Liquid Crystals

Liquid crystal is a unique organic material which exists between the solid and the isotropic liquid phase. In the context of temperature measurement, the material is in the amorphous solid phase below a certain temperature and a pure liquid beyond an upper limit. In between these temperature limits, it shows a certain molecular structure that resembles the crystalline state. Here, the incident light is scattered selectively and forms the basis of temperature measurement.

Liquid crystals possess a helical structure with a characteristic pitch. The pitch length of the helix is in the range of the wavelength of visible light. The pitch length changes with an external stimulus such as temperature. In a second family of liquid crystals, the molecular structure responds to the applied shear stress. The fundamental chemical structure is unaffected by the external stimulus and a liquid crystal coating can respond repeatedly to the physical change and can be used reliably as a temperature sensor. Since the technique involves using a (white) light source for illumination and a detector for recording the scattered light, LCT classifies as an optical technique.

There are two families of liquid crystal materials that have helical structures and are referred to as chiral-nematic and cholesteric. The molecular ordering here are quite distinct and change under the influence of electromagnetic fields, shear stress, pressure and temperature. Cholesteric crystals are suited for temperature measurement. Both the extent of the temperature range and its location in the temperature scale can be controlled by selecting the appropriate cholestric esters and their proportions in a given formulation. Liquid crystals are presently available for a temperature spectrum ranging from a few degrees below zero to several hundred degrees Celsius. A mixture can be obtained with event temperature spans as small as 1^o C to as large as 50^oC. Owing to their color changes with temperature, the cholestric liquid crystals are also called thermochromic liquid crystals (TLCs).