The other day it was convocation time here at IIT Kanpur, where I teach. The institute, the students, faculty, and all the guests who came were all dressed up in twinkling lights and smiles. An outgoing batch was to be congratulated, given some life advice, and finally their degrees.
My department head gave me a rather hurried call just the evening before “Adhip, there is a possibility that some of us may have to attend the formal programme sitting in the first few rows in the main auditorium – would you be able to sit there?’
Apart from my inability to stay awake during formal speeches, the main worry here was to find a clean and presentable set of clothes that I could wear for the occasion. My treasure hunt for a clean kurta in my apartment did lead to finding one - the only problem was that it was wrinkled beyond recognition. As you might be aware, wearing a wrinkled dress to a formal event, such as a convocation, could be considered a crime. So it was that at close to midnight, I switched on my ironing machine and decided to give my kurta a makeover.
The iron press
The iron press is a wonderful machine. There are two important ingredients to this wonder: it has a heavy metal base (often some alloy of iron, thus the name) and that it heats up. Now, have you ever wondered why our clothes are straightened when ironed?
Our clothes are made of some plant or animal fibres, like cotton, wool, etc. Fibres are long chain-molecules often made of carbon, oxygen, and hydrogen – very similar to the glucose and sugar that we eat (this doesn’t mean cotton is edible, however tempting your clothes may seem).
These molecules can be really, really long. If the water molecule, which is made of two hydrogen atoms and an oxygen atom, is the size of a pencil, a typical cellulose fibre, one of the primary molecules of cotton, would be about 100 metres long! These kinds of molecules are called polymers. Polymers make up many things around us, including plastic, soap, and even tomato ketchup. There are synthetic polymers (made in a lab, like plastic) and then naturally occurring ones, such as cotton.
When you wash your clothes, these molecules become knotted around each other – like noodles when you cook a packet, or your hair if you still have the privilege of having long hair (unlike most faculty members and a few students here). This is often because of the water molecules that become stuck between and around these molecules.
When you iron your cloth, what you are really trying to do is rearrange these long molecules into neat, straight patterns. In the process, some of these water molecules escape and evaporate. You will at times notice a damp feeling when you are ironing a cloth. Now you know why.
Polymer physics
This said, if you just place a heavy weight on cloth and try straightening it, it won’t work. Why is heat required to achieve this?
Heat is nothing but energy: it causes all the atoms and molecules to vibrate a bit and that does the trick. As we know, each of these cellulose molecules is very long – and they can have various twists and turns along their length, like one strand in a noodle.
Once in a particular shape, all the atoms and electrons inside them arrange themselves in a way that they can have as low energy as possible. But there can be another way of arranging similar twists and turns in the same system – like another noodle in your pan – with a slightly different energy or even lower than the previous way.
However, once stuck in a particular pattern, it is very difficult for all the atoms to move together. They have very little energy to do so as well as little wiggle room. The system is basically in some intertwined state, similar to cold noodles or wet hair. You may have experienced this difficulty when you tried to move a fork inside the noodles or to move a comb through tangled hair.
When you provide heat, these molecules get some energy such that the molecular bonds vibrate and change their shape. In the process, some of the water evaporates out. This is again similar to what you do when you want to fill up some food grain in a container and there is not enough space. You shake the box a little bit so that the grains move around, settle down, and you find some more free space.
Similarly, here, you provide some heat (or microscopic ‘shaking’) to cause the molecules to make some rapid twist and turns. It is at this moment that you apply pressure with the heavy metal, removing the twists and turns and straightening the fibre up. Since the molecular twists and turns are somewhat flexible when heated, they will do as you want. As soon as you take the iron away, the molecules once again cool down and get stuck in the straight pattern you had left them in, leaving your cloth wrinkle-free!
Prof. Deepak Dhar
Of course, the heat has to be such that they only move the atoms around, not so much that they break the molecular bonds. Otherwise this is how you may end up with a burnt hole in your dress.
In fact, different polymers have different temperatures at which they shake up the best, so different temperatures at which they can be most efficiently straightened. This is why there is a temperature dial on the iron. You will notice different cloth types marking how hot the iron should get, depending on the polymer you are ironing. This is because the polymers have different chemical compositions.
For example, everyday plastic melts quite easily – which is why people avoid putting hot food in a polythene container. Well, that night my ironing did work and luckily I woke up in time to attend the programme.
I heard the Infosys founder Mr. Narayan Murthy in the morning and Prof. Deepak Dhar in the afternoon – both guests at the convocation here.
Interestingly, Prof. Dhar is one of India’s most celebrated statistical physicists; statistical physics is a subfield of physics where people study things such as polymers, which he had also done in his own research. If you want to learn more about his research and his life, you can read his wonderful interview on the IIT Kanpur blog, where he also talks about his life as a student in the physics department.
The next time you dress up for a party or an occasion – such as a convocation – and take a pair of ironed clothes, just remember how the wonderful physics of iron presses and polymers are doing their sincere bit to create those joyous moments for you.
Adhip Agarwala is an assistant professor of physics at IIT Kanpur.
