Comparison of the effective dipole method for the computation of the torque with the Maxwell's stress tensor method in non-lossy systems

Abstract

In all experiments that involve electrorotation the motion of the cells is a direct result of the torque that is exerted upon them by the field. As a consequence of this the theoretical study of the origin of the torque has been of some interest. Two methods of calculating the torque have evolved: (1) an effective dipole moment induced on the cell by the external electromagnetic field is first computed and then the torque exerted on this by the latter is calculated by using standard formulae from classical mechanics; and (2) the force per unit area exerted on the surface of the cell by the electromagnetic field is calculated by using Maxwell's stress tensor. and from this the torque is obtained by taking a vector cross product with the radius vector followed by an integration over the entire surface of the cell. In this study. the torque experienced by an ellipsoidal cell in a rotating frequency-independent electrical field is calculated by using the Maxwell stress tensor approach, and the limits under which this result yields the torque, commonly computed from an effective dipole moment method, are examined. The applied electrical field is taken to be independent of the frequency in order to ensure that complications due to energy dissipation are eliminated and a simple comparison of the two methods is made possible. >