Dielectrophoretic microparticle characterization

Abstract

Dielectrophoretic (DEP) force is a result of a non-uniform electric field and the relative polarizability of a neutral particle and the fluid in which it is suspended. Measuring DEP force provides information concerning the electrical properties of the particle and thus provides one way of identifying or distinguishing one particle from another. In this study, a microfluidic DEP platform with hyperbolic quadrupole electrode geometry was implemented for particle characterization purposes. The platform was used to measure the conductivity and permittivity of polystyrene microparticles in a carrier fluid. A useful feature of the hyperpolic electrode geometry is the linearity of the electric field gradient that it produces. The linear field gradient provides a straightforward way to measure the DEP force, and consequently the electrical properties of the particle, simply by measuring the particle movement within the field. According to the simulations good linearity is achieved within the full circular area between the electrode tips of the geometry. Besides DEP force a particle may undergo many other forces during such an experiment and thus may move not only laterally between the electrodes but also wander above the electrodes. Therefore unlike previous studies the electrodes of the implemented platform were made of indium-tin-oxide (ITO) to achieve full transparency and in consequence better view of the particle motion when using common transluminescence microscopy. Electrode transparency revealed that particles have motion also in the depth direction, especially above the electrodes, and that accurate mobility measurements may require particle observation in three dimensions. The electrical properties of polystyrene microparticles were determined by measuring their mobility in a linearly increasing electric field produced by the hyperbolic ITO electrode geometry with an active region of 65 μm in radius. The experiments were done using a transluminescence video microscope and 2 μm polystyrene particles in 0.1 mM KCL dilution of 1.42 mS/m conductivity. The mobility of the particle was determined as an average of the particle's lateral displacements in consecutive video frames. Based on their mobility the polystyrene particles showed a conductivity of 3.3 mS/m and permittivity of 54 0 e 0 within the frequency range of 0.1-15 MHz.