Ferrofluids
- Magnetization curves
- Particle aggregation in ferrofluids
- AC - susceptibility
- Dynamics of ferrofluids with internal degrees of freedom
One of the most important characteristics of a ferrofluid is its magnetization curve, i.e. the dependence of the ferrofluid magnetization on the value of the applied magnetic dc-field. We simulate these magnetization curves taking into account not only the distribution of particle sizes and magnetic moments (unavoidable in real ferrofluids), but also all relevant forces acting between ferrofluid particles – steric repulsion and magnetodipolar interaction. Corresponding results allow to predict the linear and non-linear dc-susceptibilities of ferrofluids in dependence of the ferrofluid particles parameters, enabling to optimize ferrofluid properties for any desired technical application or medical diagnostic method where this highly interesting class of materials is employed.
Due to the magnetodipolar interaction between ferrofluid particles, which is attractive for favorable mutual orientations of particle magnetic moments, single particles may aggregate into chains or 3D clusters. This phenomenon is very important, because particle aggregates have a qualitative influence on the macroscopic properties of ferrofluids, like their stability and viscosity. Simulations of particle aggregation in ferrofluids, which we perform using the Langevin dynamics method, provide deep physical insights into the nature of these processes, allowing to better control them.
Our group also carries out simulations of the ferrofluid response to an oscillating external field, what allows to compute the temperature and frequency dependencies of the ac-susceptibility of ferrofluid. This characteristics plays a decisive role in many ferrofluid applications – in particular by the detection of biomolecules attached to ferrofluid particles (see next item).
In most standard ferrofluid models it is implicitly assumed that the magnetic moment of a ferrofluid particle is firmly “attached” to it, being able to rotate only together with this particle (Brown relaxation). In reality, when the particle size is not too large (below several tens of nm), magnetic moment can also rotate with respect to the particle itself, thus changing its orientation also when the particle is immobilized (Neel relaxation). Our simulations allow to study both relaxation kinds, thus enabling to calculate the dependence of corresponding relaxation times on the particle size and shape and on its magnetic parameters like magnetization and intrinsic magnetic anisotropy. These studies are highly important both from the fundamental point of view (relaxation processes in interacting many-particle systems are of a large interest for the modern non-equilibrium statistical mechanics), and for medical applications of ferrofluids like hyperthermia and on-chip diagnostics.