DEFORMATION OF A VISCOELASTIC DROPLET IN AN ELECTRIC-FIELD .1. AQUEOUS CETYLTRIMETHYLAMMONIUM BROMIDE SODIUM-SALICYLATE SOLUTION IN POLY(DIMETHYLSILOXANE)

Deformation of viscoelastic droplets in an alternating current (ac) electric field of 60 Hz was studied on aqueous solutions of cetyltrimethylammonium bromide-sodium salicylate (CTAB-NaSal) complex suspended in poly(dimethylsiloxane) (PDMS). Under an ac field of ca. 10(5) Vm-1, a droplet with the initial radius b deformed into an ellipsoid with the major and minor semiaxes X and Y, respectively. The degree of deformation D = (X - Y) / (X + Y) vs the logarithm of time curve was a double-step sigmoidal shape characterized with, at least, two strengths and two retardation times: The fast process was attributed mainly to viscoelastic deformation due to the balance of the electric stress against the elasticity of the droplet, while the slow process was governed by the balance against interfacial free energy gamma-12. The time dependence of D was simulated with a phenomenological model composed of parallel combination of a Maxwell element representing the droplet viscoelasticity with the plateau (shear) modulus G(N)-degrees and the relaxation time tau-m and a Voigt element involving gamma-12/b and eta-2 of the medium. With this model, the fast process of the deformation (or recovery) behavior may be interpreted as being governed essentially by the viscoelasticity of the droplet. With increasing time after application (switching off) of the field, viscoelastic relaxation took place within the droplet, and the following slow deformation (or recovery) process was thus governed by the same mechanism prevailing in the behavior of a viscous droplet in a viscous medium reported previously.