Liquid crystals were first discovered over 100 years ago in studies of cholesterol and related molecules. Liquid crystals are a fourth state of matter; they have greater ordering than more normal liquids but less ordering than crystalline solids. All phase changes have a characteristic phase transition temperature for a given set of conditions. Liquid crystals have separate transition temperatures where they change into “normal” liquids (melt) and into “normal” solids (solidify).
Liquid crystals have become very common in the last 20 years as displays for electronic devices. This is the result of the unusual optical and electrical properties of liquid crystals. The long thin liquid crystal molecules cause light to travel at different speeds along the molecular axis and perpendicular to that axis. This leads to their ability to rotate the plane of polarized light (see below).
• How they work
These long thin molecules also have a tendency to align parallel to an applied electrical field. This response and the optical properties of liquid crystals lead to their application in various electronic devices ranging from watches and calculators to computers and televisions.
• Polarizing Filters and Polarized Light
As shown in Figure 1, polarizing filters are materials that allow only the passage of light waves whose electric fields are oscillating in a particular plane.
Figure 1. Initially unpolarized light passing through a polarizing filter becomes polarized. The polarized light will not be transmitted by a second polarizing filter turned at 90o to the axis of the first filter.
Unpolarized light passing through a polarizing filter becomes polarized. Polarized light directed onto a second polarizer, oriented 90o to the first polarizer, will not be transmitted. Many natural substances, including liquid crystal materials, have the ability to rotate the plane of the polarized light.
For more information access the Wisconsin MRSEC website: https://mrsec.wisc.edu/