Energy-driven nematic ordering

In 1990, Janossy showed that a small amount of dye added to a nematic liquid crystal dramatically reduces the threshold intensity of the optical Freedericksz transition [68]. Subsequently, it was demonstrated that the underlying process is an optically driven Brownian ratchet mechanism [69-71]. Here, energy, but not momentum, from the radiation field causes unidirectional continuous rotation of dye molecules in the nematic, exerting a torque on the director that exceeds the direct optical torque by orders of magnitude. Similar mechanisms could, in principle, be realized in LCEs. Whether such processes are viable in overcoming the orienting effect of the network is not clear the viability of such Brownian motor processes in LCEs is an intriguing open problem. 

At present, at least three types of steady-state dielectrically driven pattern are known for nematics. The electric-field-induced periodic bend distortion in the form of parallel stripes has been observed in a homeotropi-cally oriented layer of 5-CB ( a= 13) in the presence of a stabilizing magnetic field [75, 76]. The stripes with a wavevector q were parallel to the electric field E and stationary at low fields. It was shown that a stable periodic pattern of the director minimizes the free energy of the cell when the ela.stic moduli and A 33 are similar to each other. In these experiments the Frederiks transition is of first order, the nonde-formed and deformed areas coexist at a given voltage, and the front between them may propagate along the direction y perpendicular to both fields [77]. 

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