Manipulating smectic islands using optical tweezers
 

We are using optical tweezers to manipulate and study interactions between islands on freely suspended films of smectic liquid crystal. We focus our attention on smectic C (achiral) and smectic C* (chiral) films, where the molecular orientation is tilted with respect to the layer normal of the film, and where the molecular projection onto the film, called the c-director, can be visualized in a polarized light microscope.

 

 
  Figure 1. Molecular orientation of liquid crystal in a smectic C film. The c-director "nails" represent the projection of the molecules onto the film plane, with the bars being the ends of molecules closest to the top of the film.  

Smectic liquid crystals can be made to form freely suspended films, locally quantized in thickness by an integral number N of smectic layers, on which islands, circular regions of greater thickness than the surrounding film area, can be generated. In our experiments, the diameters of such circular islands are in the range 1 < R < 100 µm and they are typically up to five times thicker (N ~ 50 layers) than the surrounding liquid crystal film of Nf ~ 10 layers.

 
 
  Figure 2. A film with an island is freely suspended from a glass holder (side view).  
 
 
  Figure 3. Profile of a 6-layer island on a 3-layer smectic film. The additional smectic layers defining the island are thought to be encapsulated within the film, as indicated.  

In smectic C films, each such island is accompanied by a topological defect pair, an s = +1 topological defect inside with an s = -1 defect nearby on the background film . These defects are created by tangential anchoring of the c-director at the edges of the islands. The resulting distortions of the in-plane orientational order of the smectic C director field result in elastic interactions between the islands, with a short-range repulsion and a long-range dipolar attraction governing their stability and leading to their organization in chain-like structures with an equilibrium island separation distance.

   
  Figure 4. Islands on a Smectic C* film seen under decrossed polarizers in a reflected-light microscope (top view). The c-director aligns tangent to the edge of islands. This boundary condition creates +1 defects inside the islands and - 1 counter defects nearby on the background film. Smectic C islands are believed to interact through these topological defects.  

Smectic islands can be manipulated using optical tweezers.1 Using an acousto-optically scanned CW YAG laser system to generate dynamically controllable, multiple optical traps, we have directly measured the repulsive and attractive elastic interaction forces between smectic C* islands and have compared the results quantitatively with theoretical calculations. Preliminary results show that interactions between chiral smectic C* islands are much higher than those between achiral smectic C islands. In fact, we rarely observe island chaining on achiral smectic C films. This surprising result contradicts the current elastic theory of inclusions on smectic C films.2

References

[1] “Manipulation of disk-shaped islands on freely suspended smectic films and bubbles using optical tweezers ,” A. Pattanaporkratana, C.S. Park , J.E. Maclennan, and N.A. Clark, Ferroelectrics 310 , 275 (2004).
[2] D. Petty, T. C. Lubensky, and D. R. Link, Liquid Crystals 28, 579 (1998).

Text and images contributed by A. Pattanaporkratana and C. S. Park.