Dynamics of liquid crystal cells

Summary of EPSRC Grant GR/M04938
Principal Investigator Professor J Roy Sambles.

In this two year research project the primary aim has been to develop the optical waveguiding technique to allow the study of dynamics using a convergent beam procedure with array detectors. Following on from earlier work at Exeter on optical waveguiding which allows the detailed director profiles in liquid crystal cells to be established we have now implemented a monochromatic convergent beam system with a time resolution of 50 microseconds which gives director details during dynamic switching. This allows the determination of viscosity coefficients and the full testing of dynamic nematic theory, including all backflow effects. Furthermore by use of the fully leaky waveguide technique standard commercial cell structures may be evaluated. In addition with the convergent beam system small areas may be explored. At present this is limited to a spot diameter of about 100 micrometres but this may be reduced further.

Principal achievements of the work are as follows:

Development of the convergent beam optical waveguide system (1-3). Use of convergent beam to explore thickness dependence of surface anchoring (4) which has shown how longer range forces may influence the strength of surface anchoring and thereby device behaviour. Quantification of the director profile during nematic relaxation and the determination of viscosity (3). First ever detailed quantification of the director 'backflow' profile during relaxation of a twisted nematic cell. Observation of the dual phase existence in thin twisted smectic A cells (5).

Five papers have so far been submitted from this work, two of which are already published.

References

[1] Smith N J and Sambles J R (1999) J Appl Phys, 85, pp 3984 - 3987 A convergent beam guided mode technique for use in liquid crystal studies.
[2] Smith N J and Sambles J R (2000) Mol Cryst Liq Cryst (Accepted) A Technique for director profiling utilising convergent beam excitation of fully leaky guided modes.
[3] Smith N J and Sambles J R (2000) Appl Phys Lett (submitted) Optically resolving dynamic processes in commercial liquid crystal cells.
[4] Ruan L Z and Sambles J R (2000) Phys Rev E (submitted) 'Thickness dependent liquid crystal anchoring energy, determined by a convergent beam waveguide technique.
[5] Ruan L Z, Osipov M A and Sambles J R (2000) Phys Rev Lett (submitted) Coexisting nematic and smectic A phases in a high twist liquid crystal cell.
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