Liquid Crystals. Iam-Choon Khoo

Liquid Crystals - Iam-Choon Khoo


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Schematic illustration of the numerical solution of the two transcendental equations.

      where V and Vc are the molar volumes at T and Tc, respectively.

      2.3.2. Nonequilibrium and Dynamical Dependence of the Order Parameter

      The temperature of the nematics can be abruptly raised by very short laser pulses [11, 12]. The pulse duration of the laser is in the nanosecond or picosecond time scale, which, as we shall see, is much shorter than the response time of the order parameter. As a result, the nematic film under study exhibits delayed signals.

Image described by caption.

      (2.21)normal upper Delta n equals StartFraction italic d n Over italic d rho EndFraction italic d rho plus StartFraction italic d n Over italic d upper S EndFraction italic d upper S period

      Unlike the change in order parameter, which is a collective molecular effect, the change in density arises from the individual responses of the molecules and responds relatively quickly to the temperature change.

      These results are reflected in Figure 2.3a and b. The diffracted signal contains an initial “spike,” which rises and decays away in the time scale on the order of the laser pulse. As shown in more detail in Chapter 9, this contribution comes from laser‐induced electrostriction or density changes that affect local order. On the other hand, the slowly rising contribution comes from laser‐induced temperature rise ΔT and the resulting order parameter change ST). The rate of change depends on the temporal characteristics of the incident laser. The buildup time is about 175 μs with microsecond infrared (10.6 μm) laser pulse (cf. Figure 2.3a) and 30 μs with nanosecond visible (0.53 μm) laser pulse (cf. Figure 2.3b).

Schematic illustration of observed buildup times of the diffracted signal associated with order parameter change as a function of the temperature vicinity of Tc; excitation by infrared microsecond laser pulses on E7 nematic film.

      More on order parameter dynamics in the context


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