What is diffusion?

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If you’re wondering “what is diffusion?” we’ll answer it for you. The concept of diffusion has an interesting history. It is widely used in many scientific disciplines. You can see the concept of diffusion working all over nature. For instance, breathing will show diffusion in some ways.. Due to the importance of this concept in science, it’s worth taking some time to understand it. In addition, you will learn a little bit about the history. At the end we’re going to cover the basic physics.

What is diffusion?

Diffusion is just a process which results from a random motion of tons of molecules. According to Wikipedia, diffusion is a process where you can see a net flow of matter from a high-concentration region to a low-concentration region. For example, the perfume of any flower will permeate any room right away. This happens even in this atmosphere full of still air. The conduction of air in any fluid involves thermal energy that is either diffused or transported from a higher temperature to a lower temperature. Another example is the operation of any nuclear reactor. A diffusing substance´s rate of flow is proportional to the concentration gradient.

Distinguishing Feature

An important feature of the diffusion process is that it results in mass transport or mixing. This does not require any kind of bulk flow or bulk motion in any way. But you should not confuse diffusion with advection, or convection. These are different transport phenomena which use bulk motion that will move particles from one spot to the other.

Widely Used Concept

The famous concept of diffusion is widely used in science such as finance, economics, sociology, chemistry, and physics. In each one of these cases, an object will spread out from a spot or location where you can find a high concentration of this object. You can use two methods to introduce a notion about diffusion: the phenomenological approach and the physical and atomistic approach. The phenomenological approach states that diffusion is the movement of any substance from a region of high concentration to another region with  low concentration of that same substance, accomplished via bulk motion.

The physical and atomistic approach says that diffusion is just the result of any random walk of any diffusing particles. In the case of molecular diffusion, we see that the moving particles are self-propelled by thermal energy. The concept of diffusion is commonly used to describe any subject matter that involves random walks of ensembles of individuals. In addition, net diffusion is a concept used in biology to describe the movement of any ion or molecule by diffusion. Oxygen can also diffuse itself into a cell membrane if this substance finds a higher concentration outside any cell than inside, then oxygen molecules will have to diffuse into these cells over time.

History

Before the theory around the concept of diffusion was built, diffusion in solids had been widely used in science. For instance, carbon diffusion was previously described as a concentration process that produces steel from an element called iron. Another example is the diffusion of color in Chinese ceramics and stained glass. Thomas Graham performed the first experimental and systematic study of the concept of diffusion in the 19th century. Because of the measurements that Graham was able to perform around the concept of diffusion, James Maxwell managed to derive the famous coefficient of diffusion just for CO2 in the air.

As you can see, the concept of diffusion is very important in science across many disciplines. You should also remember that diffusion will not use any kind of bulk flow and that it always involved mass transport as well as mixing. Another important fact that you should bear in mind is that diffusion has been used by many scientists to develop many important theories.

Bonus Physics

To understand diffusion mathematically, it is best to start with Fick’s first law:

J = -Dφ

Where J is the “diffusion flux”, which measures the amount of substance that will flow through a unit area in a unit time; D is the diffusion coefficient, ∇ is the gradient operator, and φ is the concentration. We can then use this equation to derive Fick’s second law, which is equivalent to the diffusion equation.

δφ/δt = DΔφ

i.e. the time derivative of the concentration is equal to the Laplacian (second derivative of position) of the concentration. This describes how the concentration evolves in time.

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Oliver Gauss
Oliver has a degree in physics and mathematics and has completed all but his dissertation for a Ph.D. in Physics. He believes actual science should decide scientific disagreements, and that most people who use "science" to defend their emotion-based opinions have no idea what science actually is. Oliver is the editor of WeWantScience.com and has two new sites coming out soon. Stay tuned!