Question

If I throw a ball straight up, it deflects slightly to the west due to Coriolis forces. If instead I watch a bubble float up in water, is the bubble deflected west, east, or neither?

I think the bubble also moves west, but am not sure. My reasoning is that the air in the bubble must feel a Coriolis force to the west because it is rising. Further, as the bubble rises, the water around it moves, and the net motion of the water is down. Water moving down experiences a Coriolis deflection to the east. Bubbles move the opposite direction of water, so if the water is moving east, then the bubble should move west.

Edit: One reason I'm a unsure about this problem is that a bubble does not get thrown outward by a centrifugal force. Imagine a plastic ball filled with water and spinning fast. A bubble is sitting in the water in the equatorial place, half way to the edge of the ball. There's centrifugal force out towards the wall, but the bubble does not move that way. The bubble moves up the pressure gradient. The water gets thrown to the wall, and thus there is higher pressure at the wall than at the center. The bubble moves towards the center. If a bubble moves counter to the centrifugal force, I should be careful before claiming it moves with the Coriolis force.

Answer 1

It does.

To convince yourself, remember that rising hot air does experience a Coriolis force, so I am quite sure that your bubble does too.

Also, think of what the Coriolis acceleration is - it is an apparent acceleration due to the fact that you, the observer, are in a rotating reference frame, so your definition of "straight up" is actually a curve. When the bubble goes up you see it as having a curved trajectory - but this has to do with your rotating definition of straight up. Therefore this acceleration applies to it.

or

Here is one case in which a bubble will move against the Coriolis force, although it's not exactly the same case as addressed in the question.

This reference claims that Coriolis forces used to come into play in ship navigation by deflecting the bubble in a level, but doesn't say which way.

Consider the bubble in a level. If you are in the northern hemisphere and are moving further north, the Coriolis force is to the west. The Coriolis force acts on the air in the level's bubble, but also on the water in the level. The force is proportional to mass, so by the equivalence principle we can think of it as a small gravitational force pointing west. A bubble in water moves opposite gravity, so the bubble moves east, against the Coriolis force.

The difference between this scenario and a bubble simply floating up in water is that in the level, the bubble and water move in the same direction, but not so for a bubble floating up.