Chiral topological defects in freely suspended smectic films
 

The introduction of solid or fluid inclusions into a liquid crystal (LC) induces distortions and topological defects in the molecular ordering fields of the host, thereby generating a variety of LC-mediated inclusion interactions. The interactions between colloidal particles, fluid drops, and phase-separated domains in liquid crystals depend on the LC elastic constants as well as on the anchoring conditions at the inclusion boundaries. The tendency of such inclusions to form aggregates depends intimately on the properties of the inclusions as well as those of the LC, providing exquisite control of the self-assembled colloidal structures.1

A key feature of these systems is the presence of topological defects, which are singularities of the LC director field, close to the boundaries of the inclusions.2 2D smectic LC films, freely suspended in air and with a thickness corresponding to an integral number of smectic layers3, are a particularly interesting system for inclusion studies and they enable the observation of several features of inclusion interactions not previously found in either 2D or 3D. In a few-layer thick, freely suspended tilted smectic (SmC) liquid crystal film, the local azimuthal orientation of the molecular tilt f(x,y) is a two-dimensional XY field. An island on the film (a circular region of greater thickness), presents a strong azimuthal boundary condition at its edge that forces a strength +1 chiral vortex into the XY field (inside the island). Each such vortex is paired with a strength -1 vortex in the orientation field (on the background film), forming a chiral topological dipole (Figure 1a). Chiral dipoles with the same handedness form polar chains with the dipoles pointing in the same direction and along the chain, whereas dipoles of opposite handedness form novel quadrupolar structures shown (Figure 1b)4. Figures 2 and 3 show multiple left- and right-handed islands lattices and dipolar and quadrupolar pairs of chiral islands in a freely-suspended smectic film of achiral liquid crystal.

 

Figure 1. Chiral vortices in a freely-suspended smectic film of achiral liquid crystal. Depolarized reflected light microscopy (DRLM) images of a smectic C film of racemic compound showing (a) two islands containing +1 vortices with the same handedness and (b) two islands with +1 vortices of opposite handedness. The defects in (a) form a dipolar chain, while those in (b) form a topological quadrupole. The laser illumination is obliquely incident (in the x-z plane, shown by the arrow), allowing us to differentiate between left- and right-handed islands by comparing the brightness of the brushes in their upper and lower halves.

Figure 2. Multiple left- and right-handed islands interacting on a smectic C film of racemic compound. (a) Two quadrupolar units, each comprising a pair of left- and right-handed islands and two -1 defects. (b) An extended checker board-like structure formed by multiple left- and right-handed islands.

Figure 3. Chiral vortices in a freely-suspended smectic film of achiral liquid crystal. DRLM images of a smectic C film of racemic compound showing both dipolar (bottom), and quadrupolar (top) pairs of chiral islands.

 

References

[1] P. Poulin, H. Stark, T. C. Lubensky, and D. A. Weitz, Science 275 , 1770 (1997).
[2] Y. Gu and N. L. Abbott, Physical Review Letters 85 , 4719 (2000).
[3] C. Y. Young, R. Pindak, N. A. Clark, and R. B. Meyer, Physical Review Letters 40 , 773 (1978).
[4] N. M. Silvestre, P. Patricio, M. M. Telo da Gama, A. Pattanaporkrattana, C. S. Park, J. E. Maclennan, and N. A. Clark, Physical Review E 80, 041708 (2009).

Text and images contributed by Cheol Park and Apichart Pattanaporkratana.