Next time you have kulhad chai, don’t miss the bubbly physics

If you look past the bubbles, you’ll find a teapot with holes, a paradox involving oil, and the clues to an unusual substance called a superfluid.

December 13, 2023 10:30 am | Updated December 14, 2023 11:28 am IST

Hot tea in a couple of kulhads (earthen cups), April 28, 2021.

Hot tea in a couple of kulhads (earthen cups), April 28, 2021. | Photo Credit: Harsh Pandey/Unsplash

As another hectic semester of classes and exams comes to an end here at IIT Kanpur, where I teach, everyone is breathing a sigh of relief. With winters settling in and a bit of a chill in the air, this is a perfect time to call a friend and share conversations over a hot cup of tea in Kanpur kulhads (earthen clay pots).

The teapot with holes

As you take tea in a kulhad, and if you pay attention, you will find that for a long period tiny bubbles will continue to form and appear on the surface. Where are these secret bubbles coming from?

Earthen pots, unlike steel or glass cups, are made using clay. Clay is coarse a material that, even after being fired into a pot, has tiny holes throughout, going all the way from the inner to the outer surface. These holes have trapped air inside them, such that as soon as a liquid is poured on them, the air packets try to get out and rise to the surface as bubbles.

Since air is less dense than the surrounding liquid, air bubbles experience an upward push called buoyancy. The buoyant force is the same ‘force’ that makes you feel lighter when you go for a swim in a pool or in the ocean (even though, unfortunately, you may not really have lost any actual weight).

This is also the reason you are asked to tie airbags to yourself when going for a boat ride (here of course buoyancy can save your life).

Since these tiny holes in the clay pot go all the way from the inside to the outside, why doesn’t tea just leak and fall to the ground? How can it still be inside the cup even though there are holes?

An oily paradox

All liquids flow and take the shape of a container, but not all fluids are the same. They have a property called viscosity which makes them different from each other. Viscosity tells us how easily a liquid flows. We know from daily experience that honey flows much slower than water, for example. This is because honey is more viscous than water.

Viscosity has to do with the kind of molecules that make up any liquid, how these molecules look, and how they ‘like’ or ‘dislike’ each other. You may think that a liquid made of heavier molecules’ will move slower than one made of lighter atoms. You may be motivated to think this because, in daily life, it takes a much larger force to move a heavier object (a car) than a lighter one (a bicycle).

But how easily or with difficulty a liquid flows is not necessarily due to the mass of atoms or molecules that make up the liquid. For example, take any oil in your kitchen, say mustard oil, and pour some of it in an empty and transparent glass. Now pour some water, and you will find that oil bubbles will form and move towards the surface. Just as in the case of the air bubbles in tea, where air is lighter than tea, oil bubbles that are lighter than water float upwards. (This is in fact true of most fatty substances, like the fat in milk surfacing as milk boils and then cools.)

Let’s conduct another experiment. Take two small spoonsful of oil and water and put them on two separate plates. Now tilt the plates, and you will find that the water flows much more easily compared to oil.

Then we have a puzzle: even though oil is less dense, it is also much lazier to move. In other words, it is much more viscous than water. How is this possible?

Long molecules and their tentacles

The answer to this apparent paradox has to do with oil’s molecules, which are similar to those we encounter in clothes. Specifically, just like the molecules of cotton fibre, oil’s molecules are also  quite long and can get intertwined with each other. When oil flows, this big network of molecules flows – imagine the roots of a tree slithering across a landscape. And as these big molecules move, they need to detach and reattach their ‘tentacles’ on the surface. This is what makes the flow of oil sluggish, even though its overall density is lower than that of water.

Now you can also guess why tea doesn’t leak through the clay pot. Even inside tea, all the molecules of tea, water, and milk get mixed up and intertwined. Though they can flow, they can’t pass through very small holes like the ones in a kulhad. This is because the large molecules behave like a big net, getting tangled up and snagging on each other, and get stuck in the holes.

All liquids have such molecules and thus some viscosity, be it water or honey, oil or petrol.

This makes one wonder: could there be a liquid with zero viscosity?

Superfluid helium

Superfluid helium is a liquid version of helium atoms – just like water is a liquid version of H2O (i.e. a molecule with two hydrogen atoms and one oxygen atom). This liquid form of helium forms at around 2 K (or -271 degrees C).

It appears like a liquid but has the strangest of properties. One of the most surprising is that it can never be stored in a container like a kulhad. Superfluid helium is a liquid that has zero viscosity, so it can pass through even the smallest of holes.

Why does this liquid behave so weirdly? It turns out that in order to understand the properties of superfluid helium, we can’t use the ways we thought about oil molecules. Instead, we need to use intricate concepts from quantum mechanics and condensed matter physics.

But if you are interested, you can watch some amazing demonstrations of superfluid helium in action by the physicist Alfred Leitner, who recorded some videos in the 1960s.

If you are intrigued to learn more, you should consider a course in physics.

The next time you call a friend over to share a cup of tea this winter, remember the kulhads and the amazing physics stories lying in wait inside them. In particular, while the stories of Superman are fiction (as far as we know), the stories of superfluids are all real.

Adhip Agarwala is an assistant professor, Department of Physics, IIT Kanpur.

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