Transport phenomena

In physics, chemistry, biology and engineering, a transport phenomenon is any of various mechanisms by which particles or quantities move from one place to another. The laws which govern transport connect a flux with a "motive force". Three common examples of transport phenomena are diffusion, convection, and radiation.
There are three main categories of transport phenomena:
Heat transfer,
Mass transfer, and
Fluid dynamics (or momentum transfer)
An important principle in the study of transport phenomena is analogy between phenomena. For example, mass, energy, and momentum can all be transported by diffusion:
The spreading and dissipation of odors in air is an example of mass diffusion.
The conduction of heat in a solid material is an example of heat diffusion.
The drag (physics) experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates).
The transport of mass, energy, and momentum can also be affected by the presence of external sources:
An odor dissipates more slowly when the source of the odor remains present.
The rate of cooling of a solid that is conducting heat depends on whether a heat source is applied.
The gravitational force acting on a rain drop counteracts the drag imparted by the surrounding air.
All these effects are described by the generic scalar transport equation.
The same equations governing convection in heat transfer can be applied to convection in mass transfer. When studying complex transport phenomena problems one must use tools from continuum mechanics and tensor calculus and often problems can be expressed as partial differential equations.
In solid state physics, the motion and interaction of electrons, holes and phonons are studied under "transport phenomena