Liquid crystals confined in millimetre-sized rectangular shapes towards sensing applications

The confinement behaviours of liquid crystals (LC) in chemically patterned, rectangular, millimetre-sized geometries with a free air/LC interface are presented in this thesis. Motivating this work is the development of an LC-based sensing device that does not require expensive polarised light imaging to detect analytes, is easily scalable for industrial purposes, is reusable for sustainability and has a reasonable shelf-life.

Initial work includes a derivation of a non-LC specific model to describe the height across the width of the confined LC. The model predicts maximum heights of the confined LCs throughout the thesis to be between approximately 20 - 420μm, and is validated using polarised imaging of the confinement of the nematic liquid crystal (NLC), E7. The identification of the confinement aspect ratios that have the most potential for sensing applications through confinement of the well known E7 is also completed, where a 6 x 6mm square is concluded to be most preferable of the aspect ratios tested. Therefore, it is the confinement size used throughout the rest of this thesis.

Another NLC with different physical properties to E7, MLC 7023, is then confined in the 6 x 6mm squares. Unlike the homeotropic anchoring (perpendicular to the interface) at the air/NLC interface for E7, it has been indicated in previous literature that the anchoring at the air/NLC interface for MLC 7023 is non-homeotropic. The 84° tilt angle at the air/NLC interface measured in this work suggests that using MLC 7023 broadens the range of analytes that can be detected by the system, with a suspected transition from tilted to homeotropic or planar (parallel to the air/NLC interface) on addition of an analyte. This is hoped to result in a greater optical contrast between the anchoring transitions on addition of an analyte under polarised imaging than for E7. MLC 7023 is summarised as preferable for sensing devices from the improved filling quality compared to E7 into the confinement. Additionally, the measured height of the confined NLC across the width fits well to the non-LC specific model to a greater height of approximately 125μm for MLC 7023 versus 50μm for E7.

With a motivation to improve the efficiency of the fabrication process, 6 x 6mm squares are then patterned onto surfaces fabricated with an equivalent method to the surfaces used throughout the thesis, but with the step to rub the alignment layer removed. The non-rubbed surfaces are filled with E7 and MLC 7023, with the resulting polarised images concluded as more difficult to evaluate for the non-rubbed surfaces compared to the rubbed.

Subsequently, it is quantified that the patterned substrates are reusable for at least 6 repeats with confined E7 without significant degradation, via rinsing the ‘old’ NLC off of the surface with isopropanol. It is also concluded for both E7 and MLC 7023 that the confined samples have a shelf-life of at least 14 days, with the E7 samples being resistant to changes upon heating and cooling.

A final investigation utilised a ferroelectric nematic liquid crystal (FNLC), FNLC-919, to give a novel introduction to the confinement behaviours in a 6 x 6mm chemically patterned square. The nematic phase behaved in a similar way to the E7 and MLC 7023 confined NLCs, with the height behaviour and variation of the effective birefringence in the nematic phase discussed for FNLC-919. However the two additional phases associated with FNLC-919, namely the Nx and the ferroelectric nematic phase, Nf, show interesting textures due to properties such as the polar nature of FNLCs. For example, an unexpected shrinkage along the rubbing axis is observed on cooling the smallest confined FNLC-919 sample, predominantly observed over the Nx phase (whose properties are debated in the literature). Another observation for the confined FNLC-919 systems is the ejected FNLC from the corners of the confinement, as is attributed to a competition between charges and surface tension. In the Nf phase, the FNLC textures under polarised imaging are discussed and compared to relevant literature, with the main observation being domains of twisted FNLC throughout the FNLC height.

The research discussed throughout this thesis can be used to predict behaviours of confined NLC over any width with the non-LC specific model. This, alongside the discussion of confinement behaviours for different LCs, can inform in both the planning of further research in the area and industrialisation of the system as a sensing device.