Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 193

It is possible for chiral mesogens to produce essentially achiral mesophases. For
instance, in certain ranges of concentration and molecular weight, DNA will form an
achiral line hexatic phase. A curious recent observation is of the formation of chiral
mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes
called banana LCs) have been shown to form liquid crystal phases that are chiral. In
any particular sample, various domains will have opposite handedness, but within
any given domain, strong chiral ordering will be present.
9.4.1.3.2 Industrial Applications of Liquid Crystals
LCs find wide use in displays, which rely on the optical properties of certain liquid
crystalline molecules in the presence or absence of an electric field. In a typical
device, an LC layer sits between two polarizers that are crossed (oriented at 90° to
one another). The liquid crystal is chosen so that its relaxed phase is a twisted one.
This twisted phase reorients light that has passed through the first polarizer, allowing
it to be transmitted through the second polarizer and reflected back to the observer.
The device thus appears clear. When an electric field is applied to the LC layer, all
the mesogens align (and are no longer twisting). In this aligned state, the mesogens
do not reorient light, so the light polarized at the first polarizer is absorbed at the sec-
ond polarizer, and the entire device appears dark. In this way, the electric field can be
used to make a pixel switch between clear or dark on command. Color LCD systems
use the same technique, with color filters used to generate red, green, and blue pixels.
Similar principles can be used to make other LC-based optical devices.
Thermotropic chiral LCs whose pitch vary strongly with temperature can be used
as crude thermometers since the color of the material will change as the pitch is
changed. LC color transitions are used on many aquarium and pool thermometers.
Other LC materials change color when stretched or stressed. Thus, LC sheets are
often used in industry to look for hot spots, map heat flow, measure stress distri-
bution patterns, etc. The LC in fluid form is used to detect electrically generated
hot spots for failure analysis in the semiconductor industry. LC memory units with
extensive capacity were used in Space Shuttle navigation equipment.
It is also worth noting that many common solutions are in fact LCs. Soap, for
instance, is an LC, and forms a variety of phases, depending on its concentration in
water (Friberg, 1976; Somasundaran, 2006).
9.5 aPPlIcatIonS oF emulSIonS
The area of emulsion applications is very large and cannot be described in the space
available here. However, some important areas will be highlighted to indicate the
basic criteria for emulsion technology (Friberg, 1976; Gitis and Sivamani, 2004).
9.5.1 p e r S o n a l c a r e I n d u S T r y
Human skin is the outer layer of the body, that covers and protects it from any expo-
sure to the surroundings (wind or rain, etc.). The natural substances that compose the
skin are very elaborate and complex. Further, the composition of the skin changes
with age, and is different for different people. This also true for the animal world.