A Thermochromic Mug. Image by Damianosullivan via Wikicommons CC BY-SA 3.0
About last month, some couple of my long time friends flew into town just to groove (enjoy themselves) a little, and as it was out normal practice back in the day that once a shows up, every other person available turns up. And so it was we were all gathered at the home of one of the guys at a government residential area (G. R. A). It was morning, we were all having tea, just as tea was turned into the different mugs, i notice something quite amazing -- it was an inscription which appeared on the hitherto plain black mug, it was spelt out: ENJOY YOUR TEA. This inscription appeared on all five mugs as the tea was carefully turned into it, but then it disappeared into thin air as tho it was never there and all i could see was the black mugs back.
Believe me, this was amazing but at the same time popped a whole lot of questions in my head. I couldn't question my girls didn't want to feel like the odd one out because everyone present seemed to be quite knowledgeable about it. Well, after that day, i did some little research of my own and i have come to understand that what transpired on the tea mugs were just an example of heat-sensitive paint and thermochromism.
Well just a tip off -- the paint which was used was used in coating the mugs we used in taking tea are called heat-sensitive paint and they are also used for so many other things like the mood ring (whose colour not just mugs. Now let's find out what thermochromism is.
Thermochromism -- What Is It All About?
Clip art by me
Way back secondary school, i have always loved words which ended with 'ism', they sounded kind of ambigious, thus i often resorted to my dictionary to build and strengthen my vocabulary ...and they always ended up with relatively simple meaning, thermochromism isn't any different.
To comprehend fully what thermochromism entails, let's take a look at what chromism means. According to Wikipedia
chromism is a process that induces a change, often reversible, in the colors of compounds. In most cases, chromism is based on a change in the electron states of molecules, especially the π- or d-electron state, so this phenomenon is induced by various external stimuli which can alter the electron density of substances.
Thermochromism isn't very different from what chromism is as it also falls within the ambit of chromism. In simple words, Thermochromism is the property of a particular material to change it's colour as a reaction to temperature change. Moving on, thermochromism is the science upon which heat sensitive colours operate. The paint which is seen in mood rings are heat sensitive paints, in other words, they are temperature sensitive (they react to changes in tempreature). You would readily find these heat sensitive paints in sophisticated cars. How do these heat sensitive paint work, lets find out.
Mechanism Behind Thermochromic Material Or Heat-Sensitive Paint
Heat sensitive paints are also thermochromic materials, with the aid of some pigments, they are able to modify their inherent colour, this is in line with the change in tempreature around it. Thermochromic liquid crystals and leuco dyes are the two pigments responsible for the change in colour in these thermochromic materials, lets take a look at the in details.
Nematic liquid crystal. Image by Ingo Dierking via Wikicommons CC BY-SA 3.0
Thermochromic Liquid Crystals --Most of us made use of pocket size calculators with LCD (liquid crystal display) display while in school, well for this, we are going to be needing to take our minds back and picture those calculators to get a clearer picture and understanding of how thermochromic liquid crystals work. Given that both terms liquid and crystals make this up, one might thinknof this are a firm bubble but they are happen to liquid in some sort and solid in the other. The point of interest here is a phase called nematic -- the molecules here are arranged to take the form of matches in a box and all are roughly pointing towards the same direction. Once a little ray of light shines on these nematic crystals, some of it would counter reflect back, this form of reflection is known as iridescence -- its quite the same with the surface of bubbles (try to picture this).
When light hits on these crystals, they add together by a process called interference, which produces some sort of reflection. What determimes the colour of the reflection depends on the spacing of the crystals. In other words, the liquid crystals would assume a different color depending on the temperature in they are, this is due to the fact that any bit of tempreature modification would bring about a shift in their positions, either apart ot closer. Given the nature of thermochromic liquid crystals, they provide an accurate measurement of tempreature with the ambit of certain bands thus they are used majorly for high precision works such as strip thermometers. For mugs, let's take a look at leuco dyes.
Photochromic dye transition. Image via public domain Wikicommons CC BY-SA 3.0
Leuco Dyes -- Just like me, have you ever come in contact with those mugs which happens to change colour or reflect something else when exposed to content of different temperature or perhaps a printed poster which colour is altered when touched? Well stuffs like this are just typical examples of what leuco dyes can bring about -- it just brings about marked colour differences when the temperature changes.
According to [Wikipedia]
Leuco dye (from the Greek λευκός leukos: white) is a dye which can switch between two chemical forms; one of which is colorless. Reversible transformations can be caused by heat, light or pH; resulting in examples of thermochromism, photochromism and halochromism respectively.
Leuco dyes are less complicated as to when compared with thermochromic light crystals. Given that leuco dyes are carbon based organic chemicals which simply changes color whenever tempreature around it changes. Tempreature modification makes their molecules toggle back and forth between the leuco (which is colorless) and non-leuco (which is colored) forms. The leuco and non-leuco forms absorbs and reflect light quite differently, so also, they show forth quite different colors when printed on a material.
References
How does heat sensitive paint work -- The naked scientists
How thermochromic ink work -- sfxc.co.uk
