  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.
