1. n. [Drilling Fluids]

Surface free energy that exists between a liquid and air. Surface tension can be observed as a curved meniscus in a small tube of the liquid. This energy barrier prevents a liquid (such as water) from spontaneously mixing with air to form a foam. To make a foam, as used for a drilling fluid, the liquid's surface tension must be lowered by adding a third component (a foamer) that accumulates at the interface. Foam preparation usually requires mechanical energy to break up the bulk liquid into thin films around each gas bubble.

See: emulsion,  emulsion mud,  HLB number,  interfacial tension,  invert emulsion,  invert-emulsion oil mud,  meniscus,  oil mud,  oil/water ratio