Consider the following 1D heat conduction problem. Region 1 is material 1, initially at uniform temperature T 1 , and region 2 is material 2, initially at uniform temperature T 2 . Given some period of time Δ Δ t after being brought into contact, heat will have diffused across the boundary between the two materials. The thermal diffusivity of a material is α α = λ λ / ( ρ ρ c p ) . From the heat equation (or diffusion equation), a characteristic diffusion length Δ Δ x 1 into material 1 is