Pattern formation in microdrops of magnetic fluids

The pattern formation of magnetic fluid films containing magnetic microdrops under applied perpendicular magnetic fields was investigated. At a zero field, these drops of size several mu m were initially near-circular. As the field strength increased, these drops changed to dumbbells or branched structure. When the fields strength increased again, a labyrinthine structure appeared. At even higher field strength, the connectivity of the drops broke up; and finally an equilibrium hexagonal structure formed inside the film. A new phase transition exists between the labyrinthine structure and the hexagonal structure, which has not been seen before. Also, the reverse process of pattern formation is irreversible. (C) 1999 Elsevier Science B.V. All rights reserved.