Confinement reveals hidden splay-bend order in twist-bend nematics.

Abstract

Using extensive Monte Carlo (MC) and molecular dynamics (MD) simulations, we investigate how spatial confinement reorganizes thin films of the twist-bend nematic (N_{TB}). Confinement markedly amplifies otherwise elusive splay-bend order by frustrating the three-dimensional heliconical texture of bulk N_{TB}. We find that a homochiral N_{TB} layer evolves into nonchiral splay-bend order via an interfacial smectic splay-bend-twist (S_{SBT}) band with periodically alternating handedness. Generally, for planar anchoring between parallel walls, with the helical wave vector parallel to the plates, globally polar smectic splay-bend (S_{SB}) layers form near the surfaces. Moving inward, the interfacial stack relaxes to bulk N_{TB} either directly through this S_{SBT} band or through a sequence that includes both the S_{SBT} and the nematic splay-bend-twist (N_{SBT}) phases. The N_{SBT} band is most pronounced at large molecular bend angles, i.e., near the N_{TB}-S_{A} boundary of the bulk phase diagram. Mode-resolved maps of elementary director distortions (splay, twist, bend, saddle-splay) corroborate this confinement-driven hierarchy. These results suggest thin-film routes to stabilize and diagnose modulated polar nematic and smectic organizations that are difficult to access in bulk.