Dielectrophoretic stretching of drops of silicone oil: Experiments and multi-physical modeling

Abstract

It is shown experimentally that drops of two pure silicone oils of different viscosities on a polypropylene substrate do not react to the in-plane electric field. Pre-treatment of silicone oil in a humid atmosphere at 80% relative humidity enriches oil with water-related ions and results in subsequent drop slight stretching under the action of the in-plane 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. However, a stronger stretching of silicone oil drops on the polypropylene substrate subjected to the in-plane electric field was experimentally demonstrated when 5 wt. % of TiO2 particles was suspended in oil. The particles behave as electric dipoles and, when subjected to a nonlinear symmetric electric field, experience dielectrophoretic force, which attracts them to both electrodes in air and oil. 3D simulations of the dielectrophoretically driven evolution of silicone oil drops laden with TiO2 particles also revealed a significant drop stretching in the inter-electrode direction in qualitative agreement with the experimental data. Still, numerical simulations predict an unbounded stretching with two tongues developing at the two drop sides. This prediction disagrees with the experiments where the dielectrophoretically driven stretching ceases and steady-state drop configurations without tongues are attained. This disagreement is probably related to the fact that in the experiments, TiO2 particles settle onto the substrate and are subjected to significant additional friction forces, which could ultimately arrest them.