where the derivative of the left hand side is the material derivative of the variable c. In non-interacting material, D=0 (for example, when temperature is close to absolute zero, dilute gas has almost zero mass diffusivity), hence the transport equation is simply the continuity equation: ∂ ∂ c ∂ ∂ t + v ⋅ ⋅ ∇ ∇ c = 0.