Particle characterization by AC-electrokinetic phenomena: 1. A short introduction to dielectrophoresis (DP) and electrorotation (ER)

When the response of a particle suspension to an external electric field is analyzed, measurement of the frequency dependence widens the parameter spa ce and enables access to the inherent electric properties of the particles or, for known electric properties, to layer thicknesses, etc. The common im pedance method registers the direct electric response to an applied field, whereas dielectrophoresis (DP) and electrorotation (ER) analyze the electro kinetic response of single particles. Since all responses are based on the same principles of interfacial polarization they can generally be described by a unified theory. This is demonstrated by a simple resistor-capacitor m odel describing the interdependence of the three methods. While the methods generally yield the same information, the particle properties are more cle arly reflected in DP and ER spectra. The reason is that, in contrast to imp edance, the latter methods are differential; i.e., they are based on the po larizability difference between the particles and the medium. Recently, a m ain disadvantage of the single particle methods, namely the limited externa l conductivity and frequency, was overcome by ultra-microelectrode chambers . Another drawback was the requirement of tedious microscopic measurements on single particles. Light scattering methods are proposed to improve the m easuring procedure and statistical significance. The new methods, DR/phase analysis light scattering and ER/light scattering, are presented in detail in two additional manuscripts. (C) 1999 Elsevier Science B.V. All rights re served.