Probing DLVO forces using interparticle magnetic forces: Transition from secondary-minimum to primary-minimum aggregation

The transition from secondary-minimum to primary-minimum aggregation of superparamagnetic colloidal latex particles is investigated in this study. The magnetic induction needed for this transition is calculated from the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which includes van der Waals, electrostatic, and magnetic-dipole forces as well as non-DLVO hydrophobic attraction. A chain-dipole model is used to estimate the magnetic-dipole potential. The magnetic induction at which the transition from secondary- to primary-minimum aggregation occurs is experimentally determined from visualization of chain formation and breakup. Experimental and theoretical values of transitional magnetic induction show good agreement for the small particles used in this study. The experimental value of the transitional magnetic induction of relatively large particles in a narrow capillary tube is found to deviate significantly from that predicted by theory. However, the transitional magnetic induction obtained from a wide capillary agrees well with the theoretical value. This behavior indicates that, in the narrow capillary, the magnitude of the repulsive force between the particles decreases.