Oil foaming property

Foaming normally occurs in a system such as gear box etc.The foaming takes place because the air gets entrapped in the oil specially where lot of churning/mixing is there.In many of the finished oils defoaming agents are added to take care of the foaming problem.These agents can also be added externaly.
Bacteria does not have much relation with foamin neither they cause foaming. There are standard ASTM tests to determine the degree of foaming. If you have any specific case in mind then we can discuss that.
Vijay

Hydraulic oils are designed to allow air to be releaased from the oil quickly and to rise out of solution before the return oil reaches the suction intake. Most gear oils do not have as great an ability to release oil and the additives in gear oils may increase the amount of air held and time it takes to release. Mixing even small amounts of gear oil with either hydraulic or turbine oils may cause serious foaming problems.

If the foam formed is stable it is very harmful, it reduces the heat transfer from the system and weakens the film strength at the mating surface. Thats why Anti foamant is used as an additive in lube oil. Thanks for the information that mixing of hydraulic oil or turbine oil with even small amount of Gear oil cause serious foaming problems. will somebody throw more light on this matter?