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Brian

OK, here we go, the ‘Bluffer’s Guide’ to bubble movement.  In my diagrams:
Case 1.  Everything is steady, and the bubble is central.  All the forces acting at 90 to the bubble’s surface balance out.
Case 2 The righthand side (RHS) of the bubble is marginally lower in the fluid than the lefthand side (LHS).  As pressure increases with depth there’s a buoyancy force on the bubble from the right forcing the bubble towards the centre.
Case 3:  However, as the bubble starts to move towards the centre, there’s ‘stiction’ with the glass and fluid has to move around the bubble, so there’s also viscous friction opposing its movement.  The maximum bubble speed occurs when the frictional force balances the buoyancy force. 
Case 4:  The platform suddenly accelerates to the left.  The fluid is thrown to the right because of its inertia.  This creates a false vertical, so buoyancy forces the bubble to the left.  The effect of friction is the same as Case 3.  From the above, the more viscous the fluid, the slower the bubble movement, so there will perhaps be a greater lag term between the net vertical indicated by the bubble and the true net vertical.

If you wished to impress your friends you might like to drop a few ‘buzzwords’ like Stokes’ Law, Reynolds number, and the Navier Stokes Equations, but only if they’re unlikely ask you to explain them.  I’ve struggled on and off with these over the years and understand them no more now than when I started.  Reynolds number is worth a mention, because it’s a function of bubble diameter and perhaps it helps to explain why there’s an ideal size for your bubble. Too small, too slow.  Too large, very agile, but harder to judge where to place the star.  DaveP

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