Please explain the weird behaviour of these bubbles in my tea?

Question

Here is a photo of my tea just after i made it-

When i stir it, these bubbles first are away, then they come and clump together. If i try to take them away from each other by putting a spoon inside this ball of bubbles, they again come together and stick to each other. But if some of these bubbles stick to the wall of my cup, they dont come near the centre any more! Also, I noticed that this ball of bubbles, first spins very fast then in a second it slows down, then in the next second, it again spins very fast and this goes on.

Why are these weird things happening, and what is responsible for them?

Answer 1

The bubbles stick together because of surface tension. The water surface area is smaller when the bubbles are together than when they are isolated. They stick to the sides of the cup for the same reason : part of the bubble is next to the wall of the cup, and this reduces the water surface area.

The bubbles move towards the centre when you stir the tea because the cup acts like a centrifuge. The rotating tea has a higher pressure on the outside and lower pressure on the inside. Bubbles "float" toward the low pressure region at the centre. The pressure difference is not strong enough to draw bubbles away from the side of the cup.

When the tea is rotating, as the bubbles gather together the radius of the raft of bubbles decreases and its angular velocity increases because angular momentum is conserved. However, provided the raft of bubbles is not too large, the forces between the bubbles are not strong enough to provide the centripetal force needed to keep them rotating at this speed and radius at the centre. The raft of bubbles breaks up, and groups of them are flung outward. The reverse process then happens. As the bubbles move apart to a greater radius their angular velocity decreases. Further from the centre the pressure difference in the centrifuge pushes them back towards the centre of the cup, and the raft comes back together again.

Answer 2

I'm not quite sure what is responsible for the periodic spinning motion of the bubbles, but the mutual attraction that they are feeling is fairly straight forward. The first piece of the puzzle is the water in the tea. The capillary action and surface adhesion create a meniscus of tea around each of the bubbles. Recall that the bubbles are buoyant, and so will naturally move to the highest point on the surface of the tea in their vicinity. If two bubbles are near each other, they will move towards each other because the menisci around the opposite bubble is the highest point in the immediate surroundings. The same effect occurs at the edge of the tea cup. The tea at the edge creates a meniscus around the inside of the tea cup.

The spinning motion might be caused by your stirring of the cup, and its periodic nature might be caused by constructive and destructive interference of the tea's spinning modes. But don't quote me on this. I'm not too sure about the nature of the spinning motion.