Simulation and Visualization of Magnetic Tapes and Colloidal

Inks

Pieter  B. Visscher We have developed programs for "molecular-dynamics" type
simulation of magnetic colloids on the particle level, in which
particles are modeled by Stoner-Wohlfarth (uniformly magnetized)
cylinders with hemispherical end caps.  Results  will be presented
from simulations of:
(1) aggregation from a dilute solution during solvent evaporation,
as in the production of magnetic tape,
(2) aggregation in the presence of a magnetic field (which
sometimes leads to the formation of a layered structure resembling
a smectic liquid crystal), and
(3) switching (by rotation and Stoner-Wohlfarth switching) in a
hysteresis loop.
Steric repulsion, magnetostatic forces, hydrodynamic drag due to a
polymer binder, and Brownian forces and torques are all taken into
account.  Of these, magnetostatic forces are by far the most
time-consuming, since all pairs of the N particles interact, so the
CPU time is proportional to N^2.  Recently this has been reduced to
order N (for systems of point particles) by  so-called "fast
multipole" methods, but the coefficient of N is quite large so
these methods are faster only for very large systems (N>10,000).
We have developed a simplified variant of the method that becomes
useful at much smaller N, and also is applicable to non-point
particles.