Brownian Motion and Dynamic Light Scattering

Brownian motion

Brownian motion

When reading about the principles underlying a particular analytical instrument or technique, I often come across concepts which are not fully explained but which the authors assume will be recognised by the reader. Sometimes it’s good to have a look at those concepts and make sure their significance is understood. One that I’d like to mention is Brownian motion, or Brownian movement.

Brownian motion plays a crucial part in the application of dynamic light scattering, otherwise known as photon correlation spectroscopy, in the measurement of particle sizes and distributions.

The dynamic light scattering technique involves shining a light on particles or polymers held in suspension or solution. On hitting the particles, the light scatters in all directions. Because the particles are constantly in motion (Brownian motion), the distances between them constantly vary. This results in time-dependent fluctuations in the light scattering intensity, which are analysed to obtain information on particle size.

But what is Brownian motion? It is named after an English botanist, Robert Brown, who in 1827 described how pollen grains in water, viewed under a microscope, moved in an apparently random, jiggling, zigzagging manner.

We now know that the thermal energy of a liquid or gas causes its molecules to move about and constantly bombard the particles that they surround. At any moment, if the collisions on one side of a particle outweigh those on the other, it will ‘jump’.

Increasing temperature increases the energy of the liquid or gas molecules, so increasing the speed of movement. Small particles move faster than large ones, as the surrounding molecules have more of an impact on them. We ourselves are constantly bombarded, in the same way, by molecules in the air, but we are big enough not to notice.

While Brown first observed this phenomenon in pollen, I recall that my introduction to Brownian motion involved smoke particles in air. There was a problem with the intended source of the particles that we were to observe under the microscope so our physics teacher took out a cigarette, lit up and blew smoke into the observation chamber. Little did I suspect that I would be telling that story 36 years later in an article on dynamic light scattering.