Electric field and dielectrophoretic force on particles with a surface film

This article presents the analysis of the electric field and dielectrophoretic force on spherical particles with a surface film. In the analysis, we use the method of images with the fundamental solutions given for the various types of particles composed of a core and a surface film. The concept of the apparent conductivity is introduced to clarify the difference between the particle types. The analysis shows that the response of a particle with a surface film to an external field is unique and generally cannot be obtained by replacing the particle with a homogeneous particle. Using numerical examples, we compare the field and force characteristics between the particles that are of different types but exhibit an identical response to a uniform external field. The electric field and force are found smaller on the conductor-core particle but greater on the dielectric-core particle compared with the particle without any surface film. We have calculated the electric field and force by using the boundary element method in which a surface film is treated as a zero-thickness medium. The propriety of such treatment of a surface film depends not only on the film properties but also on the external field.