Thermal effusivity is a parameter that emerges upon applying solutions of the heat equation to heat flow through a thin surface-like region. It becomes particularly useful when the region is selected adjacent to a material's actual surface. Knowing the effusivity and equilibrium temperature of each of two material bodies then enables an estimate of their interface temperature T m when placed into thermal contact. If T 1 and T 2 are the temperature of the two bodies, then upon contact, the temperature of the contact interface (assumed to be a smooth surface) becomes