  The aim of this work is the description of the chain formation phenomena
observed in colloidal suspensions of superparamagnetic nanoparticles under high
magnetic fields. We propose a new methodology based on an on-the-fly
Coarse-Grain (CG) model. Within this approach, the coarse grain objects of the
simulation are not fixed a priori at the beginning of the simulation but rather
redefined on the fly. The motion of the CG objects (single particles or
aggregates) is described by an anisotropic diffusion model and the magnetic
dipole-dipole interaction is replaced by an effective short range interaction
between CG objects. The new methodology correctly reproduces previous results
from detailed Langevin Dynamics simulations of dispersions of superparamagnetic
colloids under strong fields whilst requiring an amount of CPU time orders of
magnitude smaller. This substantial improvement in the computational
requirements allows the simulation of problems in which the relevant phenomena
extends to time scales inaccessible with previous simulation techniques. A
relevant example is the waiting time dependence of the relaxation time T_2 of
water protons observed in Magnetic Resonance experiments containing dispersions
of superparamagnetic colloids, which is correctly predicted by our simulations.
Future applications may include other popular real-world applications of
superparamagnetic colloids such as the magnetophoretic separation processes.
