Atoms at a free surface or an interface have a different local environment from atoms deep inside a uniform bulk phase. Creating additional interface therefore usually requires additional system energy.

For an interface whose state and composition are otherwise fixed, a useful approximation is

$$\Delta E_{\text{interface}}=\gamma,\Delta A,$$

where $A$ is interfacial area and $\gamma$ is the surface or interfacial energy per unit area.

If $\gamma>0$, reducing area reduces this energetic contribution. That simple tendency helps drive several important material processes:

• small particles and grains tend to coarsen when atoms are mobile;
• pores and particle surfaces provide a driving force for sintering;
• creating a new phase requires paying an interfacial-energy cost at the nucleus boundary;
• extending an ideal brittle crack creates new free surfaces and therefore costs energy.

Different interfaces have different $\gamma$. A coherent interface between similar crystal structures can have much lower energy than a poorly matched boundary, and solute segregation can change interfacial energy by changing local bonding.

Interfacial energy is not a force by itself. It enters mechanics and kinetics through how total energy changes when interface area or shape changes. The same energetic idea therefore links phase transformations, grain growth, fracture and processing.