Flow-Orientational Effect

In these studies using dynamical grating diffraction techniques, picosecond laser pulses are used to induce density, temperature, and orientational-flow effects in nematic liquid crystals. 

The flow-orientational coupling can be described by including an extra torque in the equation describing the director axis reorientational angle 0 (cf Chapter 3 also Eichler and Macdonald ), 

Under these approximations (i.e., the flow is along the x direction), the force creating the flow Fx is obtainable from the Maxwell stress tensor  

The time constant characterizing the flow process may be estimated from the dynamical equation for the flow velocity field V(r,0 discussed in Chapter 3. Assuming that the viscosity coefficient involved is p, the velocity field V(r,/) obeys a greatly simplified equation  

To estimate the flow relaxation time scale, we simply set Fstress =0 in the preceding equation. This is valid because the picosecond laser pulses, as well as the time scales associated with all the physical processes such as thermal expansion and density change that initiate the flow process, are all much shorter than the flow response time. 

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There has been much interest in flow and flow orientation effects with polymer melts containing anisometric particles which may be plate-like or fibrous. Flow-induced orientation of short reinforcing fibres is an area of considerable commercial importance, which is beyond the scope of the review [30]. 

Flexural creep behavior of nylon 6/6 based long fiber thermoplastics (LFT) was determined nsing transient and dynamic testing methods. While the eflect of increasing fiber volume fi action reduced creep, there was only a negligible effect of flow orientation effect. The creep data generated by dynamic mechaitical analysis (DMA) tests was consistent with the transient tests. 

Calvert (R-12) has tested the correlation in cross-flow packed beds, which tend to give better drainage than countercurrent beds, and has found the effect of gas-flow orientation insignificant. However, the onset of reentrainment was somewhat lower in a bed of 2.5-cm... 

In Chapter 3 it was pointed out that certain rod-like polymers showed many of the attributes of liquid crystals in the melt. In particular, these molecules were oriented in shear to such an extent that interchain entanglement was small and the melts had a low viscosity. On cooling of the melt these rod-like molecules remained oriented, effectively self-reinforcing the polymer in the direction of flow. The essential differences in the properties of liquid crystal polymers... 

There are substantial influences on a material created by the flow orientation of the molecules, so there are different properties in the flow direction and perpendicular to the flow. These differences have a significant effect on the performance of the part (Chapter 2). 

Light-scattering measurements of sphered cells are not subject to orientation effects, and a method for the rapid sphering and fixing of red cells for the purpose of measuring them ill a light-scattering flow cytometer has been developed. 

All electrooptical effects known to the present time for polymeric liquid crystals may be divided into two groups. First of all there are so called orientational effects, which are due solely to the effect of the electric field (field effect) on LC polymers, but are not a result of a current flowing. The second group of electrooptical effects is attributed to the phenomena ascribed to the anisotropy of electrical conductivity (Act) of liquid crystals. These are called electrohydrodynamic effects. 

If the director is held in a fixed orientation by a magnetic field strong enough to resist the orienting effects of flow, then shear-rate-independent viscosities can be measured in a simple shearing flow. The three simplest of these, called the Miesowicz viscosities, are obtained in each of the three director orientations shown in Fig. 10-8. These viscosities can be related in a simple way to the or,- s, namely,... 

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