Instability modes of fluid interfaces under normal AC electric fields
Final Report Abstract
In this project, we have studied how an oil-water interface reacts when exposed to an electric field. The basic configuration we studied was a layer of aqueous liquid superposed by an oil. Two classes of electric fields were considered: A. time-dependent (oscillatory) homogeneous fields due to a plate electrode; B. time-independent localized fields due to a pin electrode. In case A, we have recorded and characterized the oscillatory wave patterns (so-called Faraday waves) emerging at the oil-water interface, have extended the theory describing such waves, and have found a good agreement between the experimental results and the theoretical predictions. We have also demonstrated how the wavelength of the Faraday waves can be tuned in a continuous manner by admixing an offset to the applied oscillatory field. Based on these results, we hope that in the future electrically excited Faraday waves will be studied as intensely as their mechanically excited counterparts. In case B, a major goal was to explain a surprising effect that we had observed before the start of the project: When applying a localized electric field to an oil-water interface using a pin electrode, often a dimple forms at the interface instead of the well-known protrusion (Taylor cone). We have found an explanation for this effect that is based on small aqueous droplets that reciprocate between the pin electrode and the aqueous bath, upon which they transfer momentum to the surrounding oil phase. We hope that in the future, these insights will help explaining a number of electrohydrodynamic phenomena induced by local electric fields.
Publications
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Deformation modes of an oil-water interface under a local electric field: From Taylor cones to surface dimples. Physical Review Fluids, 6(12).
Dehe, Sebastian & Hardt, Steffen
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Controlling the electrostatic Faraday instability using superposed electric fields. Physical Review Fluids, 7(8).
Dehe, Sebastian; Hartmann, Maximilian; Bandopadhyay, Aditya & Hardt, Steffen
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The spatial structure of electrostatically forced Faraday waves. Journal of Fluid Mechanics, 939.
Dehe, S.; Hartmann, M.; Bandopadhyay, A. & Hardt, S.
