VISCOELASTIC EFFECTS ON THE DYNAMIC SUSCEPTIBILITY OF A BROWNIAN PARTICLE IN AN EXTERNAL POTENTIAL

The relaxation behavior of an assembly of noninteracting dipolar molec ules acted on by an alternating electric field and moving in a viscoel astic liquid is studied by solving the corresponding Fokker-Planck-Kra mers equation. Both inertial and viscoelastic effects an taken into ac count leading to an infinite hierarchy of differential-recurrence rela tions where the appropriate relaxation functions are characterized by three indices. An exact expression for the complex dielectric suscepti bility is found by using a matrix formulation. Dispersion and absorpti on spectra together with Cole-Cole diagrams are then plotted exhibitin g a resonant structure in the FIR region due to the forcing regime imp ressed by the electric field. From these curves, it is shown that iner tia and elasticity are strongly coupled. This coupling effect becomes much more important as the elastic relaxation time characterizing the interactions of the molecules with the thermal bath becomes greater th an the Debye relaxation time.