Dielectrophoretic Mobility: An Approach for Measuring Electrical Properties of a Single Microbead in Aqueous Medium

An approach to measure the permittivity and conductivity of a small neutral particle in an aqueous conductive medium by utilizing dielectrophoretic mobility is presented. Particle mobility is caused by the dielectrophoretic force set off by the particle polarization in a linearly decreasing electric field. Polystyrene carboxylate microbeads of various sizes are used as particles, conductive potassium chloride (KCl) dilution is used as their carrier fluid, and a transparent hyperbolic quadrupole electrode setup is used to produce the linear field gradient. The setup and method are characterized by determining the permittivity and the conductivity of the microbeads in three different KCl concentrations of differing conductivities. The effects of carrier fluid conductivity and particle size on the acquired permittivity and conductivity values are examined. According to the results, low particle concentration combined with practical carrier conductivity (≫ 1 mS/m) provides repeatable permittivity values for the microbeads, irrespective of their size as far as the field strength is high enough to overcome the uncertainty caused by the Brownian motion. Microbead conductivities show strong dependence on the carrier fluid conductivity, thus providing a potential method for monitoring the relative conductivity variations of the carrier fluid.