Dresden 2009 – wissenschaftliches Programm

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 19: Focus: Magnetic Soft Matter I

CPP 19.3: Vortrag

Dienstag, 24. März 2009, 10:30–10:45, ZEU 222

Ultra-centrifugation of single-domain magnetite particles and the DeGennes-Pincus approach to ferromagnetic colloids in the dilute regime — •Albert Philipse — Utrecht University, Van 't Hoff laboratory for physical and colloid chemistry, Padualaan 8, 3584 CH, the Netherlands

A fundamental issue for dipolar colloids is the effect of dipole moments sec on thermodynamic and transport phenomena. De Gennes and Pincus argued that the thermodynamics of ferromagnetic colloids, at sufficiently low density, can be simply modeled via an effective isotropic attraction. The present work is motivated by the insight that this very same effective attraction would also determine transport quantities such as sedimentation and diffusion. We have studied sedimentation of stable dispersions of monodisperse magnetic iron-oxide (Fe2O4) colloids, with a dipolar coupling constant tuned by the average particle size. We find that the concentration-dependence of sedimentation rates abruptly changes sign, going from pure hard spheres to even weakly dipolar particles. This marked transition does not follow from the De Gennes-Pincus approach for reasons that will be explained. Our results confirm that effective isotropic attractions are not applicable to explain either thermodynamic or transport properties of magnetic fluids.