An optical dielectrophoresis spectrometer for low-frequency measurements on colloidal suspensions

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Abstract
An optical system is described for measuring the response of suspended particles to imposed non-uniform electric fields in the frequency range 1 Hz to 4 MHz. Such dielectrophoretic measurements can provide details of the dielectric and surface charge properties of animate and inanimate particles. This simple, low-cost and rapid technique extends these measurements down to lower frequencies than previously reported, and has revealed new effects associated with particle surface charge and surface conductivity. At the lower frequencies corrections are required to take account of the electrode polarisation effects, which modify the electric field distribution in the colloidal suspension between the electrodes. Measurements are reported for silicon powder, yeast and the bacteria Micrococcus lysodeikticus and are compared with theoretical expectations. Biotechnological applications include cell separation, cell characterisation, cell-culture quality control and biomass determination.