Research articlesMagnetic detection of nanoparticle sedimentation in magnetized ferrofluids
Introduction
Ferrofluids are concentrated colloidal dispersions of magnetic nanoparticles that behave as liquid magnets in external field. Oil-based ferrofluids are used as lubricants in many applications, with the advantage that they can be magnetically kept into place [1], [2], [3], [4], [5]. Another type of application of ferrofluids exploits the phenomenon of magnetic levitation: a nonmagnetic object that would sink in a normal liquid can be made to levitate in a ferrofluid, whose apparent mass density can be tuned via the magnetization of the fluid and via the magnetic field gradient [5]. Magnetic levitation has been applied for decades in the diamond industry, to separate diamonds from gangue material [6], and currently, magnetic levitation is being developed as a technology to separate solid waste materials for recycling [7]. The separation of plastics by magnetic density separation requires new low-cost high-stability ferrofluids that are water based, to prevent the dissolution of plastic.
For optimal colloidal stability of a ferrofluid, the magnetic nanoparticles must be dispersed at the single particle level and the pair interaction upon contact between two nanoparticles must be repulsive. The stability will thus depend on the magnitude of the nanoparticle dipole moments and on the modification of the surface with possibly charged chemical groups or surfactants [8], [9], [10]. Reversible or irreversible nanoparticle structures may already be present in zero field [11], [12], they may grow in external field [13], and isotropic attraction between nanoparticles may result in macroscopic phase separation [14]. To guide the chemical development of new ferrofluids with optimal stability, it is important to have a method to characterize how rapidly the magnetic material settles towards a magnet. Moreover, this characterization should be done at the same magnetic field gradients and relatively high nanoparticle concentrations that are relevant for magnetic fluid applications. We note that precise knowledge of the magnetophoretic velocity of nanoparticles is also relevant in the framework of the magnetic capture of nanoparticles in microfluidic bioassay devices [15].
The measurement of magnetization curves is a favorite way to characterize ferrofluids, but it is not very informative about their colloidal aggregation state. With magnetic nanoparticles in the 5–10 nm diameter range prepared by coprecipitation, a magnetization curve will be largely the same whether the nanoparticles are single or clustered, as the particles mostly respond to the external field individually [16], [17]. With larger nanoparticles that form dipolar structures in zero field and which grow in external field, the structures do affect the magnetization curve [18], [19], but still the presence of nanoparticle structures cannot easily be deduced from the magnetization curve alone. Field-induced dipolar structures have been visualized by cryo-TEM [13], but this is neither a routine method and nor does it give a macroscopic characterization of stability. Small angle scattering of X-rays [20] or neutrons [21] can reveal dipolar structure formation in the presence or absence of a magnetic field, but it requires access to dedicated beam facilities. Optical imaging of a thin capillary in external field is a useful option that can be used not only to study sedimentation equilibrium profiles [22] but also to detect whether magnetophoresis is more rapid than expected in the absence of aggregates. Time-resolved optical detection of concentration profiles in external magnetic field can be applied to a wide range of systems, with particle sizes ranging from a few nanometers up to several micrometers [23].
The approach that we propose here is to monitor the external magnetic field produced by a small bottle of ferrofluid placed on top of a permanent magnet. It is a convenient and simple approach that allows easy comparison of the in-field colloidal stability of different ferrofluids. In the Theory section, the principle of our method is described, together with our mathematical approach to calculate sedimentation rates from the time-dependent magnetic data. Practical aspects of our setup are presented in the Experimental section, and the Results and Discussion compare the colloidal stabilities of a few different ferrofluids in external field.
Section snippets
Magnetic analysis of sedimentation front position
Our approach to characterize the colloidal stability of ferrofluids in external field is summarized in Fig. 1. As a column of ferrofluid is placed on a permanent magnet, the liquid becomes a magnet itself. The dipoles of the magnetic nanoparticles become aligned to an extent that depends on the magnitudes of the dipole moments and on the strength of the external field. For simplicity, a cylindrical permanent magnet is used and we consider only its axial field H, given by [24]
Experimental methods
Three different types of water-based ferrofluid were studied. Ferrofluids F1 and F2 were unfractionated prototype fluids of undisclosed precise origin (sterically stabilized iron oxide nanoparticles) kindly provided by the UMinCorp company (Rotterdam). Ferrofluid F3 contained citrate-stabilized maghemite nanoparticles at nearly neutral pH prepared by us via a recipe of Dubois et al. [14]. Magnetization curves were measured using a vibrating sample magnetometer (EZ-9 from Microsense, see example
Results and discussion
Magnetic stability analyses of ferrofluid F1 are shown in Fig. 5, before and after dilution with water by factors of 2, 4, and 8. The initially measured field scales with the initial concentration (Fig. 5a), and the relative increase in field from the sample in 100 h is the largest at the highest dilution (Fig. 5b). In terms of the model presented in the Theory section, the most concentrated sample exhibits not only the slowest sedimentation but also seems to tend towards a less strongly
Conclusions
Our magnetic approach to characterize the sedimentation rate of ferrofluids seems well suitable to compare the colloidal stability of ferrofluids in external magnetic field. Clear differences were observed between a ferrofluid with citrate-stabilized nanoparticles, which sedimented at the rate expected for single particles, and two other ferrofluids in which sedimentation was more rapid. Moreover, a trend was observed in the sedimentation rate of a ferrofluid as a function of concentration, the
Acknowledgements
The authors would like to thank Dominique Thies-Weesie for practical help and helpful discussions, Particle Solutions for help with the X-ray equipment, and Urban Mining Corporation for support and ferrofluid samples. This work is part of research programme P14-07, project number 3.1, (partly) financed by the Applied and Engineering Sciences branch of the Netherlands Organisation for Scientific Research (NWO-TTW).
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