Mass transfer refers to the movement of matter from place to place, resulting in a net change in the mass's location. The term is most commonly used in an industrial or engineering context to refer to processes involving diffusion, but in some technical or scientific areas it can also refer to other phenomena. Mass transfer takes place in a wide array of phenomena, ranging from industrial processes to Earth's climate and weather to the movements of celestial bodies.
Mass transfer through diffusion is an extremely common phenomenon in everyday life. All molecules in liquids and gases are in continuous motion, and this causes concentrations of a substance to spread themselves out until their masses are evenly dispersed, a process called molecular diffusion. This is why, for example, a drop of food coloring in water will spread until it mixes evenly with the water and why natural gas from a gas leak or carbon monoxide from a fire can spread through a building. Other characteristics of matter, such as heat, pressure, and momentum, also defuse in this way, and this can also cause the movement of mass.
Mass transfer is important in industry and engineering. Separation processes, which divide substances into separate products with different characteristics, are based on mass transfer. A wide variety of separation processes are used in modern industry and can use either purely mechanical means of transfer or incorporate chemical reactions as well. Distillation and crystallization are examples of the former, while the chemical processes used to refine crude oil are examples of the latter.
It is also important in many natural phenomena. It is essential to life, as diffusion is one of the main mechanisms by which essential materials are moved through the body. Mass transfer of particles in the atmosphere, whether through molecular diffusion or because of pressure and temperature gradients, drives Earth's weather and climate.
Mass transfer in astrophysics refers to the transfer of mass from one object, most often a star, to another object through the force of gravity. Each star's mass creates a region of space within which matter is bound to the star by its gravity. This is called a Roche lobe, which surrounds the star and varies in size according to the star's mass. Any matter from the star reaching beyond this area can potentially be pulled away by the gravity of another massive object, such as the star's companion in a binary system, resulting in a mass transfer. This is the cause of a number of astronomical phenomenon in binary star systems, such as the novae that occur when a white dwarf star accretes enough mass from its companion star to set off nuclear fusion and the x-ray emissions produced by x-ray binary systems, where one of the components is a black hole or neutron star that draws mass away from the other star.