# Diffusion

Diffusion, being the spontaneous spreading of matter (particles), heat, or momentum, is one type of transport phenomena. 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. 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 smelly 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

## 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 (see Einstein relation). The generic diffusion equation is time dependent, i.e., 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 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.

## Types of diffusion

Diffusion does not only refer to diffusion of particles, it refers to all transport phenomena occurring within thermodynamic systems under the influence of thermal fluctuations (i.e under the influence of disorder; this excludes transport through an hydrodynamic flow, which is a macroscopic, ordered phenomena).

Diffusion is the process through which an inhomogeneous thermodynamic system at local thermodynamic equilibrium returns to global thermodynamic equilibrium, through the homogeneisation of the values of its intensive parameters.