THE AGGREGATION OF SUPERPARAMAGNETIC PARTICLES IN A

Stable colloidal suspensions of magnetic nanoparticles called ferrofluids have attracted a lot of attention in recent years, since they can be used in numerous technological and medical applications [1-3]. For biomedical applications aqueous-based ferrofluids are being tested. Besides the requirements concerning their non-toxicity and biocompatibility, their behaviour in a magnetic filed is also of great importance. Under the influence of an external magnetic field, the nanoparticles of the ferrofluid undergo a major change in their ordering, forming agglomerates, which can lead to a phase separation. The transformations in the ordering of the nanoparticles in the ferrofluid and phase separation have a drastic effect on the rheological properties of the ferrofluid. In the absence of a magnetic field the thermal energy, kT, prevails over the dipole-dipole interaction between superparamagnetic nanoparticles. As a result, the nanoparticles remain separated and suspended in a carrier liquid. A drastic change in their ordering occurs when they are exposed to a magnetic field. The dipole-dipole interactions prevail over kT and the nanoparticles start to form chain-like agglomerates. This agglomeration leads to a phase separation, resulting in two phases: the “liquid phase”, containing a larger content of th