This article investigates secondary-minimum aggregation of superparamagneti
c colloidal latex particles. Chain formation and breakup are experimentally
observed by using visualization techniques. The secondary minimum of the p
otential energy between two particles is determined from potential energy c
alculations, which include van der Waals, electrostatic, and magnetic dipol
e forces. A trajectory analysis, which incorporates these interparticle for
ces, hydrodynamic resistance forces, as well as gravity and magnetic induct
ion forces, is also used to determine the secondary minimum. Furthermore, t
his study describes relative mobility functions caused by magnetic inductio
n between two approaching particles. The effects of the following factors o
n the location of the secondary minimum are investigated: external magnetic
field strength;particle size; and solution properties, such as ionic stren
gth, zeta potential, and particle magnetic susceptibility. Both potential e
nergy calculations and trajectory analysis lead to the same conclusion: the
secondary-minimum separation decreases with increasing magnetic dipole for
ce, decreasing electrostatic force, and increasing particle size and size r
atio. After the removal of the magnetic field, three regimes of chain behav
ior may be identified: (i) no breakup regime, in which chains do not break,
indicating primary-minimum aggregation; (ii) slow breakup regime; and (iii
) fast breakup regime. Primary-minimum aggregation occurs when the chains a
re formed in high-ionic-strength solutions or at the pH of zero charge. Slo
w breakup occurs when the chains are formed under a low-strength magnetic f
ield, while fast breakup occurs when the chains are formed under a high-str
ength magnetic field.