Combined AC-electrokinetic effects: Theoretical considerations on a three-axial ellipsoidal model

AC fields induce charges at the structural interfaces of particles or biological cells. The interaction of these charges with the field generates frequency-dependent forces that are the basis for AC-electrokinetic effects such as dielectrophoresis (DEP), electrorotation (ROT), electro-orientation, and electro-deformation. The effects can be used for the manipulation or dielectric single-particle spectroscopy. The observation of a particular effect depends on the spatial and temporal field distributions, as well as on the shape and the dielectric and viscoelastic properties of the object. Because the effects are not mutually independent, combined frequency spectra are obtained, for example, discontinuous DEP and ROT spectra with ranges separated by the reorientation of nonspherical objects in the linearly and circularly polarized DEP and ROT fields, respectively. As an example, the AC electrokinetic behavior of a three-axial ellipsoidal single-shell model with the geometry of chicken-red blood cells is considered. The geometric and electric problems were separated using the influential-radius approach. The obtained finite-element model can be electrically interpreted by an RC model leading to an expression for the Clausius-Mossotti factor, which permits the derivation of force, torque, and orientation spectra, as well as of equations for the critical frequencies and force plateaus in DEP and of the characteristic frequencies and peak heights in ROT. Expressions for the orientation in linearly and circularly polarized fields, as well as for the reorientation frequencies were also derived. The considerations suggested that the simultaneous registration of various AC-electrokinetic spectra is a step towards the dielectric fingerprinting of single objects.