Schematic drawing of the effects of diffusion through a semipermeable membrane.

Diffusion, being the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. Diffusion is the movement of particles from higher chemical potential to lower chemical potential (chemical potential can in most cases of diffusion be represented by a change in concentration). It is readily observed for example when dried foodstuff like spaghetti is cooked; water molecules diffuse into the spaghetti strings, making them thicker and more flexible. It is a physical process rather than a chemical reaction, which requires no net energy expenditure. In cell biology, diffusion is often described as a form of passive transport, by which substances cross membranes.

Examples of diffusion

• A balloon filled with helium will deflate a little bit every day, because helium atoms diffuse out of the balloon through its wall.
• When spaghetti is cooked, water molecules diffuse into the spaghetti strings, making them thicker and more flexible. Adding salt to the water reduces diffusion by reducing the osmotic pressure.
• Carbon dioxide bubbles in soft drinks start as small nuclei and grow because of the diffusion of carbon dioxide molecules towards them.
• Heat diffuses through the walls of a mug filled with hot coffee.
• A gas distributes itself over a room by diffusion.
• A sugar cube in a glass of water that is not stirred will dissolve slowly and the sugar molecules will distribute over the water by diffusion.
• Ink in the beaker of water is an example of diffusion. In the end, the ink particles spread evenly throughout the mass of water.

The nature of diffusion

The different forms of diffusion can be modelled quantitatively using the diffusion equation, which goes by different names depending on the physical situation. For instance - steady-state bi-molecular diffusion is governed by Fick's first law, steady-state thermal diffusion is governed by Fourier's law. The diffusion of electrons in an electrical field leads essentially to Ohm's law that is further explained by Einstein relation. The generic diffusion equation is time dependent, and as such applies to non-steady-state situations as well.

In all cases of diffusion, the net flux of the transported quantity (atoms, energy, or electrons) is equal to a physical property (diffusivity, thermal conductivity, electrical conductivity) multiplied by a gradient (a concentration, thermal, electric field gradient). Noticeable transport occurs only if there is a gradient - for example in thermal diffusion, if the temperature is constant, heat will move as quickly in one direction as in the other, producing no heat transport and change in temperature.

Diffusion occurs as a result of the Second Law of Thermodynamics, which states that the entropy or disorder of any closed system must always increase with time. Because substances diffuse from regions of higher concentration to regions of lower concentration, they are going from a state of higher order to a state of lower order, in accordance with the Second Law of Thermodynamics. Therefore, diffusion is a spontaneous, natural process, and to reverse diffusion would require the expenditure of energy to counteract the higher order of the system and prevent a violation of the laws of entropy.


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