Surface & Interfacial Tension

Surface and interfacial tensions are present whenever there is one or more condensed phase present. When there is only one condensed phase (solid or liquid), we talk of surface tension whereas if two condensed phase are in contact we talk about interfacial tension. But both the terms represent energy per unit surface area or force per unit length.
Consider the process of adhesion, that is, formation of an interface between two condensed phases (Fig. 2.1). Phases 1 and 2 are being brought under contact in a vacuum or an inert gas, thus forming an interface.

Figure 2.1. Interaction between two surfaces brought in contact in presence of vacuum or gas