Giant electro-viscous effects in polar fluids with paraelectric–modulated antiferroelectric–ferroelectric phase sequence

• Shear-driven switching between shear thinning and shear thickening behaviors. • Shear-triggered switching between bookshelf and planar geometries. • 70-fold change in viscosity by an extremely low electric field of 0.15 V µm −1 . The recently discovered ferroelectric nematic liquid-crystal material DIO exhibits an antiferroelectric (AF) phase, characterized by a sinusoidally modulated structure between the paraelectric (P) and ferroelectric (F) nematic phases. Although these sinusoidal modulated structures associated with the P–AF–F phase sequence is commonly observed in solid ferroelectrics, their presence in soft matter systems is scarce. This study is aimed at examining the macroscopic properties of DIO, identifying unique rheological properties, such as switching between shear thinning and shear thickening behaviors at certain shear rate in the ferroelectric nematic phase. Additionally, a significant electroviscous effect is observed, with the viscosity increasing by 70 times under an ultra-low electric field of 0.15 V µm −1 at the AF–F transition.