An experimental study of small Atwood number Rayleigh-Taylor instability using the magnetic levitation of paramagnetic fluids

Experiments that take advantage of the properties of paramagnetic liquids are used to study Rayleigh-Taylor (RT) instability. A gravitationally unstable, miscible combination of a paramagnetic salt solution and a nonmagnetic solution is initially stabilized by a magnetic field gradient that is produced by the contoured pole-caps of a large electromagnet. Rayleigh-Taylor instability originates from infinitesimal random background noise with the rapid removal of current from the electromagnet, which results in the heavy liquid falling into the light liquid due to gravity and, thus, mixing with it. The mixing zone is visualized by backlit photography and is recorded with a digital video camera. Several miscible, small Atwood number (A ≤ 0.1) combinations of paramagnetic and nonmagnetic solutions are used. It is found that the RT flow is insensitive to the viscosities of the fluids composing the two-fluid system, and that the growth parameter α also does not show dependence on the Atwood number when the experiments are initialized under the same conditions. It is also observed that the turbulent mixing zone grows linearly with time following a period of self-similar quadratic growth. When the width of the mixing zone becomes comparable with the cross-sectional length scale of the experimental container, the bubble front characteristic velocity approaches a constant value, similar to that observed with a single bubble rising in the confined volume, with Froude number measured in the range Fr = 0.38÷0.45. However, flow visualization does not reveal any persistent large-scale perturbations, which would dominate the flow during this stage. We believe that this phenomenon is not an attribute of the given magnetic experiments and has been observed in many other experimental studies, which involve RT instability evolving in confined volumes.