Electrically Programmable Topological Defect Lattice in a Ferroelectric Nematic

The recently discovered ferroelectric nematic (N_F) liquid crystal is a polar fluid phase exhibiting spontaneous polarization, leading to electrostatic interactions and unique phenomena absent in conventional non-polar nematics. Previous studies have reported spontaneous formation of square-lattice defect patterns in N_F materials under AC electric fields without surface pre-patterning [1, 2]. These patterns are attributed the minimization of elastic and electrostatic energies associated with polarization deformation induced by the applied field, primarily through geometric cancellation of bound charges from splay distortion [1]. Recent work suggests that material viscosity plays a crucial role in determining the periodicity and stability of these defect lattices [2]. Motivated by this, we investigate how molecular structure and viscosity influence pattern formation across a library of newly synthesized ferroelectric nematic liquid crystals structurally similar to the well-known material RM734. These materials exhibit a broad range of phase transition temperatures (10°C to 68°C). Using polarized optical microscopy, we observe the formation of square-lattice defect structures and analyze the internal flow dynamics via particle image velocimetry. This study reveals how polarization, viscosity, and flow interact in ferroelectric nematics, providing insight into electrodynamic pattern formation in polar liquid crystals.