As one other hectic semester of lessons and exams involves an finish right here at IIT Kanpur, the place I train, everyone seems to be respiratory a sigh of aid. With winters settling in and a little bit of a chill in the air, this can be a excellent time to name a good friend and share conversations over a scorching cup of tea in Kanpur kulhads (earthen clay pots).
The teapot with holes
As you take tea in a kulhad, and if you listen, you will discover that for an extended interval tiny bubbles will proceed to kind and seem on the floor. Where are these secret bubbles coming from?
Earthen pots, not like metal or glass cups, are made utilizing clay. Clay is coarse a fabric that, even after being fired right into a pot, has tiny holes all through, going all the approach from the inside to the outer floor. These holes have trapped air inside them, such that as quickly as a liquid is poured on them, the air packets attempt to get out and rise to the floor as bubbles.
Since air is much less dense than the surrounding liquid, air bubbles expertise an upward thrust referred to as buoyancy. The buoyant pressure is the similar ‘force’ that makes you really feel lighter when you go for a swim in a pool or in the ocean (although, sadly, you might probably not have misplaced any precise weight).
This can also be the purpose you are requested to tie airbags to your self when going for a ship experience (right here in fact buoyancy can save your life).
Since these tiny holes in the clay pot go all the approach from the inside to the outdoors, why doesn’t tea simply leak and fall to the floor? How can it nonetheless be inside the cup although there are holes?
An oily paradox
All liquids stream and take the form of a container, however not all fluids are the similar. They have a property referred to as viscosity which makes them completely different from one another. Viscosity tells us how simply a liquid flows. We know from each day expertise that honey flows a lot slower than water, for instance. This is as a result of honey is extra viscous than water.
Viscosity has to do with the type of molecules that make up any liquid, how these molecules look, and the way they ‘like’ or ‘dislike’ one another. You might imagine {that a} liquid product of heavier molecules’ will transfer slower than one product of lighter atoms. You could also be motivated to assume this as a result of, in each day life, it takes a a lot bigger pressure to maneuver a heavier object (a automobile) than a lighter one (a bicycle).
But how simply or with problem a liquid flows just isn’t essentially as a consequence of the mass of atoms or molecules that make up the liquid. For instance, take any oil in your kitchen, say mustard oil, and pour a few of it in an empty and clear glass. Now pour some water, and you will discover that oil bubbles will kind and transfer in the direction of the floor. Just as in the case of the air bubbles in tea, the place air is lighter than tea, oil bubbles which might be lighter than water float upwards. (This is in truth true of most fatty substances, like the fats in milk surfacing as milk boils after which cools.)
Let’s conduct one other experiment. Take two small spoonsful of oil and water and put them on two separate plates. Now tilt the plates, and you will discover that the water flows far more simply in comparison with oil.
Then we have a puzzle: although oil is much less dense, additionally it is a lot lazier to maneuver. In different phrases, it’s far more viscous than water. How is that this attainable?
Long molecules and their tentacles
The reply to this obvious paradox has to do with oil’s molecules, that are much like these we encounter in garments. Specifically, similar to the molecules of cotton fibre, oil’s molecules are additionally fairly lengthy and might get intertwined with one another. When oil flows, this massive community of molecules flows – think about the roots of a tree slithering throughout a panorama. And as these massive molecules transfer, they should detach and reattach their ‘tentacles’ on the floor. This is what makes the stream of oil sluggish, although its total density is decrease than that of water.
Now you may guess why tea doesn’t leak by means of the clay pot. Even inside tea, all the molecules of tea, water, and milk get blended up and intertwined. Though they will stream, they will’t go by means of very small holes like the ones in a kulhad. This is as a result of the massive molecules behave like a giant internet, getting snarled and snagging on one another, and get caught in the holes.
All liquids have such molecules and thus some viscosity, be it water or honey, oil or petrol.
This makes one surprise: might there be a liquid with zero viscosity?
Superfluid helium
Superfluid helium is a liquid model of helium atoms – similar to water is a liquid model of H2O (i.e. a molecule with two hydrogen atoms and one oxygen atom). This liquid type of helium varieties at round 2 Ok (or -271 levels C).
It seems like a liquid however has the strangest of properties. One of the most shocking is that it may possibly by no means be saved in a container, together with in a kulhad. Superfluid helium is a liquid that has zero viscosity, so it may possibly go by means of even the smallest of holes.
Why does this liquid behave so weirdly? It seems that so as to perceive the properties of superfluid helium, we will’t use the methods we considered oil molecules. Instead, we have to use intricate ideas from quantum mechanics and condensed matter physics.
But if you have an interest, you can watch some wonderful demonstrations of superfluid helium in motion by the physicist Alfred Leitner, who recorded some movies in the Nineteen Sixties.
If you are intrigued to study extra, you ought to think about a course in physics.
The subsequent time you name a good friend over to share a cup of tea this winter, keep in mind the kulhads and the wonderful physics tales mendacity in wait inside them. In specific, whereas the tales of Superman are fiction (so far as we all know), the tales of superfluids are all actual.
Adhip Agarwala is an assistant professor, Department of Physics, IIT Kanpur.
