In 1827, Scottish botanist Robert Brown looked through a microscope at pollen grains suspended in water, and discovered the pollen was moving in a random fashion – tiny particles did not slow or stop, but were in constant motion. This phenomenon, we now call Brownian Motion, is not unique to pollen but is commonly observable in daily life. It is not specific to biology either but instead has been proven mathematically and is due to physics. Most people might have noticed dust particles dancing in a ray of light in a dark room or the diffusion of pollutants/smoke in the air or diffusion of calcium in bones – these are all examples of Brownian motion.
Brownian motion is the random movement of particles due to the bombardment by the molecules that surround them. Understanding Brownian motion is important because it provides us with the evidence that atoms exist. Einstein’s mathematical model of Brownian motion from 1905 is one of his least well known but very important contributions to physics. It described how tiny visible particles suspended in a liquid are bombarded or moved by invisible water molecules around them causing them to jiggle. The model explained this motion in detail and was able to accurately predict the irregular random motions of the particles which could be directly observed under a microscope. Einstein’s theory of Brownian motion offered a way to prove that molecules exist despite the fact that molecules are too small to be seen directly. Soon after a French physicist J.B. Perrin conducted a series of experiments that confirmed Einstein’s predictions. The theory also helped to understand how particle size is related to their speed of movement.
While Brownian motion of small particles has been observed quite easily using a light microscope and studied for the past 200 years, the mechanism that drives Brownian motion is not well understood. What we do know is that Brownian motion is caused both by the structure and physics of fluids, i.e., liquids and gases. According to kinetic theory as proposed by J.C. Maxwell, L. Boltzmann and R.J.E. Clausius, all matter is in motion; atoms and molecules especially within liquids and gases are in constant vibrating motion. These particles will travel in straight lines until redirected by a collision. Particles within gases and liquids are constantly moving, colliding, and moving toward equilibrium.
There are mainly 4 factors that affect Brownian motion: temperature, particle number, particle size, and viscosity. The larger the particle or molecule and the more viscous the dispersion medium, the slower the Brownian motion will be. Smaller particles are “kicked” further by the solvent molecules and move more rapidly. In addition, a high temperature and a high number of particles, all increase the rate of motion.
Given particle speed of movement or Brownian motion can be correlated to particle size, various analytical measurement techniques have been developed that exploit this relationship.
Dynamic light scattering measures Brownian motion and relates this to the size of the particles. DLS, sometimes referred to as Photon Correlation Spectroscopy or Quasi Elastic Light Scattering (QELS), is a non-invasive, well-established technique for measuring the size and size distribution of molecules and particles dispersed in a liquid typically in the submicron region and extending to lower than 1nm using the latest technology pioneered by the manufacturer MalvernPanalytical. Typical applications of dynamic light scattering are the characterisation of particles, emulsions or molecules which have been dispersed or dissolved in a liquid. Common samples analysed by DLS include colloidal silica, titanium dioxide, ceramics, carbon dots, lipid nanoparticles, proteins and adeno-associated virus (AAV). The sensitivity of modern systems is such that it can also be used to measure the size and concentration of macromolecules in solution with little dilution using small sample volumes (3µL).
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