Surface Energy and Surface Rheology Relation to Foam Properties

摘要

Foams made from pure fluids are generally very unstable : bubbles obtained by shaking pure water last only for a few secodns. When a surface active substance is added to water, bubbles obtained by shaking pure water last only for a few seconds. When a surface active substance is added to water, bubbles lifetime can become much longer. The increase in surface neergy due to the increase in surface are after the creation of thebubbles is substancial : for instance if one cm~3 of solution with a surface of about one cm~2 is shaken to produce a foam with bubbles of millimetric size, the air-solution area increases by a factor of ten and the surface energy, which is proportional to the area, increases by the same factor. Of course, the surface tension of hte solution is decreased by the presence of the substance, but the state of equilibrium is the state of minimal energy and is in any case the state of minimal area : the foam can never be stable. The difference between water and surfactant solutions lies therefore in the time scales involved in bubbles lifetime. Surface tension is not the main surface characteristic property there, and other properties such as surface elasticity, surface viscosity, dynamic surface tension, become extremely important. In this chapter, we will first define all these surface properties, indicate how they can be determined and discuss how they influence the foam properties.