Smectic membranes in aqueous environment

Abstract

We present a study of thermotropic smectic liquid crystal films in aqueous environment. Macroscopic freely suspended films in water with a size up to 7.4 × 15 mm2 were prepared with the help of a surfactant, which ensures a strong homeotropic anchoring at liquid crystal/water interfaces. The films were studied by optical microscopy and ellipsometry. Attention was paid to the stability and the thinning transitions which occurred at temperatures above the bulk smectic- A -isotropic transition temperature. In addition, we investigated the formation and rupture kinetics of thin smectic membranes separating water droplets in microfluidic devices. Besides possible applications in discrete microfluidics, smectic films in aqueous environment may expand the general range of possible studies of freely suspended smectic films.

Surface tension, disjoining pressure and layer compression of free-standing smectic films in water

“…Notice that according to the experimental data [4], the thin cyanobiphenyl smectic film immersed in water ruptures when it is heated to ≈2 K above T AI (bulk) ≈ 323 K, or at T(R) ≈ 325 K. This result strongly suggests that the first layer-by-layer thinning step 9 → 8 should take place at minimal positive value of  =  AI (N = 9) −  AI (bulk) (see Ref. [12], Figure 1(a) and (c)).…”

“…It should be pointed out that most LC molecules tend to align planar at the interface to an immiscible liquid, such as water, thereby preventing the generation of freely suspended films in their usual configuration in which the smectic layers are parallel to the two film surfaces. However, a homeotropic anchoring at LC/water interfaces can be attained by the use of suitable surfactants [4]. Moreover, by controlling the amount of surfactant adsorbed at the interface, it is possible to tune the anchoring strength W 0 , i.e., the strength of the ordering surface field.…”

“…Competition between surface and finite-sized effects leads to unusual physical properties of free-standing smectic films (FSSFs), both in air and water environments [1][2][3][4][5]. In particular, it has been shown that the surface film tension acts to promote smectic order at the surface resulting in the surface layers ordering at a higher temperatures than that in the interior [3,6].…”

“…Recently, the thinning effect in thin smectic films, composed of decylcyanobiphenils (10CB) and dodecylcyanobiphenils (12CB) molecules in water on heating to the isotropic temperature has been observed [4]. It was shown that these stable smectic films can be prepared in water with the help of a surfactant that induces homeotropic anchoring of LC molecules at the film/water interfaces.…”

“…It was shown that these stable smectic films can be prepared in water with the help of a surfactant that induces homeotropic anchoring of LC molecules at the film/water interfaces. These films undergo a one-step thinning transition in which the film thickness decreases in a stepwise fashion until the films rupture at ∼2 K above T AI (bulk) [4]. This behavior is different from the case of FSSFs in air: on heating to the isotropic phase, the FSSFs in air either just rupture, for the vast majority of LC compounds, or show a layer-by-layer thinning transition, for LC compounds possessing the partially fluorinated alkyl chains.…”