Images of Patterns near Onset of Electroconvection (EC) in Nematic Liquid Crystals

Nematic liquid crystals (NLC) consist of elongated molecules which tend to align their long axes relative to each other, but which have their centers of mass randomly distributed in space as for an ordinary liquid. The local average direction of the molecular axes is called the director. When the NLC is confined between parallel glass plates, it is possible to cause the director to be aligned globally in a particular direction parallel to the plates. For the images shown here, the director will be in the horizontal direction. When an alternating voltage of frequency f and amplitude V is applied between the plates, convection occurs in the NLC for a wide range of fluid properties and above a critical value of V. As for RBC, the pattern close to onset usually consists of straight rolls; but unlike the RBC case, the orientation of the rolls is uniquely determined by the fluid properties and the applied voltage. In the example above the roll wavevector is parallel to the director. Such rolls are called "normal" rolls. They tend to occur at relatively large f.

K.M.S. Bajaj and G. Ahlers, unpublished.

For relatively small f, the roll wavevector in many cases has a non-zero angle relative to the director. This angle depends on f. Such rolls are called "oblique" rolls. Since the director only selects an axis in space and not a direction along that axis, there are always two types of oblique rolls, called "zig" rolls and "zag" rolls.

K.M.S. Bajaj and G. Ahlers, unpublished.

As for the straight rolls in the isotropic RBC case, dislocations also occur in the anisotropic EC. But in a sense they are simpler because they always travel along the roll axis (i.e. they "climb" and do not "glide").

K.M.S. Bajaj and G. Ahlers, unpublished.

When the zig and zag rolls occur simultaneously but in different parts of a sample, a boundary between them must exist. This boundary can take the form of a simple domain wall parallel to the director (left image), or it can consist of a vertical or oblique sequence of dislocations (right image).

K.M.S. Bajaj and G. Ahlers, unpublished.

The two types of domain walls can occur in various combinations which depend on the history of the sample and which evolve in time. These combinations can provide some esthetically appealing patterns, as shown here.

K.M.S. Bajaj and G. Ahlers, unpublished.

They become even more appealing when they are rendered in color!

K.M.S. Bajaj and G. Ahlers, unpublished.