Electrokinetic effects on the dielectric response of colloidal particles: Dielectrophoresis and electrorotation

Electrokinetic effects on a.c. field dielectrophoretic and electrotational phenomena have been examined. Asymptotic methods have been used to carry out computations on the net electric force and torque acting on a spherical colloidal particle and its attending electric double layer. The dielectrophoretic force and the electrorotational torque are shown to depend on the real and imaginary parts of the (complex) particle dipole coefficient respectively. Moreover, it is demonstrated that the dipole coefficient that appears in the force and torque expressions is equivalent to that manifested by the particle in a uniform a.c. field. Thus the force and torque calculations reduce to a problem that has been solved previously in connection with dielectric dispersions in colloidal suspensions. The results for aqueous KCl solutions indicate that at modest ζ potentials (around 25 mV), the dielectrophoretic and electrorotational spectra are influenced strongly by ε{lunate}_Pε{lunate}, the ratio of the particle and solution dielectric constants. At higher ζ potentials (100 mV), the spectra of particles with very thin double layers are affected by Stern layer ion transport below forcing frequencies of 1 MHz.