Using heat-sensitive micro-capsules to make temperatures visible

Basic principle

If you scatter a small amount of liquid crystal capsules in water and shine a slit of light, the temperature of the water on the plane that is lit up becomes visible.

Liquid crystal capsules

The liquid crystal capsules used for visualization are made by putting liquid crystal that changes color as the temperature changes into resin micro-capsules. The capsules are only tens of microns in size, and they are shipped in a slush solution (mixed with water). When using them with oil, you first dry out this slush solution and use the resulting powder-like substance. They are not mass produced, so all production is on a made-to-order basis, and it is possible to make capsules with many different temperature reaction levels. Only small amounts are used for experiments, so batches of around 100 grams are sufficient, however production costs aren't really related to volume, so it costs approximately 100,000 yen (US$900) for each type of capsule made. (Made by Japan Capsule Products.)

Color characteristics

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Originally, these capsules were designed for uses such a mixing with paint in mind, so the data on color producing temperature ranges provided for reference by the maker are in terms of "the color produced on a black paper background with light reflected at 180 degrees (i.e. light shone from directly behind the observer)". In the experiment, diffused light reflected at approximately 90 degrees is observed, so the color producing temperature range is narrower than the maker's data, and tends to the lower temperature limit. In other words, the lower limit of temperature where color changes doesn't change much, but the upper limit drops considerably. Further, the color produced changes considerably depending on the angle of observation, so care is needed to get consistent results.

The trick to making temperature visible

Using this technique, if you put in too many capsules, the working fluid becomes cloudy, and it becomes impossible to get a clear view of the temperature ranges. The trick is to put in an amount so small that under normal lighting you can barely tell that capsules have been added. Making this small volume of capsules light up enough for the purposes of the experiment requires a very strong source of slit light.

Light source

A linear lighting system that uses fiber optic cables can be used as a light source compact enough to fit on a rotating table and strong enough for the purposes of the experiment. (Molitex) This is a device that passes light from a halogen light source through a bundle of fiber optic cables, which are formed into a line at the other end, and shine that light onto a cylindrical lens which condenses the light rays. You can get quite a strong light source using this system from a 50W light bulb. However, this lighting apparatus was originally designed to light up a linear section of a certain part of an automation system, so not much attention was paid to avoiding unwanted leakage of light. In order to prevent light other than the desired plane of light shining from the device, it is better to put another slitted panel in front of the cylindrical lens.