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