From phase space representation to amplitude equations in a pattern-forming experiment

We describe and demonstrate a method to reconstruct an amplitude equation from the nonlinear relaxation dynamics in the neighbourhood of the Rosensweig instability. A flat layer of a ferrofluid is cooled such that the liquid has a relatively high viscosity. Consequently, the dynamics of the formation of the Rosensweig pattern becomes very slow. By a sudden switching of the magnetic induction, the system is pushed to an arbitrary point in the phase space spanned by the pattern amplitude and magnetic induction. Afterwards, it is allowed to relax to its equilibrium point. From the dynamics of this relaxation, we reconstruct the underlying fully nonlinear equation of motion of the pattern amplitude. The measured nonlinear dynamics helps us to select the best weakly nonlinear expansion that describes this hysteretic transition.