Magnetic Sedimentation Velocities and Equilibria in Dilute Aqueous Ferrofluids

Abstract

Dilute ferrofluids have important applications in the separation of materials via magnetic levitation. However, dilute ferrofluids pose an additional challenge compared to concentrated ones. Migration of the magnetic nanoparticles toward a magnet is not well counteracted by a buildup of an osmotic pressure gradient, and consequently, homogeneity of the fluid is gradually lost. Here, we investigate this phenomenon by measuring and numerically modeling time-dependent concentration profiles in aqueous ferrofluids in the field of a neodymium magnet and at 10 T in a Bitter magnet. The numerical model incorporates magnetic, frictional, and osmotic forces on the particles and takes into account the polydispersity of the particles and the spatial dependence of the magnetic field. The magnetic sedimentation rate in our most stable ferrofluids can be understood in terms of the magnetophoresis of separate nanoparticles, a best-case scenario when it comes to applications.

Ferrofluids and bio-ferrofluids: looking back and stepping forward

“…The synthesis of magnetite nanoparticles by chemical co-precipitation is a scalable and cost-effective procedure as it does not need or produce toxic intermediates, and does not require precursor complexes and it proceeds at moderate temperatures below 100 °C. 76–78…”

“…75 The synthesis of magnetite nanoparticles by chemical coprecipitation is a scalable and cost-effective procedure as it does not need or produce toxic intermediates, and does not require precursor complexes and it proceeds at moderate temperatures below 100 °C. [76][77][78] An interesting type of multicore magnetic nanoparticle is nanoflowers, the assembly of monocrystalline grains to be used as the magnetoresponsive component of a bio-ferrofluid. Nanoflowers have an exceptional hyperthermic efficacy, and their specific loss power (SLP) is one order of magnitude higher than that reported for the single core MNPs of the same size under the same AC field exposure, so they have shown outstanding interest since their first discovery.…”

“…Remarkable colloidal stability was achieved under extreme field conditions for dilute aqueous citrate stabilized ferrofluids designed for multifraction magnetic density separation on an industrial scale. 77,78 The colloidal stability of the citrate coated maghemite ferrofluid was tested at fields of up to 10 T and gradients of up to 100 T m −1 . The ferrofluids with particles in the 5-10 nm diameter range (ideal ferrofluid) show no aggregation and only low sedimentation rates which are compatible with their use in large-scale magnetic density separation.…”

“…It is well-known that a stable ferrofluid reacts quickly as a whole to a magnet, while the magnetophoretic motion of individual particles begins in a poorly stabilized system and needs longer time. Very good colloidal stability is required and can be tested by monitoring sedimentation in a strong magnetic field even at 10 T. 77,78 The lack of magnetoviscous effect also indicates the ideal behavior of "true" ferrofluids. 162,163 In the case of aqueous bio-ferrofluids, however, one faces several extra problems.…”

Non-Invasive Manipulation of Two-Phase Liquid–Liquid Slug Flow Parameters Using Magnetofluidics

“…The magnetically active particles in these fluids are so small that they are not subject to sedimentation effects, allowing them to remain suspended due to Brownian motion. Ferrofluids can therefore be used for applications where a continuous and homogeneous phase is mandatory [20]. Kurto glu et al clearly showed that a homogenous system of magnetically active nanoparticles can be used for this purpose [21].…”