The key difference between Brownian motion and diffusion is that in Brownian motion, a particle does not have a specific direction to travel whereas, in diffusion, the particles will travel from a high concentration to a low concentration.
Brownian motion and diffusion are two concepts that associate with the movement of particles. The existence of these two concepts proves that the matter is consisting of smaller particles, which we can separate from each other. It also proves that there is enough free space between the atoms or molecules within a substance (solid, gas or a liquid) which allows other particles to travel through them.
What is Brownian Motion?
The botanist Robert Brown presented the concept of Brownian motion in 1827. He observed pollen grains in water under a microscope and saw that the pollen grains are moving here and there (random movement) in water. He named this movement as the Brownian motion. However, Einstein is the one who explained this movement.
According to the explanation of Einstein, he describes some properties of atoms. Though they believed the existence of atoms at that time; there was no proves for it. Brownian motion is a proof for the existence of atoms. Every matter around us consists of atoms. Therefore, even the pollen grains and water contains atoms. Furthermore, Einstein described that the movement of the pollen grain is due to its collisions with the water molecules which we can’t see. When water molecules hit the pollen grains, it bounces off, and we can see it under the microscope. Since we can’t see the water molecules; we tend to think that the pollen grains are moving on their own, which is not so.
Moreover, by studying the Brownian motion, we can predict some of the properties of the water molecules such as their speed of movement. Similarly, the particles in the air also show Brownian motion. For example, a dust particle in the air is moving around randomly due to the collisions with the gas molecules.
What is Diffusion?
Diffusion is the travelling of particles from a higher concentration to a lower concentration. In other words, it is the movement of particles from the regions of higher chemical potential, to areas of lesser chemical potential. Therefore, this concept is similar to the travelling of heat from a hot object to a cooler object.
Furthermore, osmosis is a type of diffusion that describes the movement of water. When water is moving from one cell to another, it flows according to a water potential gradient that is from a high water potential to low water potential. Moreover, diffusion is an important concept in biological systems. Plants and animals absorb and distribute most of the nutrients, gases and water through diffusion. For instance, inside a cell, the oxygen content is lower than in blood capillaries, and the carbon dioxide concentration is higher than in the blood capillaries. Therefore, by diffusion oxygen transfers into a cell from blood capillaries, and carbon dioxide comes out from the cell.
What is the Difference Between Brownian Motion and Diffusion?
Brownian motion is the erratic, random movement of microscopic particles in a fluid, as a result of continuous bombardment from molecules of the surrounding medium. Whereas, diffusion is the movement of a substance from an area of high concentration to an area of low concentration. Therefore, the key difference between Brownian motion and diffusion is that in Brownian motion, a particle does not have a specific direction to travel whereas in diffusion the particles will travel from a high concentration to a low concentration. However, the particle movement is random in both scenarios.
Moreover, another significant difference between Brownian motion and diffusion is that the diffusion takes place according to a concentration or potential chemical gradient. But, Brownian motion is not governed by such factors. Brownian motion of a particle occurs according to the motion of other particles in the medium.
Below infographic provides more details on the difference between Brownian motion and diffusion.
Summary – Brownian Motion vs Diffusion
In summary, the key difference between Brownian motion and diffusion is that in Brownian motion, a particle does not have a specific direction to travel whereas, in diffusion, the particles will travel from a high concentration to a low concentration. However, the particle movement is random in both scenarios.
1. Alfred Rudin, Phillip Choi, in The Elements of Polymer Science & Engineering (Third Edition), 2013.
2. Helmenstine, Anne Marie, Ph.D. “An Introduction to Brownian Motion.” ThoughtCo, Jan. 5, 2019. Available here