Double Rosensweig instability in a ferrofluid sandwich structure.

We consider a horizontal ferrofluid layer sandwiched between two layers of immiscible nonmagnetic fluids. In a sufficiently strong vertical magnetic field the flat interfaces between magnetic and nonmagnetic fluids become unstable to the formation of peaks. We theoretically investigate the interplay between these two instabilities for different combinations of the parameters of the fluids and analyze the evolving interfacial patterns. We also estimate the critical magnetic field strength at which thin layers disintegrate into an ordered array of individual drops.