Secondary-minimum aggregation of superparamagnetic colloidal particles

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.