Thermometers liquid crystal

Mood rings change color based on your body temperature. The piece that changes color is a liquid crystal thermometer, which is the same technology that is used in some disposable medical thermometers as well as the adhesive thermometers commonly used in aquariums. 

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 ... 

Reliable micro-scale measurement and control of the temperature are required in developing thermal micro-devices. Available measurement techniques can be largely classified into contact and non-contact groups. While the resistance thermometer, thermocouples, thermodiodes, and thermotransistors measure temperature at specific points in contact with them, infrared thermography, thermochromic liquid crystals (TLC), and temperature-sensitive fluorescent dyes cover the whole temperature field (Yoo 2006). 

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... 

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

Liquid crystals are now part of our everyday life. They are widely used in displays for digital watches, calculators and lap-top computers, and in televisions (Figure 1.8). They are also useful in thermometers because liquid crystals change colour as the temperature rises and falls. 

Thermal transmission methods are relatively new techniques for adhesive inspection. Heat flow is determined by monitoring the surface temperature of a test piece a short time immediately after external heating or cooling has been applied. Subsurface anomalies alter the heat flow pattern and, thereby, affect the surface temperature. The surface temperature difference can be detected by thermometers, thermocouples, or heat-sensitive coatings. Liquid crystals applied to the joint can make voids visible if the substrate is heated. 

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. 

Ireland PT, Jones TV (1987) The time response of a surface thermometer employing encapsulated thermochromic liquid crystal. J Phys E Sci Instrum 20 1195-1199... 

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