Drop motion, deformation, and cyclic motion in a non-uniform electric field in the viscous limit

Abstract

Drop motion and deformation of a conducting drop in a perfect (or leaky) dielectric fluid and a leaky dielectric drop in a leaky dielectric fluid, in a non-uniform electric field is presented. The investigated non-uniform electrode configuration is of the pin-plate type. Systematic experiments and comparison with existing analytical models is carried out. The main results are summarized as follows: (i) The dielectrophoretic motion of a conducting drop in a non-uniform electric field is explained reasonably well assuming a spherical drop, although deviations are observed at large deformations. Thus dielectrophoretic motion shows a weak shape dependence. (ii) The deformation of a conducting drop in a non-uniform electric field has comparable contributions from the uniform and the non-uniform components of the applied field. (iii) The leaky dielectric nature of the medium results in three different states for a conducting drop (a) no movement, (b) near electrode cyclic motion, and (c) cyclic motion between the electrodes. The frequency of cyclic motion decreases with electric field for near electrode motion. On the contrary it increases with the applied field for electrode-electrode cyclic motion. The leaky dielectric system showing positive dielectrophoresis leads to the drop getting attached to the pin electrode causing emulsification at large field. A leaky dielectric drop suspended in a dielectric, system exhibiting negative dielectrophoresis shows oblate deformation which is augmented by the plate-drop hydrodynamic interaction.