Effects of Freeze-Thaw Cycles on the Rheological Properties of Magnetorheological Fluids

Abstract

Magnetorheological fluids (MRFs) are smart materials whose rheological properties can be rapidly and reversibly controlled by an external magnetic field, enabling their widespread use in dampers, brakes, and vibration-control systems. However, the deployment of MRF-based devices in cold and alpine regions exposes them to repeated freeze-thaw cycles, which may substantially affect their microstructure, stability, and performance consistency. To elucidate the damage-evolution mechanism of periodic phase transitions on the comprehensive rheological service performance of MRFs in extreme alpine environments, a silicone oil-based MRF containing 30 vol.% carbonyl iron particles (CIPs) was prepared. A periodic freeze-thaw cycle (FTC) accelerated-ageing experiment (0, 1, 3, and 5 cycles) was designed for extreme cold working conditions. Steady-state continuous shear and dynamic oscillatory strain sweep tests were systematically conducted using a rotational rheometer to quantitatively investigate the impact of freeze-thaw damage on the matrix flow resistance, ultimate shear load-bearing capacity, and three-dimensional magnetic chain network rigidity of the material. The results indicate that, with an increasing number of freeze-thaw cycles, the zero-field apparent viscosity exhibits an anomalous upward shift across the entire shear-rate range, representing an irreversible thickening phenomenon. At a baseline shear rate of 100 s-1 , the viscosity after 5 FTCs increased from an initial value of 0.86 Pa \(\cdot\) s to 1.32 Pa \(\cdot\) s,, yielding an irreversible thickening amplitude of 53.49%. Extrapolation using the Herschel-Bulkley model reveals that the core dynamic yield stress undergoes monotonic, non-linear degradation and collapse with the accumulation of freeze-thaw cycles, decreasing from 18.70 kPa to 14.15 kPa after 5 FTCs, a reduction of 23.96%. Dynamic oscillatory strain sweeps further confirm that freeze-thaw damage causes the critical distortion threshold of the magnetic chain network to shift significantly to the left, indicating premature occurrence. Furthermore, the storage modulus plateau of the 1-FTC sample experiences a non-linear, abrupt decrease. Inverse microscopic mechanism analysis demonstrates that the phase-transition lattice stress induces the mechanical peeling of the oleic acid steric hindrance dispersion layer on the particle surfaces. This triggers irreversible, secondary, large-scale disordered agglomeration of the particles, which subsequently reconstructs into a weakened magnetic chain network replete with lattice vacancies, stress concentration points, and chain-bending defects under an external magnetic field. This study provides theoretical support for the formula modification of high-performance anti-freezing MRFs and the long-term all-weather service evaluation of devices under extreme operating conditions.