In liquid crystals a bulk polarisation can be induced by the application of an electric field via a number of mechanisms, which include: the electronic polarisation of individual atoms; the electronic polarisation of individual molecules; and the re-orientation of permanent molecular dipoles. The response of these different polarisation mechanisms occurs on different timescales. The electronic polarisation mechanisms exhibit a resonant response to an applied a.c. electric field. Below this resonant frequency such mechanisms respond to the applied field and contribute to the dielectric susceptibility of the material. When these materials consist of molecules containing a permanent dipole, bulk polarisation may also be induced by partial re-orientation of these dipoles to align with an applied field. However, there is a time lag for the re-orientation of the dipoles due to the viscosity of the fluid. In Debye theory, where it is assumed that the magnitude of the induced polarisation evolves exponentially with time, this results in a well defined relaxation frequency, below which the orientational mechanism contributes to the dielectric susceptibility, but above which it does not.
In calamitic nematic liquid crystals a lower relaxation frequency is generally observed when the a.c. electric field is applied parallel to the director (the average orientation of the molecular long axis) compared to when the field is applied perpendicular to the director. This is because rotation about the short molecular axis is hindered when compared to rotation about the long axis. Typical values are above 100KHz for relaxations in the dielectric susceptibility measured parallel to the director and above 100MHz for relaxations in the dielectric susceptibility measured perpendicular to the director.
In so called 'dual-frequency' or '2f' materials the relaxation in of the parallel susceptibility occurs at frequencies on the order of 1KHz at room temperature, well below normal values. At low frequencies well below fc, the relaxation frequency, the dielectric susceptibility anisotropy is positive. At frequencies well above fc the value of is negative.
Figure: Experimental measurements of susceptibility (red circles) with fitted model (black lines).
Our present work consideres a model of this phenomenum which we use to investigate switching in a variety of liquid crystal devices filled with a dual frequency material. These include Freedericksz, HAN and 45 degree tilt cells as well as zenithally bistable devices. We have found that using tailored voltage pulses it is possible to switch between bistable states.
Reference:
N. J. Mottram and C. V. Brown, Pulsed addressing of a dual frequency nematic liquid crystal (2005).