Some Example of Surface Force Effects

In many industrial devices, bubbles are formed by forcing a gas through an orifice into a stagnant pool of liquid. For gas being forced through a circular, horizontal orifice, this situation is shown in Fig. 17.11. 

If the bubble is spherical, then the buoyant force on the bubble is 

If the liquid wets the orifice, including the vertical par of the hole, as shown in 

The bubble breaks away from the orifice when the buoyant force exceeds the surface force. Assuming that breakaway begins when these forces are just equal, we can equate these two forces and solve for. the bubble diameter  

As shown in Fig. 17.11, this treatment assumes that the gas does not wet the orifice and that instead a thin film of liquid wets the inside of the orifice. This is commonly observed for gas-liquid systems and for some liquid-liquid systems. However, if the fluid flowing through the orifice wets the surface of the orifice, then much larger bubbles result [7]. 

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