How diapers use quantum physics to hear the call of nature | Explained

How diapers use quantum physics to hear the call of nature | Explained

Still wondering what you should gift your friends who have recently become parents? Think no more and buy a load of diapers. This sleep-inducing, time-relieving and calming device often provides young parents with the only solace amid the screaming and food-throwing of their adorable babies. Simply wrap this soft, powdery white packet around the baby's butt, and from a few hours to a whole night it will ensure that the baby – and consequently you – will be able to get some more sleep.

But have you ever wondered how a diaper absorbs so much liquid? We all know that some things around us can absorb water, like our cotton clothes, and some things that don't, like our rubber shoes. What is it about a diaper that absorbs so much water?

It contains some of the most beautiful examples of physics that we see around us. But before we do that, let's find out why some things absorb water and some things don't.

The Cotton Affair

Whether something absorbs or repels water has to do with subtle forces and the nature of matter. Water, that ubiquitous, life-giving fluid we have become so accustomed to, is a remarkable thing. Its smallest component is a molecule made up of two hydrogen atoms and one oxygen atom. While every atom here is charge-neutral – that is, it has an equal number of positively charged protons and negatively charged electrons – something strange happens. Two electrons, one from each hydrogen atom, decide to shift slightly toward the larger oxygen atom (due to the pull exerted by the latter). Thus the oxygen atoms become more negatively charged and the hydrogen atoms more positively charged.

Thus we can think of each molecule as a big-headed monkey with two arms.

When you place your cotton handkerchief on the water that you accidentally spilled on the table, the water molecules meet the molecules of the cotton fibers. These fibers are large networks of molecules called polymers, and they have a bunch of little positively and negatively charged ions all over them. So as you place the cotton on the water, the water molecules look at this large network like a herd of hungry monkeys meeting a dense forest of trees.

The water molecules now experience a force that makes them move towards ions, climbing up different molecules of cotton. In this process water is absorbed and your handkerchief becomes wet.

This also means that whether a material will absorb water or not depends on the kind of ions it is made of. For example, synthetic rubber is also made of a kind of network of molecules, but it deliberately contains ions that repel water molecules. So water molecules are not at all interested in entering them. Cotton absorbs water quite well and so it is no wonder you see it everywhere, including the cotton balls in handy medical kits.

Diaper molecules

Cotton works great when you need to soak up a small amount of water, but when you need to soak up the fluids your baby produces overnight, something more remarkable is clearly needed. That magical substance is called super-absorbent polymer (SAP). If you want to see this substance, take a pair of scissors and cut up an unused diaper or sanitary pad. Underneath all the cotton tissue and fragrances, you will see a white powder. This is SAP.

The molecular structure of this compound again resembles the intricate meshwork of a tree. As soon as it comes in contact with water, the water molecules flow in and settle inside. The oxygen atoms in particular are attracted to the meshwork due to the presence of an important ion called sodium in the SAP – the same sodium that is in your salt and is often off the charts if you don't hydrate yourself properly in the summer.

Sodium and water have such a love for each other that they stick to each other when they get the chance. This is why salt – as a compound of sodium and chlorine – dissolves in water. Here, the sodium ions move into the water molecules, leaving behind the chlorine ions, and the salt dissolves in the process.

Anyway, because of the oxygen, water molecules bind to sodium ions in the SAP trees. However their monkey hands are still free. They start to grab each other and form a strong network that can no longer move, i.e. it is rigid. The whole network swells up, trapping water molecules inside it, forming a gel.

Thus, sap is a magical compound that can absorb large amounts of water, sometimes even more than its own weight.

Conflict in quantum physics

You may still be wondering why sodium ions have such an affinity for water. What other kinds of ions could we use that would change this physics? What if there was another liquid to be absorbed instead of water?

The reason the sodium and oxygen atoms get close to each other is that they want to share an electron. As children we are told to think of the electron as a tiny table-tennis ball that revolves around a much larger nucleus, like a basketball. Like everything else in this world, this is not the full picture.

The electron is actually a wave, and due to the laws of quantum physics it can be shared by two atoms at the same time. In fact, nature prefers this arrangement: letting the electron stay in the shared world of oxygen + sodium. And this is what drives water molecules towards the sodium ions in the diaper.

If you want to understand this better, you should consider taking a graduate course in physics, like the one here at IIT Kanpur, where some of us teach.

The next time you diaper your baby and plan to put him to sleep, don’t forget to thank those tiny electrons and quantum physics for actually paying heed to nature’s call.

Adhip Agrawal is an Assistant Professor of Physics at IIT Kanpur.

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