Liquid-crystalline ferroferric oxide nanocomposites: self-assembly and magnetorheological effects

Abstract

Magnetorheological (MR) fluids are a class of smart materials whose yield stress increases considerably when an external magnetic field is applied. The MR effect and sedimentation stability are two major factors for good MR fluids. In order to synthesize novel MR materials with high yield stress and enhanced sedimentation stability, a series of liquid-crystalline (LC) ferroferric oxide nanocomposites (Fe₃O₄@PILCs) with a core–shell structure were self-assembled by magnetic Fe₃O₄ nanoparticles, polyethylenimine and a cholesteryl-based LC material bearing sulfonic and carboxylic acid groups. The chemical structure, surface morphology, magnetic performance, thermal properties, and liquid-crystalline behavior of the Fe₃O₄@PILCs were characterized by various equipment and instruments. The MR effect, magnetic field response, and sedimentation stability of the Fe₃O₄@PILC-based MR fluids were investigated by various technical methods. The Fe₃O₄@PILC nanocomposites showed a smectic A LC phase when they were measured by the heating and cooling cycles. The MR fluids fabricated using the Fe₃O₄@PILC nanocomposites as suspended particles and silicone oil as a carrier liquid showed Bingham flow characteristics when an external magnetic field was applied on them, and it exhibited a maximum MR efficiency of 90 under 128 kA m⁻¹ magnetic field strength. The enhanced MR effect of the Fe₃O₄@PILC-based fluids was derived from a synergistic effect between the LC orientation and the Fe₃O₄ particle rearrangement along the magnetic field direction. In comparison with pristine Fe₃O₄-based fluids, the Fe₃O₄@PILC-based MR fluids showed excellent sedimentation stability, which is attributed to the surface modification of Fe₃O₄ particles reducing the density, the sulfonic and carboxylic acid groups acting as surfactants and stabilizers, and the excellent compatibility between the organic polymer shell structure and silicone oil preventing the Fe₃O₄ core from settling.