Study of dielectric properties of electrorheological fluids under high electric fields via normal stress

The mechanical properties of electrorheological fluids (ERFs) are primarily governed by the polarization characteristics of dielectric particles during their formation of chain–columnar structures under an applied electric field. Based on the equivalence between normal stress (σn) and electric field energy density, this study proposes a novel method for the in situ characterization of the effective dielectric constant (ɛeff) of ERFs under high electric fields. This methodology was employed to investigate the dielectric response behaviors of Fe–TiO2-ERF and H2O–TiO2-ERF under high electric fields. The results indicate that the normal stress of Fe–TiO2-ERF exhibits significant relaxation characteristics, revealing a process in which particle polarization intensity gradually strengthens and eventually stabilizes alongside the evolution of chain/columnar structures. Within the range of E = 0.5–2 kV/mm, ɛeff increases with field strength, manifesting as nonlinear polarization that reflects the progressive densification of the microstructure. At E ≥ 2 kV/mm, ɛeff remains constant, exhibiting linear polarization and indicating that the structural density has reached a steady state. A linear relationship is observed between ɛeff and the volume fraction under high electric fields, which suggests that both the formed chain/columnar structures and the single-particle polarization states are independent of concentration. In contrast, H2O–TiO2-ERF maintains nonlinear polarization characteristics even at high field strengths, evidenced by σn∝E1.8 and a decrease in ɛeff with increasing electric field. This finding provides the first experimental evidence of orientation polarization in polar-molecule ERFs under strong electric fields.