Free standing film, smectic

The formation of the TGBA phase from the isotropic liquid can also be observed in free standing films. The defects formed in this case tend to be filamentary in nature, see Plate 8. Such a texture is not normally seen in free standing films of other liquid crystals, and therefore the presence of this defect texture is diagnostic for the characterization of TGB phases. In addition to the observation of such filamentary textures in free standing films, filaments were also observed embedded in homeotropically aligned smectic A phases, see Plate 9. 

Figrrre 9.7. A cross-section of a free-standing film of a smectic A phase. 

In spin-coated or free-standing films, the smectic layers are aligned parallel to the surface [163, 164]. In such a homeotropic arrangement the tilt direction is not defined and the polar axis of different domains is randomly distributed over the film. If the substrate is water-soluble, it can be dissolved and freely suspended films can be obtained [165]. 

In conclusion, the physical origin of the surface order in free-standing films is not fully understood. It seems that the smectic C (see Sec. 4.6) as well as the hexatic B smectic phases are stabilized by a free surface with respect to the smectic A phase (see data reported above). X-ray measurements have shown that the layer fluctuations become very small close to the surface of the films [128], and some theoretical arguments [129] are in favor of a quenching of the layer fluctuations, resulting in an enhancement of the hexatic surface order. 

These uniform free-standing films offer a well-defined smectic layer structure in which the layer normal is perpendicular to the plane of the film. As a result, the films have proven to be very beautiful systems for investigating numerous physical properties, in particular two-dimensional behavior, related to various smectic mesophases. 

To reduce the free energy contributed by the surface tension term, the molecules at the liquid crystal/vapor interface favor a layer structure. In the smectic phase, the outermost layers favor a better molecular packing than exists in the interior. The enhanced surface order has been reported for various liquid crystal phases, for example the surface SmA order on the bulk isotropic or nematic sample [50] the surface SmI order on a SmA film [47] the surface SmB gx order on a SmA film [45,48] the surface SmI on a SmC film [17,93] the surface B on a SmA film [49] the surface crystal E order on a SmBhex film [100]. Realizing the importance of the surface tension in characterizing the liquid crystal free-standing films, we... 

As early as the 1920s, Georges Friedel recognized that one could prepare free-standing films (just like soap films) from smectic liquid crystals. (That could be the reason for calling them smectic, which means soap in Greek). A simple technique was developed, and systematic studies were started at... 

In the effort to make free-standing films of the first B1 material it was found that instead of films they form free-standing strands [25] just like columnar liquid crystals of disc-shape molecules [95], Such observation was confirmed with other B7 materials [98], and later it was found that the B7 and B2 type banana-smectics also form strands of fibers [99]. Although the most stable fibers are the B7 materials with slenderness ratio as large as 5000, the B7 and B2 fibers also have aspect ratios over 1000 and 100, respectively. The values are orders of magnitudes larger than of the Rayleigh-Plateau limit [100] of Newtonian isotropic fluids and of nematic and smectic liquid crystals of rod-shape molecules [96]. 

These considerations have led to several attempts to measure the thickness change of films in dependence on an applied electric field. In a first experiment, a Michelson interferometer was used for the determination of the thickness change in a 75-nm thick free-standing smectic film of FLCE2.1 (Scheme 1). With this method, a strain of 4% at a field of 1.5 V/pm was measured [170], which would correspond to a = 55 x 10 In a second experiment (with the similar... 

Theoretical models of free-standing smectic films have been recently developed [15] that extend the smectic bulk free energy to include the effect of the surface tension y at the free surfaces. For an (N+ l)-layer freestanding film, the free energy can be written as... 

There are a number of reasons for the interest in free-standing smectic membranes. First, thin smectic membranes are models of two-dimensional fluids (SmA and SmC) and crystals (SmB). By varying the film thickness, one therefore can study the crossover from three- to two-dimensional behavior as well as the influence of surfaces on the morphology and the phase behavior. 

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