Fundamental physical quantities such as mass, momentum and energy can be exchanged between portions of a system, or created during transformations through different processes such as convection, diffusion and chemical reactions. Convection is a type of mass exchange that is dependent upon the mean material velocity. Momentum and energy transport are generally associated with mass transport. Aside from mean material motions, momentum and energy can also be exchanged through molecular interactions. Indeed, molecular quantities are transferred by collisions between molecules, as explained in Sect. 3.3.1. Molecular collisions lead to momentum transfer due to viscosity and to internal energy transfer through thermal conduction. Self-diffusion is the diffusion of molecules in a one-component medium. Chemical mixtures involve the diffusion of species, and the mass diffusion flux has already been defined in Sect. 2.1.3. All of these diffusive fluxes have associated transfer coefficients.