Metamorphosis of Trilobite-like Drops on a Surface: Electrically-driven Fingering

         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 super-critical electric field the Maxwell stresses become dominant and not only stretch the drop as a whole, but also trigger growth of multiple fingers crawling towards 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 and also provides a theoretical framework for this novel type of fingering.