Removal of Bubbles by Buoyancy Effects

Bubbles can be removed from melts either by physically rising to the surface, or by chemical dissolution of the gas into the surrounding melt. Since the density of a bubble is less than that of the surrounding liquid, a bubble will automatically rise to the surface and burst unless prevented from doing so by some external agent. The basic principle of buoyancy is given by Stokes Law, which applies to the velocity, V of a solid sphere in a liquid of a different density  

Equations 3.2 and 3.3 indicate that the velocity of rise of a bubble is inversely proportional to the viscosity of a melt, i.e., that bubbles will rise faster in a more fluid melt, and directly proportional to the density of the melt, i.e., bubbles rise more rapidly in a more dense melt than in a less dense one. Although these predictions are generally true, there 

These equations also predict that the rate of removal of bubbles will be proportional to the square of the bubble radius or diameter. Procedures which increase bubble size will therefore rapidly accelerate fining. Conversely, these equations indicate that bubble rise is not a very efficient process for the removal of very small bubbles. It follows that very fine seed cannot be efficiently removed from viscous melts by simple bubble rise in the absence of any other processes. 

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