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Title: Flexoelectricity in nematic liquid crystals
Author: Kischka, Claudius
ISNI:       0000 0004 2727 8678
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2011
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Flexoelectricity in liquid crystals is thought to be due to a coupling between dielectric properties and shape anisotropy of the molecules and described by the flexoelectric coefficients e1 and e3. Two experiments are needed to measure e1 and e3 and it is usual to measure the difference (e1 − e3) and the sum (e1 + e3) and then calculate e1 and e3. The first experiment to measure the difference (e1 − e3) uses a TN structure with an in-plane applied electric field. Due to the dielectric coupling, the director aligns with the electric field and due to the flexoelectric effect, the director tilts out of plane. This tilt is measured optically using two laser beams at oblique incidence, e.g. 45◦ . Using a theoretical model the experimental data is fitted and the difference (e1 − e3) extracted. The second experiment to measure the sum (e1 + e3) uses a Pi cell. Applying an ac voltage the transmission through the device is a repeating oscillating signal which contains 1st and 2nd harmonics. The 1st harmonic corresponds to the flexoelectric effect and the 2nd harmonic to the dielectric effect. Using a lock-in amplifier, the harmonics were measured and the sum (e1 +e3) extracted using a theoretical model to fit the experimental data. Unfortunately, the data proved the experiment to be unreliable and another method was developed, which uses a HAN cell. The third experiment uses simple pulses in a HAN cell and also measures the sum (e1 + e3). The big disadvantage of the HAN cell is an internal voltage, which is created by the homeotropic alignment layer and the flexoelectric polarisation. The internal voltage has the same effect on the director profile as the flexoelectric effect, which is a big problem in measuring flexoelectricity. Using a material, which is non ionic and has no flexoelectricity, the internal bias could be measured and taken into account. Applying short dc pulses of opposite sign, the flexoelectric effect can be ii observed by the optical response and can be measured. Using these experiments, a number of investigation are being carried out such as the correlation between flexoelectricity and the molecular structure, ions, elastic properties, molecular orientation, dielectric anisotropy ∆ϵ, and order parameter S. The results showed that flexoelectricity only depends on ions and dielectric properties which was very interesting and surprising at the same time.
Supervisor: Elston, Steve Sponsor: EPSRC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Nematic liquid crystals ; Liquid crystals--Electric properties