Flow-reorientation coupling

Generally speaking, excitation of a medium by short laser pulses can be used to study dynamic properties of the medium over a very wide time range. Here, we have shown that nanosecond-pulse excitation can yield information about the dynamics of molecular reorientation on the -10-sec time scale, and thermal effect on the 10—l(X)-msec time scale. The power of this technique lies in the fact that a single 6-function-like laser pulse may induce a number of fundamental excitation modes of vastly different time constants. Consider, for example, molecular reorientation coupled with flow induct by a picosecond laser pulse in a liquid crystal. It can be shown that, aside from the thermal effect, the transient behavior will manifest itself with three characteristic time constants ... 

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 ),... 

We use here the Ericksen-Leslie continuum theory to describe the effect. The rotational motion of the director (i.e., molecular reorientation) is driven by the pump laser pulse, but it is also coupled with the translation motion (flow) of the fluid through viscosity. Thus, with a finite pump beam, a rigorous theoretical calculation would require the solution of a set of coupled three-dimensional nonlinear partial differential equations for the angle of... 

The coupling of the axis of rod-shaped molecules is the basic characteristic that leads to a liquid which displays many of the. properties of a crystalline solid, such as birefringence and electric and diamagnetic anisotropy. The response of liquid crystals to applied forces may be weakly elastic but, in nematics at least, true flow is retained. This combination of anisotropy and liquid flow leads to some interesting properties of liquid crystals, since it becomes possible to reorient the director by imposing an external electric field. 

Field-Induced Reorientation without Flow Coupling ... 

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