Equivalent multipole-moment theory for dielectrophoresis and electrorotation in electromagnetic field

The equivalent multipole-moment theory is extended for the calculation of dielectrophoretic force (F) and electrorotational torque (T) exerted on a spherical particle under electromagnetic field. The field distortion produced by the particle is replaced by a set of current multipoles, and F and T on the particle are calculated as the summation of F and T on the multipoles. Three components are identified for F, namely, dielectrophoretic (DEP) force, travelling-field DEP (TWD) force, and the optical pressure which is non-existent in static case. It is found that DEP is related with the imaginary part of the scattering coefficient, while DEP, TWD, and T with the real part. DEP, TWD and T agree with those under electrostatic field at the limit of infinite wavelength. The mathematical expressions obtained are in terms of field derivatives, and can be applied for arbitrary field distribution in a straightforward manner.