![]() ![]() ![]() The same effect occurs at the edge of the tea cup. 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. Recall that the bubbles are buoyant, and so will naturally move to the highest point on the surface of the tea in their vicinity. The capillary action and surface adhesion create a meniscus of tea around each of the bubbles. The first piece of the puzzle is the water in the tea. 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. 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. As the bubbles move apart to a greater radius their angular velocity decreases. The raft of bubbles breaks up, and groups of them are flung outward. 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. 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. The pressure difference is not strong enough to draw bubbles away from the side of the cup. Bubbles "float" toward the low pressure region at the centre. The rotating tea has a higher pressure on the outside and lower pressure on the inside. The bubbles move towards the centre when you stir the tea because the cup acts like a centrifuge. 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 water surface area is smaller when the bubbles are together than when they are isolated. ![]() The bubbles stick together because of surface tension. ![]()
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