Dielectrophoretic Stretching of Drops of Silicone Oil: Experiments and Multi-Physical Modeling*

The present work explores the electrically-driven stretching of silicone oil drops without and with metal-oxide particles on a dielectric surface, which experience significant dielectrophoretic forces resulting in body forces pulling the suspension drops. The experimental evidence reveals that drops of two pure silicone oils of different viscosities on polypropylene substrate do not react to the in-plane electric field. On the other hand, pre-treatment of silicone oil in a humid atmosphere at 80% RH (relative humidity) enriches oil with water-related ions and results in a slight drop stretching under the action of the in-plane external electric field. These phenomena demonstrate that the original silicone oils do not contain a sufficient concentration of any ions and counter-ions for the appearance of any Coulomb force or Maxwell stresses, which would result in drop stretching. Nevertheless, additional experiments show a significant drop stretching on polypropylene substrate subjected to the in-plane electric field when 5 wt.% of TiO₂ particles was suspended in silicone oil. Here, the metal-oxide microparticles behave as electric dipoles and experience forces of dielectrophoretic origin when subjected to a nonlinear symmetric electric field which attracts them to both electrodes in air and oil. Furthermore, 3D simulations of the dielectrophoretically-driven evolution of silicone oil drops with TiO₂ particles also revealed a stronger drop stretching along the inter-electrode direction, in qualitative agreement with the experimental data.

The authors acknowledge the financial support from the National Science Foundation award CBET-1906497.