Fully Stable and Homogeneous Lyotropic Liquid Crystal Alignment on Anisotropic Surfaces

Abstract

Lyotropic chromonic liquid crystals have great potential in both biosensing and optical devices due to their biocompatible nature and strong optical characteristics. These applications, however, demand a homogeneous and stable alignment on anisotropic surfaces, a challenge that, so far, has not been solved adequately. In this work, it is shown how to drastically increase the quality of in-plane alignment and stability of these liquid crystals on conventional rubbed polyimide substrates by the addition of a small amount of a nonionic surfactant. Samples with surfactant show excellent alignment that is stable for months, while control samples without surfactant show much poorer alignment that further deteriorates in days. Also, well-aligned dry films of chromonics can be prepared following this approach. It is demonstrated how to obtain high-quality alignment by controlling the concentration and the nature of the surfactant, in particular its molecular structure and hydrophilic/ lipophilic balance (HLB value) and other critical parameters are discussed. It is believed that this approach may very well be essential for advancing the applicability of these water-based, biocompatible, and often highly dichroic materials for a wide range of uses.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.201701209. many new and exciting applications are emerging that require novel materials properties and new engineering methods. The last decade has seen an increasing interest in synergizing unique LC characteristics with aqueous environments, for instance, for the development of biological sensing applications, [1][2][3][4][5][6][7] (biocompatible) templates which align and manipulate (living) anisotropic materials, [8][9][10][11][12][13] and highly dichroic LCs for facile fabrication of optical elements. [14][15][16][17][18] This particular field of application requires different, water compatible materials. Excellent candidates are so-called lyotropic chromonic liquid crystals (LCLCs), which are rigid, plank-like molecules consisting of aromatic cores with hydrophilic groups on the periphery that, in water, spontaneously stack face-to-face in columnar assemblies due to π-π stacking and hydrophobic interactions. At certain concentrations and temperatures, these stacks form liquid crystalline phases [14,15] such as nematic and columnar phases. Typical members of the class of LCLCs are highly conjugated dyes that are functionalized with water-soluble (often ionic) groups. [14] The rigid, aromatic cores of LCLCs give these materials excellent optical properties, such as strong linear dichroism, high birefringence, and in some cases high absorbance in the visible range.Typically, LCLCs are applied either in closed cells where one benefits from the dynamics of the anisotropic solution or, alternatively, as a thin film after water evaporation from an anisotropic LCLC solution. In any application, however, high-quality, homogeneous, and stable macroscopic alignment is ...

Fabrication of Lyotropic Alignment Layers for Thermotropic Liquid Crystals Facilitated by a Polymer Template

“…The former result proves the high alignment quality of the DSCG alignment layer even on a sample area of 1 cm 2 , the latter result indicates that the contrast ratio for the thermotropic liquid crystal sample is smaller. However, the intensity ratio of ( I || / I ⊥ ) = 22.7 ± 3.3 observed for ZLI‐1993 is similar to the result that was found for lyotropic liquid crystal alignment on anisotropic surfaces, where ( I ⊥ / I || ) ≈ 0.02, i.e., ( I || / I ⊥ ) ≈ 50, was reported in the best case …”

“…However, LCLCs, aqueous solutions of aggregates of (often biocompatible) polar organic molecules, have been recognized as a useful link between living organisms and photonics, which can be used, for example, for biosensing . Thus, LCLC alignment techniques based on rubbing, rubbed polymer layers, or patterned substrates have been studied, as well . Recently, aligning interactions between thermotropic and lyotropic liquid crystals have been reported.…”

“…Deviations of the director from a single alignment axis reduce the contrast ratio, which is very important for practical applications. To characterize this possible practical limitation, we adopted a method that was recently suggested for describing the alignment of lyotropic chromonic liquid crystals . For this purpose, the intensity transmitted through the sample was measured with parallel polarizers (intensity I || ) and with crossed polarizers (intensity I ⊥ ), when the alignment direction of the sample was aligned parallel to the plane of polarization of the incident light.…”

Thermotropic Liquid Crystal-Assisted Chemical and Biological Sensors

“…Progress was made in improvements of the homogeneous and stable alignment on anisotropic surfaces [ 38 ]. This is a critical requirement for the applications mentioned above to mitigate the occurrence of false optical signal.…”

Liquid crystal templating as an approach to spatially and temporally organise soft matter

“…123,124 In the latter case, a surfactant is needed which anchors to the hydrophobic command layer interface and couples the in-plane organization of the patterned layer to the hydrophilic LCLC. 125,126 Electric and magnetic fields. Besides directing surfaces, both electric and magnetic fields can be used to align LC templates, which in turn align the dispersed materials by reorientational shear forces and elastic interactions.…”