Electric field-induced assembly of magnetic nanoparticles from dielectric ferrofluids on planar interface

Electric fields, similar to magnetic fields, could be the driving force inducing the assembling of magnetic nanoparticles at the interface. Using specular neutron reflectometry, the assembly of superparamagnetic nanoparticles of a dilute classical ferrofluid (magnetite coated with oleic acid in transformer oil) on the planar surface of a metal electrode (copper) was observed when an out-of-plane electric field was applied to the interface. Starting only from the wetting layer of magnetic nanoparticles on the electrode surface in the absence of an electric field, the evolution of the interface structure with an increase in the electric field strength is traced by analyzing changes in the scattering length density depth profiles derived from neutron reflectivity curves. At a sufficiently strong electric field, a second adsorption layer is revealed, which becomes even more saturated than the initial one. The cause of the observed effects is related to the polarization of the particles in the electric field and their interaction as dipoles. Relaxation mea-surements (after the field is switched off) prove that the local electric field induced by strongly interact-ing electric dipoles in concentrated adsorption layers is an important feature determining the enhancement of adsorption of nanoparticles.(c) 2022 Elsevier B.V. All rights reserved.