The primary requirement for foaming is "film elasticity" so that the bubbles can respond to stresses and changes. A foam starts as spherical, wet bubbles, which drain to give a network of films defining polyhedral bubbles. The curvature of the walls indicates that unequal pressures exist in different bubbles. The foam bubbles must contain stale air because of the immediate release of carbon dioxide as the foam is broken. The effect on aeration and agitation is shown by the mixing power requirements.



This figure shows a wattermeter record of mixing power. Mixing power drops gradually because of a build up of air entrainment which lowers the liquid density and partially floods the impellers. Defoaming gives a sharp increase in power because of de-gassing. Approximately 20% more power is required to stir large fermentors just after a shot of oil.