CONICET | Buscador de Institutos y Recursos Humanos

We have used low-field 1H nuclear-magnetic resonance (NMR) spectroscopy and molecular dynamics (MD)to investigate the aggregation dynamics of magnetic particles in ionic ferrofluids (IFFs) in the presence ofmagnetic field gradients. At the beginning of the experiments, the measured NMR spectra were broad andasymmetric, exhibiting two features attributed to different dynamical environments of water protons,depending on the local strength of the field gradients. Hence, the spatial redistribution of the magneticparticles in the ferrofluid caused by the presence of an external magnetic field in a time scale of minutescan be monitored in real time, following the changes in the features of the NMR spectra during a period ofabout an hour. As previously reported [Heinrich et al., Phys. Rev. Lett., 2011, 106, 208301], in thehomogeneous magnetic field of a NMR spectrometer, the aggregation of the particles of the IFF proceedsin two stages. The first stage corresponds to the gradual aggregation of monomers prior to and during theformation of chain-like structures. The second stage proceeds after the chains have reached a criticalaverage length, favoring lateral association of the strings into hexagonal zipped-chain superstructures orbundles. In this work, we focus on the influence of a strongly inhomogeneous magnetic field on theaforementioned aggregation dynamics. The main observation is that, as the sample is immersed in a certainmagnetic field gradient and kept there for a time tinh, magnetophoresis rapidly converts the ferrofluid into anaggregation state which finds its correspondence to a state on the evolution curve of the pristine sample ina homogeneous field. From the degree of aggregation reached at the time tinh, the IFF sample just evolvesthereafter in the homogeneous field of the NMR spectrometer in exactly the same way as the pristinesample. The final equilibrium state always consists of a colloidal suspension of zipped-chain bundles withthe chain axes aligned along the magnetic field direction.