The primary study of this thesis is the response of the nematic director to pressure driven flow. Dynamic flow experiments using optical conoscopy and pressure gradient measurements are used to explore the physics behind the flow alignment seen to occur for some nematic liquid crystals. New research into the techniques and methods for aligning the director at a glass interface is also presented, the results of which are used towards the latter end of this thesis in the production of a highly novel flow cell. A bespoke technique for fabricating robust liquid crystal flow cells is also presented. The observation of flow alignment for the nematic liquid crystal 5CB is detailed for pressure driven flow via optical conoscopy when the director is initially aligned planar homogeneously at 45◦ to the direction of flow. The results of this experiment are compared to the theory of Ericksen and Leslie through a one dimensional dynamic model that provides simulated director profiles and corresponding simulated conoscopic images. Good agreement between the data and simulation is observed, whereby the director is seen to rotate to become parallel to the flow direction whilst exhibiting no net tilt distortion at all flow rates. The presence of small surface pretilt from a rubbed planar aligning polyimide layer and its effect on director rotation is also examined for cells that are rubbed in both the parallel and anti-parallel directions. The result observed is a striking difference in the mean director rotation when initially aligned close to normal to the direction of flow. The results of these experiments are also compared to the theory of Ericksen and Leslie through the one dimensional dynamic model. Good agreement is seen, highlighting the dramatic effect that a small amount of surface pretilt can have on the overall director orientation, whilst also demonstrating the need for caution when assuming that rubbed conventional alignment techniques provide true planar orientation. Two methods for producing intermediate or large pretilt angles at liquid crystal align- ment surfaces are also examined. Here, two recipes involving the commercial polyimides Nissan SE-1211, Nissan SE-130 and Nissan SE-4811 are experimentally investigated, with results showing the ability to tune the director pretilt angle as a function of the rubbing strength used to align the sample. The results also show an interesting depen- dance on the material upon which the aligning layer is deposited for the recipe involving Nissan SE-1211. Here, vastly different pretilt angles are observed for cells constructed with glass and indium tin oxide (ITO) layers. Finally, the large pretilt angles produced from the recipes mentioned above are also used to fabricate pressure driven flow cells exhibiting large pretilt angles on both sur- faces, constraining the director to align in a splayed state. When aligned parallel to the flow direction, experiments examining the valve-like nature of the director profile suggest that a preferential flow direction exists in what here is termed the ‘diode cell’. Measurements of the pressure gradient required to achieve a constant volumetric flow rate through the cell are compared for flow in both directions relative to the splayed di- rector profile. A striking difference is observed for flow ‘with’ the splay and ‘against’ the splay, leading to the realisation of a cell exhibiting a preferential flow direction through surface treatment. Again, results are compared to the theory of Ericksen and Leslie through the one dimensional dynamic model, showing good agreement.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:573807 |
| Date | January 2012 |
| Creators | Holmes, Christopher J. |
| Contributors | Sambles, J. Roy |
| Publisher | University of Exeter |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10036/4009 |
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