Polarization-driven twisted states in ferroelectric nematic liquid crystals under confinement

Ferroelectric nematic liquid crystals (FNLC) are 3D fluids with a giant spontaneous electric polarization (P) in the order of several microcoulombs per centimeter squared. In an unconstrained sample this high P has recently been shown to twist the nematic director field in order to reduce the electrostatic energy [P. Kumari et al., Science 383, 1364 (2024)]. By studying an FNLC, namely AUUQU-2-N, in a wedge cell with continuously increasing thickness, we now show that the polarization-driven twist modes depend on the local distance d between the lower and the upper plates of the wedge. For planar and parallel anchoring conditions of the nematic director we find a uniform, non-twisted director field at small d below 2 µm and a likely 2π-twisted director field above a certain critical thickness of about 5 µm. At intermediate d we observe locally twisted director fields but with zero total twist between the lower and the upper surface. We coin these twisted director configurations with alternating twist sense "mesotwisted". In view of these polarization-driven twist instabilities in FNLCs, the uniform state at small d might be considered as a surface-stabilized ferroelectric nematic, an interesting analogy to surface stabilized ferroelectric chiral smectics.