Air occlusion and foaming
Hydraulic fluids - Influencing factors

An oil's tendency to foam depends on its nature and the additives it may contain. Foam stability also depends on the fluidity of the oil. A viscous oil keeps air in solution more easily, and surface foam disperses with greater difficulty. Heating the oil increases its fluidity and reduces foam persistence.

Foaming can have very serious consequences. In a hydraulic system, the presence of foam results in inefficient power transfer, and the compressibility of the liquid phase behaves erratically. As the fluid undergoes alternating compression and expansion, occluded air can escape and accumulate at high points. The presence of these air pockets results in jerky operation, or is signaled by water hammer. If air accumulates in the pump, cavitation may occur.

In a pump, dispensing a mixture of oil and air results in a drop in effective oil pressure,...