Cholesteric response time

CLCs can be fabricated by two main methods. The first and simplest way is based on chiral mesogens, which can exhibit the cholesteric phase at a certain temperature range. The cholesteric phase produced by this method consists of pure materials and thus may exhibit advantages such as good uniformity, enhanced stability, and fast response time to stimuli. However, they are typically produced at... 

The simple expression for / means that the response time will be inversely proportional to the field, r oc 1/ PE) when there is flexoelectric coupling. This is the same dynamics as in ferroelectric liquid crystal (FLC) switching. In the cholesteric case the expression for / is somewhat more complicated and, as we will see below, it turns out that r is independent of E. 

For instance, the third harmonic of the distorted helix relaxes nine-times faster than the first one and this agrees with experimental data showing submillisecond response times of the cholesteric helix in the external electric field. 

The response time of cholesteric liquid crystals is finite and comparatively long because the visible color change occurs in two stages. An instantaneous change in the temperature of a cholesteric liquid crystal causes it to assume a new pitch. However, this requires that some of the material flows to a different configuration. The rate at which such flow occurs is limited by the material viscosity in the direction of flow. 

The time constant of ordinary cholesteric materials is in the order of 0.1 s. For example, for cholesteryl nonanoate it is 0.1 s and for cholesteryl oleyl carbonate 0.2 s. These response times do not indicate a limitation if the materials are to be observed by eye motion pictures at 64 frames/s have been successfully made and information determined from single frames. 

The flexoelectrooptic effect is a field-sensitive electrooptic effect (it follows the sign of the field), which is fast (typically 10-100 p.s response time) with two outstanding characteristics. First, the induced tilt (p has an extremely large region of linearity, i.e., up to 30° for materials with dielectric anisotropy Ae=0. Second, the induced tilt is almost temperature-independent. This is illustrated in Fig. 39 for the Merck cholesteric mixture TI 827, which has a temperature-independent pitch but not designed or optimized for the flexoelectrooptic effect in other respects. 

Although by far nematics are the most extensively used ones, other phases (smectic, cholesteric, etc.) of hquid crystals and mixed systems such as polymer-dispersed liquid crystals capable of field-induced reorientation have also been employed for electro-optical studies and applications. They are basically based on the same basic mechanism of field-induced director axis reorientation similar to nematic hquid crystals i.e., the response is Kerr like in that it is independent of the direction of the electric field. In general, nematic liquid crystal electro-optics devices switch at a rate of several terrs of hertz, corresponding to response times from a few to tens of microsecorrds. 

The first surprising conclusion is that a model system with just the three elements that we have characterized as the Broken Symmetries of Life, also knows time. Its nonchiral analogue, the traveling nematic-isotropic phase bormdary does not know time. The argument given is that the existence of an intrinsic length, p, in cholesteric liquid crystals, implies a frequency for its response to perturbations in its structure. 

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