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 affects molecular organization within thin films of the nematic twist-bend (${\mathrm{N}}_{\mathrm{TB}}$) phase. Our simulations show that confinement markedly amplifies the otherwise elusive splay-bend order, primarily by suppressing the intrinsic three-dimensional heliconical structure characteristic of bulk ${\mathrm{N}}_{\mathrm{TB}}$. Remarkably, when the ${\mathrm{N}}_{\mathrm{TB}}$ phase is confined between parallel walls imposing planar anchoring, and the bulk wave vector is oriented parallel to the walls, a smectic splay-bend (${\mathrm{S}}_{\mathrm{SB}}$) phase spontaneously emerges near the confining surfaces. This intermediate structure subsequently transforms into the bulk ${\mathrm{N}}_{\mathrm{TB}}$ phase either directly via a smectic splay-bend-twist (${\mathrm{S}}_{\mathrm{SBT}}$) phase or through a sequence involving both the ${\mathrm{S}}_{\mathrm{SBT}}$ and the nematic splay-bend-twist (${\mathrm{N}}_{\mathrm{SBT}}$) phases. Notably, the ${\mathrm{N}}_{\mathrm{SBT}}$ phase becomes particularly pronounced as the molecular bend angle approaches its maximum attainable value in bulk ${\mathrm{N}}_{\mathrm{TB}}$; this regime occurs in close proximity to the $\mathrm{N}\text{–}{\mathrm{S}}_{\mathrm{A}}\text{–}{\mathrm{S}}_{\mathrm{SB}}$ triple point on the bulk phase diagram. Our findings reveal a compelling and intricate interplay among chirality, confinement, and molecular ordering, further evidenced by the calculated elementary director distortions. Crucially, this study opens promising avenues for experimental exploration: confined thin-film geometries serve as powerful model systems for revealing and characterizing novel nematic and smectic liquid-crystal phases that remain elusive in, or currently inaccessible to, bulk experiments.

arXiv comment7 pages, 7 figures, revised fig. 3, results unchanged

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