Magnetic Faraday – Instability

– In a magnetic fluid parametrically driven surface waves can be excited by an external oscillating magnetic field. A static magnetic field changes the restoring forces and damping coefficients of the various surface waves. This property enables the excitation of both subharmonic and harmonic responses of the standing waves. Introduction. – " Most fluids, if not all, may be used to produce these crispations, but some with particular advantages; alcohol, oil of turpentine, white of egg, ink and milk produce them ". What Michael Faraday describes here is the parametric excitation of surface waves, nowadays known as the Faraday-Instability [1]. Today it is a popular experimental system for the study of parametrically excited instabilities, pattern formation and spatio-temporal chaos [2]. Following Faraday's suggestion we use a fluid which was not available to him: a superparamagnetic fluid, more popularly known as ferrofluid or magnetic fluid. Its particular advantage is that the properties of the fluid can be tuned in a wide range by an external magnetic field, and that Faraday's parametric excitation can be achieved by temporal modulation of that magnetic field, i.e. without a mechanical driving. The mechanism of the parametric driving is however very similar to the mechanic case: the spontaneously created surface deformation of a standing wave is amplified by the concentration of the magnetic field in the neighborhood of a wave crest (demagnetization). This amplification takes place twice during the oscillation cycle of the standing wave: peaks which are half a wavelength apart are amplified at time differences of half a driving period T D. The idea to excite surface waves magnetically in a ferrofluid is fairly similar to the electric excitation of dielectric fluids [3] and thus not new [4, 5]. In those experiments a magnetic field perpendicular to the surface of the fluid was used. Related to this idea is the excitation