Nanocharacterization of Lyotropic Liquid Crystalline Systems - Self-Assembled Supramolecular Architectures: Lyotropic Liquid Crystals - page 99

Figure 4.2 Optical textures observed under crossed polarizers for lyotropic liquid
crystals from the classical work of Rosevear (Rosevear, 1968): (1-3) lamellar phases
(denoted “neat” phases in the original work) and (4-6) hexagonal phases (denoted
“middle” phases in the original work).
Isotropic phases (e.g., cubic and micellar phases) do not alter light polariza-
tion and hence appear dark under cross polarizers. In such systems, it is often
still possible to distinguish phases from one another based on qualitative
observation of phase viscosity by, for example, observing the appearance of
entrained bubbles. Specifi cally, high-viscosity phases (e.g., cubic phases) typi-
cally inhibit bubbles from adopting a fully spherical geometry.
A particularly useful implementation of polarized light microscopy is the
water penetration or water fl ooding experiment. Here, a small sample of the
amphiphile of interest is placed on a microscope slide, and a cover slip is used
to form a sandwiched disk of amphiphile. A small quantity of water is then
introduced at the edge of the cover slip, and capillary force draws water toward
the sample. The microscope is focused at a position where the water contacts
the sample. A surfactant-water concentration gradient is established at this
interface, which results in the appearance of different phases depending on