First-order transition in the instability of a magnetic fluid interface

We have observed the surface instability of the interface between a highly concentrated ferrofluid and a weakly concentrated one. Our experimental data clearly show the first order character of the transition between a flat interface and the observed regular pattern of peaks. A large hysteresis effect is observed because of the high value of the relative permeability (μ ∼ 40) of our concentrated ferrofluid.