Secondary-minimum aggregation of superparamagnetic colloidal particles

This article investigates secondary-minimum aggregation of superparamagnetic colloidal latex particles. Chain formation and breakup are experimentally observed by using visualization techniques. The secondary minimum of the potential energy between two particles is determined from potential energy calculations, which include van der Waals, electrostatic, and magnetic dipole forces. A trajectory analysis, which incorporates these interparticle forces, hydrodynamic resistance forces, as well as gravity and magnetic induction forces, is also used to determine the secondary minimum. Furthermore, this study describes relative mobility functions caused by magnetic induction between two approaching particles. The effects of the following factors on the location of the secondary minimum are investigated: external magnetic field strength; particle size; and solution properties, such as ionic strength, zeta potential, and particle magnetic susceptibility. Both potential energy calculations and trajectory analysis lead to the same conclusion: the secondary-minimum separation decreases with increasing magnetic dipole force, decreasing electrostatic force, and increasing particle size and size ratio. After the removal of the magnetic field, three regimes of chain behavior 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 are formed in high-ionic-strength solutions or at the pH of zero charge. Slow breakup occurs when the chains are formed under a low-strength magnetic field, while fast breakup occurs when the chains are formed under a high-strength magnetic field.