Structure of the magnetic fluid film under alternating magnetic fields

Structural formation in the magnetic fluid film under alternating magnetic fields, H_ac, perpendicular to the plane of the film, is investigated. For a given alternating frequency of the field, the structure in the magnetic fluid film evolves from a monodispersed state to a disordered column pattern, and then to an ordered structure when the amplitude of H_ac increases from zero. Under the application of H_ac, the diameter of a magnetic column varies periodically with the same period as that of the H_ac, whereas the distance d between two neighboring columns remains fixed. This implies that the magnetic particles of columns disperse into the liquid and then agglomerate to form the original column at the same location. This process repeatedly proceeds under the application of H_ac. The experimental results also show that the oscillating range of the column diameter reduces when the alternating frequency of H_ac rises, and the averaged diameter of each column becomes larger.