Experimental investigation of the electrostatically forced Faraday instability*

We study the instability of an interface between a dielectric and a conducting liquid subjected to a spatially homogeneous harmonically oscillating electric field, and show the analogies of the resulting wave structures to the mechanically induced Faraday instability. We use high-speed imaging in combination with an algorithm to evaluate light refraction at the interface to extract the instability wavelengths and modes, and investigate the influence of the liquid viscosities on both the critical instability voltage and the emerging interfacial wavelength. We demonstrate agreement between the experimentally obtained interfacial wavelengths and theoretical predictions accounting for viscosity effects. In addition, we show that depending on the influence of the boundary of the circular domain, the instability can exhibit either discrete modes of the surface harmonics, or boundary-independent patterns. Validating the theoretical predictions of the instability modes and demonstrating the analogies between mechanical and electrostatic forcing is important for a better control of electrostatically driven instabilities.