High-performance bidisperse magnetorheological fluids via combining modified carbonyl iron powder and octahedral/sheet-like Fe3O4 nanoparticles: Preparation and cross-scale experimental validation for comprehensive performance

Magnetorheological (MR) fluids have garnered sustained attention for their reversible magneto-responsive properties, and bidisperse systems integrating micro- and nano-sized magnetic particles have emerged as a prominent research focus for optimizing comprehensive performance. However, existing research rarely systematically investigates the influence of nanoparticle micro-morphology on the synergistic regulation of MR fluid performance, and comprehensive evaluations often lack integration of thermal stability and thixotropy-key metrics for practical engineering service. To address these gaps, bidisperse MR fluids were prepared by combining micrometer-sized sodium lauryl sulfate (SDS)-modified carbonyl iron powder (mCIP) and nanoscale Fe3O4 with two distinct morphologies (octahedral and sheet-like structure). Cross-scale experimental characterizations were conducted: microscale analyses (scanning electron microscope, energy dispersive X-ray spectrometry, Fourier transform infrared spectroscopy, vibrating sample magnetometer) clarified particle microstructure, elemental composition, and magnetization properties; macroscale tests evaluated sedimentation stability (60-day observation), thermal stability (0–80 °C), thixotropy (three-interval test), zero-field viscosity (1–100 s−1), and MR effect (shear yield stress under 0–1.0 T). The results showed that SDS modification reduced mCIP agglomeration, while octahedral Fe3O4 filled mCIP gaps to form a “micron-nano” hybrid network. The optimal formulation (mCIP: octahedral Fe3O4 = 8:2) exhibited low sedimentation rate (17.24% at 60 days), favorable thermal stability and thixotropy, balanced zero-field viscosity (1.74 Pa·s @ 100 s−1), and shear yield stress (21.2 kPa @ 1.0 T). In contrast, sheet-like Fe3O4 induced stacking-induced agglomeration, worsening stability. This study provides a feasible strategy for morphology-dependent optimization of high-performance bidisperse MR fluids and supports the development of long-service-life MR devices in engineering and biomedicine.