Programming emergent symmetries with saddle-splay elasticity

Abstract

The director field adopted by a confined liquid crystal is controlled by a balance between the externally imposed interactions and the liquid’s internal orientational elasticity. While the latter is usually considered to resist all deformations, liquid crystals actually have an intrinsic propensity to adopt saddle-splay arrangements, characterised by the elastic constant . In most realisations, dominant surface anchoring treatments suppress such deformations, rendering immeasurable. Here we identify regimes where more subtle, patterned surfaces enable saddle-splay effects to be both observed and exploited. Utilising theory and continuum calculations, we determine experimental regimes where generic, achiral liquid crystals exhibit spontaneously broken surface symmetries. These provide a new route to measuring . We further demonstrate a multistable device in which weak, but directional, fields switch between saddle-splay-motivated, spontaneously-polar surface states. Generalising beyond simple confinement, our highly scalable approach offers exciting opportunities for low-field, fast-switching optoelectronic devices which go beyond current technologies.

Periodic Arrays of Chiral Domains Generated from the Self-Assembly of Micropatterned Achiral Lyotropic Chromonic Liquid Crystal

“…In terms of this viewpoint, liquid crystals (LCs) that are sensitive to external fields such as geometrical confinement and electrical, mechanical, and chemical perturbations are fascinating materials, which have been extensively studied in the past decades. 7 − 10 For instance, chiral symmetry breaking happens when LC molecules have bent shapes 11 − 13 or are placed in confined geometries. 14 − 17 Among the various kinds of LC types, lyotropic chromonic LCs (LCLCs), dissolved in water, are frequently used to explore spontaneous chiral symmetry breaking.…”

“…In terms of this viewpoint, liquid crystals (LCs) that are sensitive to external fields such as geometrical confinement and electrical, mechanical, and chemical perturbations are fascinating materials, which have been extensively studied in the past decades. − For instance, chiral symmetry breaking happens when LC molecules have bent shapes − or are placed in confined geometries. − Among the various kinds of LC types, lyotropic chromonic LCs (LCLCs), dissolved in water, are frequently used to explore spontaneous chiral symmetry breaking. − LCLC molecules have rigid planar shapes due to polyaromatic cores, which spontaneously aggregate “face-to-face” through π–π stacking interaction to form columns when they are dissolved in an aqueous medium. At a low concentration, the LCLC aggregates are short and oriented randomly, which is the isotropic (Iso) state.…”

Structural Landscapes in Geometrically Frustrated Smectics

“…External constraints may force deformations of the smectic that are incompatible with the layer constraint, leading to geometric frustrations and the spontaneous assembly of a wide variety of textures with characteristic defect structures of the smectic phase [4,6]. Driven by advances in surface control, there has been considerable renewed interest in exploiting the ability of smectics to repeatably self-assemble over device length scales by using surface patterning [7,8], topographical features such as grooves [9][10][11][12] or posts [13,14], confinement in droplets [15][16][17], or curved surfaces more generally [18], to produce emergent patterns [19,20] that are optically active as lenses, gratings [21], photonic crystals [20], or lithographic templates [22]. Moreover, defect structures in the texture act to efficiently trap dispersed microparticles or nanoparticles, making smectics useful for hierarchical [23][24][25] or synergistic [26] assembly processes that could potentially be adopted for metamaterial, sensor, or solar cell production.…”

Role of Stimuli on Liquid Crystalline Defects: From Defect Engineering to Switchable Functional Materials

“…The last example of using geometric confinement on defect engineering is illustrated in Figure 7 [67]. Compared to the previous cases, the geometry provokes three-dimensional distortion of the molecular directors, particularly at the post's edge.…”

“…Therefore, specially designed topographic patterns with additional chemical treatment could generate three-dimensional molecular distortion that results in various defect structures stabilized with high uniformity. The last example of using geometric confinement on defect engineering is illustrated in Figure 7 [67]. Compared to the previous cases, the geometry provokes three-dimensional distortion of the molecular directors, particularly at the post's edge.…”