Advanced magnetic properties in textured ferrofluids by nanoscale self-assembly of nanoparticles – PROMAFAO

The PROMAFAO project aims at a qualitative leap in the understanding of the structural and magnetic properties of textured ferrofluids. Ferrofluids are colloidal suspensions of magnetic nanoparticles dispersed in a liquid (water, oil,…). Particles are composed of a ferro/ferrimagnetic material (g-Fe2O3, CoFe2O4, MnFe2O4, …) with a typical diameter in the 5-25 nm range, each of them being regarded as a single magnetic dipole. Ferrofluids have found for fifty years numerous applications in biomedicine (contrast agents, hyperthermia therapy), technology (microfluidic devices, loudspeakers, car dampers) or art (where their spectacular surface instabilities are highlighted) that take advantage of magnetically driven changes in the macroscopic properties. This effect finds its origin in the nanoscale organization of particles under an applied magnetic field, which aggregate to form linear chain-like structures resulting from strong dipole-dipole interparticle interactions. When the latter are large enough, nanoparticles can self-assemble into chains or rings even at zero magnetic field. Recent investigations have underlined the relevance of studying field-induced structures in ferrofluids, in order to optimize the efficiency of biomedical applications, develop new magneto-optical applications or understand original phase transitions occurring with temperature. Imagine that we could tailor the formation of nanoparticle assemblies in ferrofluids, using the anisotropic, reversible and instantaneous character of magnetic dipole–dipole interactions: this would open bright perspectives to engineer ferrofluids for targeted applications and likely result in novel magnetic responsive materials with yet undiscovered exciting properties.
In PROMAFAO, ferrofluids offering a large interplay of magnetic dipole interactions will be prepared using texturation strategies that have not been yet tested experimentally. We will act on the “elementary brick” (i.e., the individual nanoparticle) using synthesis procedures fully mastered by the team, through solvent effects and by mixing two ferrofluids with different magnetic properties: such magnetic binary ferrofluids (like MnFe2O4-CoFe2O4) will be the central focus of the project because they offer an unprecedented interplay of dipole interactions and the potential applications connected to their texturing are currently largely under explored. Chaining phenomena occurring with and without an external magnetic field will be characterized, in the liquid and frozen states of selected ferrofluids, as well as their magnetic properties. An original cross-curricular combination of in-situ advanced magnetometry, microscopy and spectroscopy techniques will be used for the first time to build a comprehensive multi-scale picture of these ferrofluids, from the collective behavior down to the nanoparticle level.
The project stems from our recent proof-of-concept results obtained by cryogenic electron microscopy and a novel magnetic x-ray spectroscopy providing element selective information, which offers the very first opportunity to disentangle the contribution of both magnetic components in binary ferrofluids. PROMAFAO involves technical innovations addressing the challenges (i) to prepare frozen ferrofluids (used as a “snapshot” of the liquid state) in a reproducible way and (ii) to investigate, using dedicated sample holders, undamaged liquid or frozen samples where particle assemblies and interparticle interactions have been preserved. It will imply theoretical developments allowing detailed, quantitative interpretation of magnetic spectral signatures. Our results will enable to understand how nanoscale particle organization controls the collective magnetic properties of ferrofluids, which will be keys for the smart design of future magneto-intelligent fluids.