Something you might keep in mind about bubbles especially if you are to animate them. 

 

Whatever object is creating the initial stream of bubbles; the bubbles will be very small at first creation.  As the bubbles move away from the origin they will merge with other bubbles to make larger bubbles.  At a certain point when merging they become so large they get very buoyant and that will make them move or rise toward the surface more rapidly? 

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