Pattern and wavenumber selection in ferrofluids

The formation of patterns of peaks on the free surface of a ferrofluid subject to a magnetic field normal to the undisturbed interface is investigated theoretically. The relative stability of ridge, square, and hexagon planforms is studied using a perturbative energy minimization procedure. Extending previous studies the finite depth of the fluid layer is taken into account. Moreover, adding the wavenumber modulus $k$ to the set of variational parameters also the wavenumber selection problem is addressed. The results are compared with previous investigations and recent experimental findings.