Liquid Crystal Thermometers

In a Nutshell

Liquid crystal thermometers tint in response to temperature because the pitch of their helix-shaped crystals changes. Specifically, the pitch represents the distance between each “rung of the ladder.”

Notably, this color change is reversible, meaning the crystals respond equally well to both increases and decreases in temperature. Consequently, this provides a cost-effective and continuous high-resolution reading with a latency of just a few seconds.

In addition, liquid crystal thermometers offer a safer alternative to mercury and ‘Galinstan’ (gallium, indium, and tin) models. Moreover, they are easier to operate than infrared thermometers, since the latter rely heavily on accurate device pointing.

Thermotropic vs thermochromic liquid crystals

Since we are talking about temperature changes and changes in material properties, let’s clarify a couple of important terms [4]:

• Thermochromic liquid crystals exhibit a change in colour when the temperature changes. (From ‘khroma’, Ancient Greek for ‘colour’).
• Thermotropic liquid crystals exhibit a change in transparency. (From the word ‘tropic’ which comes via Latin from Ancient Greek ‘tropē’, meaning ‘to change direction’).

Both effects can exhibit themselves simultaneously within the same material and both are reversible.

How liquid crystal thermometers work

The liquid crystals we find in thermometers are in the ‘cholesteric’ phase. This means that they form parallel layers, rotated with respect to one another and somewhat similar to a ‘spiral staircase’.

The image below shows the various phases of liquid crystals, shown below from left to right:

• smectic
• nematic
• cholesteric

These correspond to the phases through which a liquid crystal changes with increases in temperature (going from left to right).

In other words, when cold, a liquid crystal will typically be in the smectic phase. As the temperature rises, it will transition into the nematic phase, then to the cholesteric phase (also called the ‘chiral nematic’ phase).

After that, any further temperature increases will turn the material into an isotropic liquid.

Phases of liquid crystals, appearing in the left-to-right order corresponding to temperature increase

For cholesteric liquid crystals, the helical structure expands with temperature, much like the stretching of a spring. When the distance between consecutive layers matches certain wavelengths of light, colours are produced matching those same wavelengths by diffraction of the incident light beam.

The temperature changes can be the result of thermal conduction, convection or radiation.

These changes in colour are sensitive to excessive ultraviolet light and extreme temperatures, which can cause liquid crystals to degrade.

Liquid crystal thermometer strips

The most common form factor for liquid crystal thermometers is a self-adhesive strip which contains a series of temperature-sensitive elements with microencapsulated thermochromic liquid crystals sitting on a black background.

Each liquid crystal element has been engineered with a defined pitch (using additives) so that successive elements ‘turn green’ with step changes in temperature. Thus, as the temperature rises, these elements ‘light up’ to reveal the number that is hard-wired under them.

The liquid crystals are thus black below (and above) their designed temperature range, and produce the colours of the rainbow, passing through the midpoint green, as the temperature rises or falls within the temperature range.

As one element of liquid crystal turns green, the one above it may turn slightly tan coloured and the one below it may become slightly blue, depending on how the molecules have been engineered.

Telatemp liquid crystal thermometer strip, image attribution

The image below shows the breakdown of a typical liquid crystal strip showing the liquid crystals encapsulated in the middle layer. The white graphic print has prefixed numbers and the black backing film is placed under the liquid crystals.

The liquid crystals are of course the only part of the structure that actually changes colour, illuminating the white numbers located above them, one by one, as the temperature increases.

Thermochromic liquid crystal strips, by Spotsee, image attribution

Real-World Applications

Real-world applications of liquid crystal thermometers outside of medicine include monitoring of:

• refrigeration rooms
• industrial processing plants
• automotive workshops
• pharmaceuticals
• oil & gas
• freight forwarding
• and mining.

Some manufacturers customise products according to client needs, for example adjusting temperature ranges between 0º to 30ºC, 60ºC to 90ºC, or 25ºC to 100ºC, depending on the application.

Hospital Case Study

A study was conducted in a paediatric ward at Port Moresby General Hospital (Papua New Guinea) on a sample of 200 children suffering from feverish symptoms. Staff used liquid crystal thermometers in parallel with mercury glass thermometers to compare accuracy and ease of use.

Body temperature is a crucial component of patient health and the mercury glass thermometer is the de facto standard.

However, mercury glass thermometers are fragile and glass breakages can release toxic chemicals, with subsequent injury to skin and mucous membranes.

As a result, mercury glass thermometers are often replaced by tympanic membrane thermometers in some developed countries, but are quite expensive suffer from variable accuracy [1].

Hence the interest in lower-cost temperature measurement solutions for developing countries, such as Papua New Guinea.

In the study, medical staff valued liquid crystal thermometers for their ease of use, reusability, accuracy, safety and robustness. Further benefits include the lack of toxic materials, easier storage requirements and no calibration needed.

The study concluded that liquid crystal thermometers are in fact equivalent to glass mercury and tympanic thermometers, with accuracies between 0.1ºC to 0.2ºC.

Some disadvantages of liquid crystal thermometers include the necessity to read their temperature immediately on removal from the patient. They do also require acceptable lighting and good eyesight in order to read the device.

References

1. Comparison of the Use of Liquid Crystal Thermometers with Glass Mercury Thermometers in Febrile Children in a Children’s Ward at Port Moresby General Hospital, Papua New Guinea, Journal of Tropical Pediatrics, URL

2. Liquid Crystal Lab Notes, Yerkes Winter Institute, URL

3. Thermotropic and Thermochromic Polymer Based Materials for Adaptive Solar Control, National Library of Medicine, URL

4. Thermochromic and Thermotropic Materials, Arno Seeboth, Detlef Lötzsch (2013), ISBN 9789814411028, URL

5. IUPAC Compendium of Chemical Terminology, or ‘Gold Book’, International Union of Pure and Applied Chemistry, URL

6. Preparation of a Liquid Crystal Thermometer, Univ. of Wisconsin – Madison, URL

7. Thermotropic Liquid Crystals for Temperature Mapping, National Library of Medicine, URL

8. Liquid Crystal Modelling, University College London, URL