Birefringence, — continued

Such measurement provides the magnitude of birefringence, but not its sign. In addition, identical transmission values will be observed for multiple birefringence orders, that is, whenever the optical path difference, dAn, becomes a multiple of X. The main interest of this method arises from its excellent time resolution, below 1 ms, that is readily achieved using a low-power (e.g., 5 mW) continuous-wave laser and a photodiode. If the sample is initially isotropic, it is possible to follow the birefringence order to obtain quantitative results. For improved accuracy, a second (reference) photodiode or a beam chopper and a lock-in amplifier can be used. 

Tumbling regime At very low shear rates, the birefringence axis (or the director) of the nematic solution tumbles continuously up to a reduced shear rate T < 9.5. While the time for complete rotation stays approximately equal to that calculated from Eq. (85), the scalar order parameter S,dy) oscillates around its equilibrium value S. Maximum positive departures of S(dy) from S occur at 0 n/4 and — 3n/4, and maximum negative departures at 0 x — k/4 and — 5it/4, while the amplitude of oscillation increases with increasing T. 

From the measurements of the birefringence in Fig. 12a further important aspect has to be mentioned. With increasing temperature An does not continuously tend to zero at the phase transformation temperature Tc but vanishes discontinuously. At the phase transformation the nematic phase, having a finite birefringence, coexists... 

Figure 3. Birefringence (An) versus incident exposure in (J/cm2) for polarized UV(A) and polarized pulsed laser ( ) exposure. The continuous 325 nm power density was 0.2 W/citp, while the 575 nm pulsed laser intensity (8 ns pulses at 10 Hz) was 1.6 GW/cm2. (Reproduced with permission from Ref. 8.

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