Phase transitions in liquid crystals
Phase transitions in liquid crystals
A liquid crystal is a substancl that flows like a liquid but still maintains some of the ordered structure characteristic of crystals. Materials that qualify as liquid crystals are of anisotropic shape (usually rod-shaped or disc-shaped), and are called mesogens. Their shape is of particular importance because these material undergo ordering phase transitions. Liquid crystals can be divided into two broad categories: thermotropic and lyotropic. Thermotropic liquid crystal phases are formed by pure mesogens. Here phase transitions are a function of temperature and heat is either consumed or generated during the transition. Lyotropic liquid crystal phases arise from mesogens in a solvent. Here concentration controls the phase transition. Some of the most prominent liquid crystal phases are the following:
Nematic Phase: In the nematic phase the mesogens all point in the same direction, essentially expressing the molecular anisotropy as a phase anisotropy. This preferred direction of orientation is simply called the director of the nematic state. Experimentally the director can be observed with polarization microscopy or birefringence experiments.
Smectic Phase: In this phase the mesogens have orientational as well as positional order. This means that not only do they point in the same direction, but they are also orderly layered perpendicular to the director. In a smectic A phase the molecules are, on average, normal to the layers. In a smectic C phase, the director is tilted with respect to the layers.
Chiral Nematic (Cholesteric) Phase: In a cholesteric phase the nematic molecules have chirality, i.e. they have a preferential twist with respect to one another. The cholesteric phase was named after the cholesterol molecule, where this structure was first observed!
Chirac Smectic (Smectic C*) Phase: In the chiral smectic phase, the tilt direction of the mesogens rotates as one progresses through the layers.
Perhaps the most popular application of liquid crystals is the LCD srceen , short for Liquid Crystal Display. In this application, the anisotropy of liquid crystal molecules is exploited, since it leads to a susceptibility to electric and magnetic fileds. A less obvious and somewhat surprising application of the liquid crystal doctrine is in the biological field. The phospholipid and water mixture that forms the cell membrane is thought to be a liquid crystalline phase. Moreover, numerous cellular protein components are rod-shaped and in in vitro studies undergo isotropic to nematic phase transitions at critical concentration.
Text adapted from: Hampley, I.W., 'Introduction to soft matter', Willey & Sons, England (2007)
Polymer Dispersed Liquid Crystals
In polymer dispersed liquid crystal devices (PDLCs), liquid crystals are dissolved or dispersed into a liquid polymer followed by solidification or curing of the polymer. During the change of the polymer from a liquid to solid, the liquid crystals become incompatible with the solid polymer and form droplets throughout the solid polymer. The curing conditions affect the size of the droplets that in turn affect the final operating properties of the "smart window".
Typically, the liquid mix of polymer and liquid crystals is placed between two layers of glass or plastic that include a thin layer of a transparent, conductive material followed by curing of the polymer, thereby forming the basic sandwich structure of the smart window. This structure is in effect a capacitor. Electrodes from a power supply are attached to the transparent electrodes. With no applied voltage, the liquid crystals are randomly arranged in the droplets, resulting in scattering of light as it passes through the smart window assembly. This results in the translucent, "milky white" appearance. When a voltage is applied to the electrodes, the electric field formed betwen the two transparent electrodes on the glass cause the liquid crystals to align, thereby allowing light to pass through the droplets with very little scattering, resulting in a transparent state. The degree of transparency can be controlled by the applied voltage. This is possible because at lower voltages, only a few of the liquid crystals are able to be aligned completely in the electric field, so only a small portion of the light passes through while most of the light is scattered. As the voltage is increased, fewer liquid crystals remain out of alignment, resulting in less light being scattered. It is also possible to control the amount of light and heat passing through.
When a sufficient voltage is applied, the PDLC becomes transparent and allows light to pass, while the PDLC is opaque and scatters light without an applied voltage. The PDLC is suitable for an electrooptic material of a light modulation device used in high-brightness projectors, because it exhibits high optical transparency without a polarizer.