Answer:
Dr. Tsuge Hideki kindly answered the above question.
  
 
  In general, the word “bubble” is used for two types of bubbles - air bubbles and foam bubbles. Bubbles formed when air is provided to a fish tank, bubbles blown out in a Jacuzzi, bubbles formed when a bottle glass is produced… these are all air bubbles. An air bubble is a part of air in liquids or solids. A second type is foam, i.e., bubbles which form on the liquid of beer, or bubbles made by soap water. In this type of bubbles (foam), air is separated by liquid membranes or lamella. When you pour beer into a glass, you can see that fine CO₂ bubbles rise up in the liquid, and are combined with foam on the surface of liquid. The property of forming foam is called “foaming power”. (Photo: Beer and foam, from Bier-Reise Corporation, with permission) 
 
  Foam is not formed when air is blown into pure water. In this case, air bubbles immediately break away from the liquid surface. Solute, such as egg white or soap, which can be adsorbed on the surface of bubbles, is necessary in generating foam. These surface-active compounds are adsorbed on bubbles, forming foam. The foam is divided into two types according to its lifetime; stable foam and unstable foam. The lifetime of stable foam is measured in units of hours or days. The lifetime of unstable foam is usually seconds or minutes.  
  
  If you observe the generation of foam inside a cylindrical container, you will notice that as the liquid inside the membranes of foam at the top of the container is drained by gravity, the membrane of foam at the bottom of the container becomes thicker, and the thickness is different according to location and is changed by time. The factors which influence the stability of foam vary according to the thickness of the membrane; viscosity is the main factor when the membrane is thick. On the other hand, surface tension is the main factor when it is thin. When the thickness of the membrane reaches 5 – 10 nm, the membrane of foam is finally disrupted. 
   
   
Acknowledgement
Dr. Tsuge Hideki
He is studying the fundamental phenomena of bubbles and its applications.
 
*Please do not use the photo above without the permission from Bier-Reise Corporation. 
 
 

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