Positive Dielectrophoretic Mobilities of Single Microparticles Enhanced by the Dynamic Diffusion Cloud of Ions

Dielectrophoretic behavior of single polystyrene-carboxylate microparticles and polystyrene microparticles was studied with a quadrupole microelectrode. For positive dielectrophoresis (DEP) of polystyrene-carboxylate particle observed in the lower frequency region (≤10 kHz), a DEP mobility coefficient ( α ) was almost proportional to the square of ac voltage ( U rms, root mean square), as was expected from standard theories of DEP. The α values did not depend on the particle radius ( r e ), but depended on the kind of electrolyte in the order HCl ≫ KCl ≈ KOH ≥ tetrabutylammonium chloride, showing a specific enhancement by H + . Since this observation was against the standard theories, we introduced a DEP radius ( r DEP ) of a Debye-type function instead of the particle radius to fit the observed α values in the lower frequency region. The positive DEP behaviors were reproduced by a function of r DEP and the surface conductivity of the negatively charged particles. As for negative DEP observed in the higher frequency region (≥56 kHz), proportional relationships between α and U rms 2 and ones between α and r e 2 were obtained as predicted by the standard theories. The obtained r DEP values increased with a decrease in the ac frequency, and the r DEP value in the HCl system was larger than those for the other systems. These suggested that the r DEP was controlled by the moving distance of cations swung by the applied ac electric field in the dynamic diffusion cloud around the particles.