Magnetically controlled cluster formation/dissociation in high-moment nanoparticle-based ferrofluids.

Ferrofluids (FFs) based on high-moment nanoparticles have emerged as an important class of smart nanomaterials, because of their fast response to moderate strength magnetic fields. Understanding the mechanism of cluster formation stimulated by external magnetic fields, followed by cluster dissociation, is pivotal for FFs' magnetic manipulation. A new strategy is proposed here, using the diffusion limited cluster aggregation (DLCA) model, to investigate the characteristics and the optimum conditions for the formation of field-driven high-moment structures in a fluid at room temperature. The conditions for fast cluster dissociation after removing the field, studied for the first time, suggest a completely reversible process, resulting in the initial FF structures; therefore, the FFs can be reused. Two representative cases of high-moment material-based FFs are investigated: (1) the CoFeO multicore particle-based FFs and (2) the FeCo alloy nanoparticle-based FFs. In both cases, each particle inside the fluid is covered with an organic surfactant shell. The numerical simulations demonstrate that (a) high magnetic moment plays a significant role in the cluster aggregation rate in the presence of the field and (b) steric interactions from the surfactant coating result in complete reversibility of the cluster process. The results open new perspectives for novel FF-based applications.