Freely suspended films of smectic liquid crystal

Freely suspended smectic films offer a wealth of information on the physics of liquid crystals in two dimensions. The first detailed studies of smectic films were made at Harvard in the 1970's1,2.

Drawing Films

Within an oven, set to the appropriate temperature for the desired phase, thin liquid crystal  films can be drawn over an aperture in a microscope coverslip (the film holder). The smectic layers within the film are aligned with the plane of the film holder and the layer thickness is quantized, i.e. fractional layers do not exist. The layer thickness is defined by the liquid crystal molecule and the temperature dependent phase. Regions of different thickness may exist in the film, isolated regions of greater thickness being referred to as islands, and isolated thinner regions as holes.

Film Thickness Measurement

Film thickness is measured via laser reflectivity, illustrated at right. For very thin films, the intensity of the reflected beam is, to first order, quadratic in the film thickness3.

 

 

 

Depolarized Reflected Light Microscopy (DRLM)

Incident polarized light is rotated in the direction of the larger index of refraction (i.e. along the c-director) when it is reflected from a tilted phase film. Electrodes evaporated onto the film holder allow an electric field to be applied across the film, providing a means of aligning the liquid crystal molecules. Depending on the dominant polarization, either parallel or transverse to the molecular axis, the c-director will be forced to align parallel or perpendicular to the electric field.Thus by decrossing the analyzer, the dominant polarization and c-director orientation may be determined 4.

Odd/Even Effects in Tilted Phase Films

At the air/ liquid-crystal interface there exists a preferential molecular orientation, whereas in the bulk there is no such up/down preference.This broken symmetry enables the determination of the structures in various phases based on odd/ even layering effects. For example, in an anticlinic SmCA film of an achiral material there is a net longitudinal polarization in even layered films, whereas in odd layered films the net polarization is zero. In the antiferroelectric SmCA* phase, with chiral liquid molecules in a similar anticlinic arrangement, even layered films again have longitudinal polarization, but odd layered films possess a transverse polarization. These effects are observed with DRLM and can be correlated with film thickness measurements5.

 

π & 2π Walls

Within a single layer domain the c-director may reorient through an angle of 180° or 360° in a feature referred to as a  π or 2π wall. These walls are characterized by stripes, when observed using DRLM, that indicate the gradual rotation of the c-director from one side of the wall to the other. In polar films these walls may be squeezed down by applied electric fields1.

References

[1] C. Rosenblatt, R. Pindak, N. A. Clark, R. B. Meyer, Physical Review Letters 42, 1220 (1979).
[2]  J. D. Litster, Physics Today 35, 5 (1982).
[3] C. Rosenblatt, N. M. Amer, Journal of Applied Physics 36, 432 (1980).
[4] D. H. Van Winkle, N. A. Clark, Physical Review Letters 53, 1157 (1984).
[5] D. R. Link, J. E. Maclennan, N. A. Clark, Physical Review Letters 77, 2237 (1996).

Text and images contributed by Andrew Cavanagh, Darren Link and Joe Maclennan.