Self-assembly and dynamics of magnetic holes
  Nonmagnetic particles in magnetized ferrofluids have been denoted magnetic holes and are in many ways ideal model systems to test various forms of particle self assembly and dynamics. Some case studies to be reviewed here include:   Chaining of magnetic holes   Braid theory and Zipf relation used in dynamics of magnetic microparticles   Interactions of magnetic holes in ferrofluid layers   The objectives of these works have been to find simple characterizations of complex behavior of particles with dipolar interactions. 
Arne T. Skjeltorp (Insitute for Energy), Jorgen Akselvoll (Insitute   for Energy), Kai De Lange Kistiansen (UIO, Amks), Geir...