using a dielectrophoresis chamber that incorporated interdigitated, castellated micro-electrodes.

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.

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