Cholesteric liquid crystal thermometers

As the pitch of many cholesteric liquid crystals is a function of temperature, this phenomenon has been exploited to make thermometers. 

The reflective wavelength of cholesteric liquid crystals varies according to temperature. Such an effect has been made useful in thermography. It has been applied in the diagnosis of cancers by displaying the skin temperature distribution. It has also been applied to test faults in integrated circuits. The applications also include thermometers and temperature warning indicators and non-destructive detection. 

Another type of thermotropic liquid crystals is called cholesteric liquid crystals. The color of cholesteric liquid crystals changes with temperature and therefore they are suitable for use as sensitive thermometers. In metallurgy, for example, they are used to detect metal stress, heat sources, and conduction paths. Medically, the temperature of the body at specific sites can be determined with the aid of liquid crystals. This technique has become an important diagnostic tool in treating infection and mmor growth (for example, breast tumors). Because locahzed infections and tumors increase metabolic rate and hence temperature in the affected tissues, a thin film of liquid crystal can help a physician see whether an infection or tumor is present by responding to a temperature difference with a change of color. 

Explain how cholesteric liquid crystals can be used as a temperature sensor (liquid crystal thermometer). How how does the phase change as a function of temperature ... 

Thermotropic cholesterics have several practical applications, some of which are very widespread. Most of the liquid crystal displays produced use either the twisted nematic (see Figure 7.3) or the supertwisted nematic electrooptical effects.6 The liquid crystal materials used in these cells contain a chiral component (effectively a cholesteric phase) which determines the twisting direction. Cholesteric LCs can also be used for storage displays utilizing the dynamic scattering mode.7 Short-pitch cholesterics with temperature-dependent selective reflection in the visible region show different colors at different temperatures and are used for popular digital thermometers.8... 

Temperature is measured by thermometers. The first temperaturemeasuring device was a special type of liquid thermometer, the thermoscope (discovered by Galilei in the sixteenth century). These days there are gas thermometers, liquid thermometers, infrared thermometers, liquid crystal (LC) thermometers (cholesteric), thermocouples, and resistance thermometers (these latter ones are the most important in thermal analysis). 

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