Ultracentrifugation of Single-Domain Magnetite Particles and the De Gennes−Pincus Approach to Ferromagnetic Colloids in the Dilute Regime

Abstract

We report an analytical ultracentrifugation study on sedimentation in dilute stable dispersions of uniform, magnetic iron oxide (Fe3O4) colloids. On increase of the dipolar coupling constant, tuned by the average particle size, the linear concentration dependence of the sedimentation velocity shows an abrupt transition from the hindered sedimentation expected for hard spheres to a marked acceleration already for weak dipolar interactions. This transition is not reproduced by sedimentation theory derived from an effective, isotropic pair correlation function of the type proposed by De Gennes and Pincus, for reasons which are made clear. Accelerated settling, instead, follows a scaling based on the mass action law for dimer formation with dipoles in head-to-tail configuration. Our work illustrates that orientational averaging of dipole interactions of ferromagnetic colloids in the dilute regime is inapplicable, with an obvious implication for the possible existence of isotropic gas-liquid criticality for such colloids.

Effects of Interparticle Attractions on Colloidal Sedimentation

“…( 1) suggests that attractions should increase the sedimentation velocity, while added repulsions should decrease it compared to the pure HS case. Experiments on dilute suspensions with inter-colloid attractions [12,13] or long-ranged electrostatic repulsions [14] are consistent with this picture. However, these theories and experiments are only relevant for very dilute suspensions.…”

Sedimentation of nanoparticles in nanoscale colloidal suspensions

“…Such a nonmonotonic dependence on particle volume fraction for sedimenting nanoparticles has been observed for the first time in the present simulation. Experiments on the behaviour of colloidal sedimentation velocity with interparticle attractions [42,43] are also consistent with this picture. Considering the hydrodynamic situation described in the preceding paragraph and the corresponding simulation results, one may obtain the average distance between particle centers (d) as a function of effective particulate radius for which the sedimentation velocity reaches its maximum.…”

Second virial coefficients of dipolar hard spheres

“…It is conjectured that the electrical dipole moment is due to bare electrical charges on the nano-particle. Calculation of the corresponding λ and its comparison to λ obtained via equation (10) or (24) from an experimental second virial coefficient might be an option to validate this conjecture.…”