Agglomeration and magnetic self-assembly of bare and polymer-modified Fe3O4 nanoparticles under ambient and applied fields

This study aims to elucidate how surface modification affects the agglomeration, self-assembly, and magnetic field-induced structural transformation of colloidal Fe3O4 at the microscopic scale. Although colloidal Fe3O4 are often surface-modified with polymers to improve colloidal stability and surface functionality, their influence on magnetic separation behavior remains unclear. Herein, a comparative study was performed by examining the assembly behavior of bare and surface-modified Fe3O4 nanoparticles under ambient and externally applied magnetic fields. Under ambient condition, bare Fe3O4 nanoparticles at 100 mg/L and 1000 mg/L formed noticeable particle agglomeration. At 10,000 mg/L, these particles further self-agglomerated into irregular, fractal-like structures without specific orientation. In contrast, surface-modified Fe3O4 at the same concentration self-assembled into smooth, thread-like structures, possibly due to polymer entanglement. Upon application of external magnetic field, both systems formed thick, linear chains and migrated toward the magnetic region. Notably, after removal of the external magnetic field, the surface-modified Fe3O4 chains underwent pronounced disintegration, manifested as sinusoidal bending and branching into hairy-like morphologies, whereas the bare Fe3O4 chains remained largely intact. These findings demonstrate that surface modification not only governs colloidal stability but also critically alters the reversibility of magnetic self-assembly, providing new insight for designing colloidal systems for controlled magnetic separation. © 2026 Elsevier B.V.