Can microbubbles be pelleted by centrifugation?
In short, no. At least not a pellet that you would see at the bottom of your tube when you spin down cells.
Instead, what happens is anything with a density greater than water sinks and sinks faster when in a centrifuge. And anything with a density less than water rises and rises faster when in a centrifuge.
So, microbubbles all go to the same place in a tube, but instead of being at the bottom they are at the top. This is also true for any cells that are attached to the microbubbles, they float to the top when centrifuged.
While at an entirely different scale, another way to think about this is from the time you might have spent in a pool in your childhood. Granted, what a cell on a microbubble experiences is at a much lower Reynolds number, it can still serve as a useful analogy.
If you were to strap water wings on and jump into a swimming pool, you would indeed briefly sink before returning to the surface. This in general is not the case with microbubbles. Swimmers will initially sink because of the downward momentum they accumulate on the way into the water. If you were somehow able to increase the gravitational pull at your local pool once your in the water, you would bob up even with the stronger gravity. In fact, you would float with greater force.
This is true of our microbubbles. Centrifugation only drives them more quickly to the top of the sample.