Material transfer in a phase
Mass transfer - Mass transfer kinetics between two phases

In a fluid consisting of a single hydrodynamically immobile phase, or in a stationary flow (all the quantities describing the fluid are independent of time), as soon as there is heterogeneity in concentrations, there is a spontaneous evolution towards uniformity of these concentrations: this is due to molecular diffusion. This phenomenon tends to bring all systems to thermodynamic equilibrium, and is therefore general. Indeed, uniformity of concentration within the same phase is...