On the Orientation of Nonspherical Particles in an Alternating Electrical Field

Abstract

It is shown in general that any nonspherical particle imbedded in a medium with a different complex dielectric constant will experience orienting forces from an impressed alternating electrical field. The direction of stable orientation is determined by the geometry of the particle, by the dielectric properties of the material involved, and also by the frequency of the field. A continuous change of frequency may result in a continuous rotation of the particle as well as in a sudden ``turnover'' covering an angle of 90 deg. The case of an ellipsoid suspended in an isotropic medium is discussed in more detail. If the complex dielectric constant of the ellipsoid can be represented by a tensor with principal axes coinciding with the geometrical axes, the direction of stable orientation always is given by one of these axes. However, the choice of ``stable axis'' may be frequency dependent, so that turnover effects may occur as the frequency is changed. Such behavior is possible even if the dielectric properties of the ellipsoid are isotropic. Similar effects are displayed by a system approximating the passive electrical properties of biological cells in an electrolyte solution, thus accounting for some unusual experimental observations of this kind.