The dielectrophoretic levitation of latex beads, with reference to field-flow fractionation

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.