Drop-on-demand printing of liquid crystals and composites

This thesis focuses on the development and investigation of drop-on-demand (DoD) printing of liquid crystal (LC) and polymer composites with a focus on the manufacture of new photonics technologies that encompass a range of length scales. Both inkjet and electrohydrodynamic printing are employed as advanced material deposition tools with the end goal of forming droplets on both the micro-and nano-scale. The benefits of using these two forms of printing lie in their ability to deposit viscous and complex fluids in a non-contact, maskless, scalable and additive manner. These advantages have led both techniques to be considered for the manufacture of a range of different applications including printed electronics, printed photonics, displays and biotechnology. The first part of the thesis investigates the droplet generation process of a nematic liquid crystal (LC) when ejected from a piezoelectric-driven inkjet nozzle. For this study, high speed shadowgraphy imaging technique is used to visualise the droplet formation process and to determine the range of experimental conditions (e.g., temperature, voltage waveform applied to the piezoelectric transducer) where stable and reproducible inkjet printing of a nematic LC can be performed. These findings are then explained in terms of the fluid dimensionless numbers. Using the experimental conditions identified in the first part of the study, the thesis then demonstrates the precision offered by printing nematic LC microlens arrays for potential micro-optics applications. With the accuracy afforded by DoD inkjet printing, microlenses with a variety of different diameters and focal lengths in the range of 120 – 255 μm and 220 – 463 μm, respectively, are deposited precisely between in-plane electrodes. These microlenses are then subjected to an in-plane electric field and their resulting behaviour characterised with the aid of polarising optical microscopy and measurements of the focal length. The LC director is observed to align with the electric field direction leading to tuning of the focal length. Moreover, a bifocal behaviour is observed due to the formation of two separate domains with different alignments of the LC director over a range of electric field amplitudes. Subsequently, these two different LC domains gives rise to different refractive index profiles. These experimental findings are supported by finite element modelling of the LC director for different electric field amplitudes. Following the successful demonstration of electrically tunable microlenses, this thesis then employs DoD inkjet printing to manufacture large-area printed photonic devices. This is achieved by printing a polymer dispersed liquid crystal (PDLC) mixture to form arrays of droplets that make up emblems/logos that can be made to disappear with the application of a voltage. After device assembly and subsequent illumination with ultraviolet light, the PDLC droplets are shown to scatter the incident light in much the same way as that observed for conventional PDLC thin films. By applying an electric field across the device, the printed droplets become transparent and the scattering disappears. The thesis investigates the electro-optic properties of a single printed PDLC droplet and through the formation of arrays a smart window prototype with a switchable embedded image is then demonstrated. The final part of the thesis introduces the electrohydrodynamic (EHD) printing of liquid crystalline materials, which unlocks the possibility of forming nanoscale droplets for nanophotonic technologies including diffractive optic elements and optical circuits. To demonstrate EHD printing, a custom-built printing system is presented that is capable of depositing a nematic LC at room temperature. The response of the LC meniscus to an applied electric field is investigated using shadowgraphy imaging and the drop generation process is discussed in the context of the formation of a Taylor cone. By adjusting the input waveform and using different diameter nozzles, nematic LC droplets with droplet footprint diameters ranging from 2 – 8 μm are printed. EHD printing is then employed to form arrays of droplets that result in the appearance of shapes and alphanumeric characters at the microscale.