Lifting degeneracy in nematic liquid crystal viscosities with a single optical measurement

Characterization of optically thin cells and experimental liquid crystals

The current development of new liquid crystal devices often requires the use of thin cells and new experimental materials. Characterizing these devices and materials with optical methods can be challenging if (1) the total phase lag is small ("thin cells") or (2) the liquid crystal optical and dielectric properties are only partially known. We explore the limitations of these two challenges for efficient characterization and assessment of new, to the best of our knowledge, liquid crystal devices. We show that it is possible to extract a wealth of liquid crystal parameters even for cells with a phase lag of ΔΦ≈π, such as E7 liquid crystal in a 1.5 µm cell, using cross-polarized intensity measurements. The reliability of the optical method is also demonstrated for liquid crystals without precise values of dielectric or refractive index coefficients.

Nanoparticle-Induced Property Changes in Nematic Liquid Crystals

Doping liquid crystals with nanoparticles is a widely accepted method to enhance liquid crystal’s intrinsic properties. In this study, a quick and reliable method to characterise such colloidal suspensions using an optical multi-parameter analyser, a cross-polarised intensity measurement-based device, is presented. Suspensions characterised in this work are either plasmonic (azo-thiol gold AzoGNPs) or ferroelectric Sn₂P₂S₆ (SPS) nanoparticles in nematic liquid crystals. The elastic constants and rotational viscosity showed nonlinear dependence on the concentration of AzoGNPs, initially increasing at lower concentrations and then decreasing at higher concentrations, indicating some degree of particle aggregation. For the SPS suspension, the elastic constant decreased with doping, while the rotational viscosity increased, in agreement with previous findings. Through viscosity measurements, the stability of SPS suspension over ten years is also highlighted.

Increased Nematic-Isotropic Transition Temperature on Doping a Liquid Crystal with Molecularly Rigid Carboxylic Acids

A thermotropic nematic liquid crystal (LC) becomes an isotropic liquid at the nematic-isotropic transition temperature (T_NI), which depends on the molecular order of the mesophase. By means of a polarized optical microscope and a differential scanning calorimeter, it was found that doping the nematic liquid crystal 4-n-pentyl-4'-cyanobiphenyl (5CB) with molecularly rigid carboxylic acids (benzoic, 1-naphthoic, 2-naphthoic, and biphenyl-4-carboxylic acids) increases T_NI without modification of the nematic-isotropic transition enthalpy. This increment in T_NI is due to the increased order caused by the formation of molecularly rigid and elongated dimers of carboxylic acids in the nematic LC, as confirmed with infrared spectra. Furthermore, T_NI increased with the length of the molecularly rigid dimers at the same concentration level. Conversely, doping the LC with molecularly flexible acids caused lowering of T_NI. A quantitative correlation was established between the T_NI increase of the rigid carboxylic acids and the length of the dimers of these acids; a predictive model for these ΔT_NI values as a function of acid molar fraction was developed. It was also demonstrated that the doping of 5CB with rigid carboxylic acids increases the rotational viscosity of the liquid crystal.

Wide area mapping of liquid crystal devices with passive and active command layers

We track the non-uniformity of a wide area liquid crystal device using multiple cross-polarized intensity measurements. They give us not only accurate estimates of the core physical liquid crystal parameters, such as elastic constants, but also spatial maps of the device properties, including the liquid crystal thickness and pretilt angle. A bootstrapping statistical analysis, coupled with the multiple measurements, gives us reliable error bars on all the measured parameters.