The Diffusion Coefficient or diffusivity expresses the rate or how fast a material can diffuse into another. In this article we will discuss about the topic “is diffusion coefficient constant?”.
At the beginning of this article we should properly know about the Diffusion and Diffusion Coefficient. So here first we will discuss about their definitions and then discuss the related factors to know whether a Diffusion Coefficient remains constant or not.
What is Diffusion?
Diffusion is the process where atoms, molecules or particles combine together as a result of their random motion or kinetic energy associated with them.
Diffusion is the process by which one matter is transported from one part of a system to another due to the random molecular motion.In other words diffusion is the outcome of spreading up of atom or molecules in a system from higher concentration to lower concentration region.
Dissolving of salt in water, escaping of air from a balloon, spreading of ink in a glass of water all are the example of diffusion encountered in our daily life.
In the above figure we can see an example of diffusion where blue ink is diffused in water giving a blue colour to the whole mixture.
If we spray a perfume in a room, a fragrance spreads throughout the entire room is also an example of diffusion.
Case hardening process is the example of Diffusion in solids where diffusion of carbon molecules take place through the surface of the steel to make the surface strong.
As diffusion phenomenon occurs between two regions of different concentration, it also occurs across two membranes, between outside and inside of cells
What Is Diffusion Coefficient?
Including Fick’s first and second law Diffusion constant is an important parameter in lots of equations of Physics and Chemistry
Diffusion Coefficient depicts the diffusion mobility, the higher the Diffusivity or Diffusion coefficient of a substance in comparison to the other in a pair of species, the faster they mix into each other.
Diffusion coefficient is the parameter which measures how quickly one matter diffuses into the other. The S.I unit of Diffusivity is m2/s.
We can get the mathematical expression for Diffusion Coefficient from Fick’s laws.
Fick’s Laws
Diffusivity or Diffusion Constant(D) is the constant of Proportionality encountered in Fick’s Law. The easiest explanation of Diffusion is given by Fick’s laws.
According to Fick’s First law of diffusion, the molar flux due to diffusion is proportional to the concentration gradient where molar flux refers to the amount of substance flow through a unit area within a unit time interval and concentration gradient is the change in the value of quantity.
Fick’s first law can be used to derive his second law which in turn is identical to the diffusion equation.
A diffusion process that obeys Fick’s laws is called normal or Fickian diffusion; otherwise, it is called anomalous diffusion or non-Fickian diffusion.
According to Fick’s Second law of diffusion, The rate of change of concentration of the solution at a point in space is proportional to the second derivative of concentration with space.
From Fick’s first law, we know that molecules move from a region of higher concentration to lower concentration that means it relates directly the diffusive flux to the gradient of concentration.
Where J: diffusion flux
D: diffusivity
x: position
From Fick’s second law, we can predict the change in concentration along with time.
Where, D: diffusivity
t: time
x: position
Factors Affecting Diffusion Coefficient
Diffusion process is influenced by various factors. The changes in these factors bring changes to the rate of Diffusion.
The factors affecting Diffusion Coefficient are listed below:
- Molecular Size of diffusing matter: Heavier particles move slowly in comparison to the lighter particles. Since diffusion process is fully associated with the movement of molecules, so the molecular size affects the rate of diffusion. Therefore, heavier molecules diffuse more slowly than lighter molecules.
- Temperature: A faster moving molecule posses higher kinetic energy, as soon as the temperature is added to a system the molecules inside the mixture moves faster due the higher kinetic energy experienced by each molecule. Molecules with higher kinetic energy can diffuse at a faster rate.
- Pressure: Generally pressure is considered as a influencing parameter in case of diffusion in gases, an increase in pressure results a higher rate of diffusion. Because when pressure increases the gas molecules come closer to each other and they have to move a shorter distance to get diffuse.
- But as liquids are incompressible in nature the affect of pressure in case of liquid is negligible
- Surface Area: For a large surface area of interaction results faster rate of diffusion.
- Viscosity of the solvent: If the viscosity of the medium through which the particles have to move or diffuse is too thick or very viscous, then the rate of diffusion automatically slows down. In case of a less viscous medium the particles move more easily.
- Properties of the solution(concentration, pH etc.): Diffusion is the process by which one matter is transported from one part of a system to another due to the random molecular motion. For a greater difference in between the concentration of two regions, faster will be the movement of the molecules. As soon as the distribution of molecules reaches the equilibrium condition, the rate of diffusion becomes slower.
All the factors give a combined affect on diffusion coefficient.
Is Diffusion Coefficient Constant?
No, Diffusion Coefficient is not constant.
Diffusion occurs due to the random motion of molecules in a mixture and the overall rate of diffusion is influenced by many factors. Diffusion Coefficient or Diffusivity depend on various factors like size of molecules, pressure, temperature, viscosity, surface area etc. Therefore, Diffusion Coefficient does not remain constant.
Generally all these factors give a combined affect on Diffusivity.
To know more about Diffusion(Click here)
- 9 Facts On Boiler Steam Temperature: Various Types & Details
- 11 Facts On Boiler Steam Drum: What, How, Material, Function
- Is Boiler A Heating System:7 Facts Beginner’s Should Know
- Is Boiler Closed System Or Machine? 5 Facts You Should Know
- Is Boiler Energy Efficient ? 7+ Facts You Should Know
- 23 Facts On Radial Stress: The Complete Beginner’s Guide
I am Sangeeta Das. I have completed my Masters in Mechanical Engineering with specialization in I.C Engine and Automobiles. I have around ten years of experience encompassing industry and academia. My area of interest includes I.C. Engines, Aerodynamics and Fluid Mechanics. You can reach me at