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 [Formula: see text] 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, [Formula: see text] particles settle onto the substrate and are subjected to significant additional friction forces, which could ultimately arrest them.

Control of surface wettability by means of electrowetting-on-dielectric (EWOD) is among the most effective methods of active enhancement of surface wettability. Here, electrohydrodynamics of drop impact onto a dielectric surface with electrodes embedded in the dielectric (or aligned and attached to it) is experimentally investigated. Drop impact of different liquids (water, n-butanol, and motor oil) onto different substrates (stretched Teflon, parafilm, and polypropylene) is studied. Water drop impact onto stretched Teflon (the only Teflon which revealed significant electrowetting) and un-stretched parafilm surfaces is studied in detail. The results for water drop impact indicate that drop spreading on such non-wettable surfaces can be significantly enhanced by the electric field application. In particular, water drop rebound can be suppressed by the electric force. Furthermore, impact dynamics and spreading of hydrocarbon liquids with electric field are explored. Partial suppression of splash phenomena was also observed with the application of the electric field in addition to enhancement of spreading. In addition, the experimental results for water drops are compared with the Cahn−Hilliard−Navier−Stokes (CHNS) simulations for static contact angles and drop impact dynamics, and the results are in close agreement for water drops. This study demonstrates that electrowetting-on-dielectric holds great promise for coating and spraying technologies.

The experimental evidence reveals that sessile drops on a dielectric horizontal substrate subjected to sub-critical in-plane electric field acquire steady-state configurations where a balance between the pulling-outwards electric Maxwell stresses and the restoring surface tension has been attained. On the other hand, the experiments show that in supercritical electric field the Maxwell stresses become dominant and not only stretch the drop as a whole but also trigger growth of multiple fingers crawling toward electrodes on both sides of the drop. This makes the drops with fingers stretched along the electric field lines similar to some trilobites known from their imprints in petrified sediments studied in paleontology. It is shown experimentally and theoretically that fingers are triggered during the encounters of the spreading drop outlines with minor surface imperfections. Such surface defects (existing originally or pre-notched on purpose) result in fingers which can grow being directed by the electric-field lines. The present work details multiple experimental observations of the trilobite-like fingering with several types of commercially available paints (colloidal dispersions) and also provides a theoretical framework for this novel type of fingering.

The creeping-flow theory describing evolution and steady-state shape of two-dimensional ionic-conductor drops under the action of surface tension and the subcritical (in terms of the electric Bond number) electric field imposed in the substrate plane is developed. On the other hand, the experimental data are acquired for drops impacted or softly deposited on dielectric surfaces of different wettability and subjected to an in-plane subcritical electric field. Even though the experimental situation involves viscous friction of drops with the substrates and wettability-driven motion of the contact line, the comparison to the theory reveals that it can accurately describe the steady-state drop shape on a non-wettable substrate. In the latter case, the drop is sufficiently raised above the substrate, which diminishes the three-dimensional effects, making the two-dimensional description (lacking the no-slip condition at the substrate and wettability-driven motion of the contact line) relevant. Accordingly, it is demonstrated how the subcritical electric field deforms the initially circular drops until an elongated steady-state configuration is reached. In particular, the surface tension tends to round off the non-circular drops stretched by the electric Maxwell stresses imposed by the electrodes. A more pronounced substrate wettability leads to more elongated steady-state configurations observed experimentally than those predicted by the two-dimensional theory. The latter cases reveal significant three-dimensional effects in the electrically driven drop stretching. In the supercritical electric fields (corresponding to the supercritical electric Bond numbers), the electrical stretching of drops predicted by the present linearized two-dimensional theory results in splitting into two separate droplets. This scenario is corroborated by the predictions of the fully nonlinear results for similar electrically stretched bubbles in the creeping-flow regime available in the literature as well as by the present experimental results on a substrate with slip.

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