When a ferrofluid layer is subjected to a uniform and vertically oriented magnetic
field, an interfacial instability occurs, above a critical value of the magnetic field,
giving rise to a hexagonal array of peaks. On increasing the magnetic field, a smooth
morphological transition from the hexagonal array to a square array was observed
above a second threshold. The hexagon–square transition phenomenology, in addition
to the role of penta–hepta defects initially present in the hexagonal pattern, was
investigated. Furthermore, the pattern and wavenumber selection was studied by two
different procedures: first by imposing jumps in field intensity and second by varying
the magnetic field in a quasi-static way. The results obtained were very different
for the two procedures. They indicated that the square pattern was a metastable
state induced by the compression of the hexagonal pattern on increasing the control
parameter. This hypothesis was confirmed by performing an additional experiment
where the pattern was isotropically compressed. In this experiment, the transition was
induced at a constant magnetic field lower than the transition onset value. However,
the theoretical values for stability domains of hexagons and squares proposed in the
literature were found to not agree with the experimental values.