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Brownian Motion

Brownian motion of a particle

What is Brownian Motion?

Brownian motion is the random movement of particles in a liquid or gas. This movement occurs even if no external forces applied. Particles are never staying completely still. Instead, the movement occurs because of particles colliding with each other in a liquid or gas. Similar to how billiard balls hitting cause them each to change direction, the same is true of molecules.  Particle one hitting particle two will cause both particles to shift their momentum(direction and speed).

Video of particle tracking due to brownian movement
Yellow particles collide randomly with black particles and their path is tracked in blue. (Source: Wikipedia Commons)

The steps the particle takes are non-correlated. Each step is random and independent of the previous step. Brownian movement is often modeled using a ‘random walk’. The distance of a particle from its starting position will be a Gaussian distribution, with the width of the Gaussian increasing over time. This means that as time goes on, the particle is more likely to be further away from its starting location.

Distribution of particle location overtime due to Brownian motion
Probability of finding a particle at a certain distance after different amounts of time. The blue line is after the least amount of time has passed. The orange line is after the most amount of time. (Source: Wikipedia Commons)

Other names for Brownian motion include Brownian movement and pedesis (Greek for ‘leaping’).  Brownian motion is also called thermal noise because of its relationship to temperature. As the temperature increases, there is more energy in the system, and motion increases.

Factors that Increase Motion of Particles

  • Increase Temperature: The higher the temperature, the more energy each particle has.
  • Less Viscous Solution: The more viscous a solution is the more energy it takes for a particle to move through it. Think about trying to move through molasses (very viscous) compared to water (less viscous). The water will be easier to move through. The same is true on the smaller scale with individual particles.
  • Smaller Particles: particle speed is inversely related to the size of the particle. Therefore, smaller particles will move faster and travel further than larger particles.

Brownian Motion in Colloids

A colloid is a homogeneous mixture with large particles suspended in a solution of another substance. Milk and fog are common examples of colloids. The larger particles stay suspended in solution and do not settle out to the bottom as predicted due to gravity.  Instead, Brownian movement is what keeps the particles in solution.

The effect of all the smaller particles hitting the larger particles is enough to counteract gravity and cause the large particles to stay in solution.

Examples of Brownian Motion

Brownian motion can be hard to observe. All particles in a liquid or gas are moving due to Brownian motion.

Diffusion happens in part due to Brownian motion. Particles move away from their original position and randomly distribute.

A common experiment where Brownian movement can easily be tracked is watching fluorescent dyes in a solution. The individual particles are tracked by detecting the photons or light released by single molecules as they move through the solution.

Video of fluorescent beads moving in solution
Fluorescent beads can be seen moving in solution. (Source: Wikipedia Commons)

The short movie above shows fluorescent beads moving through a solution. This movement is due to Brownian motion. The beads each have a random trajectory and do not all move in the same direction.

Who Discovered Brownian Motion?

The Scottish botanist Robert Brown was the first one known to investigate Brownian motion in 1827. He was examining plant seeds and was puzzled by their movement even when they were dead. From this observation, he deduced there was some other phenomenon occurring.

Following Brown’s discovery, Einstein continued investigating this phenomenon. And in 1905 he published the first paper on the topic. Jean Baptiste Perrin then furthered Einstein’s studies. And for his work, Perrin was awarded the Nobel Prize in physics in 1926.

In the early 1900s, Norbert Wiener was also an important figure in learning about Brownian motion. Wiener focused on the mathematical models of Brownian motion and other stochastic processes.

Many other scientists have also focused on studying Brownian movement and it is still an ongoing research area today.

Other Resources and References:

For a deeper dive into the complex topic of Brownian movement see the links below:

Further Reading

Learn more interesting chemistry by exploring some other topics!

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