On a fine morning in 1827, botanist Robert Brown had no idea that a simple observation through his microscope would lead to the discovery of invisible matter.
As he carefully examined a slide containing pollen grains suspended in water, Brown noticed a peculiar movement. The tiny particles did not remain still; instead, they followed a constant, unpredictable zigzag pattern, as if they were colliding with unseen forces, abruptly changing direction upon impact. This phenomenon later became known as Brownian motion—a groundbreaking observation that hinted at the presence of invisible entities.
Decades later, in 1905, Albert Einstein built upon Brown’s discovery in his Ph.D. thesis, using Brownian motion as evidence for the existence of atoms and molecules. He explained that the jittery movement of pollen grains was caused by random collisions with invisible water molecules—a physical manifestation of molecular activity. To further verify this theory, scientists conducted experiments using inorganic particles such as soot, which also exhibited the same zigzag motion, strengthening the case for atomic theory.
However, for a substance to show continuous zig-zag Brownian motion, the particle's size must be between 10-9 to 10-6 m while suspended in a dispersion medium. Such particles with that size range are called colloids, and the dispersion medium is called the colloidal sol.
The striking of the colloidal particles with the smaller molecules of the dispersion medium occurs with varying velocity and direction. This causes the colloidal particles to bounce off with different forces from the molecule's surface that drive such zig-zag motion.
Therefore, the particles move continuously and do not settle, creating a stirring type of effect. The stirring effect is responsible for the stability of the sol so that they do not separate into two parts- the solid particle and the liquid.
The Brownian motion is independent of the colloidal particle's nature but highly depends on the size of the particle. The bigger the size of the particle and higher the dispersion medium's viscosity, the slower the particle's motion.
The Brownian motion, therefore, explains the dynamic nature of the molecules present in a dispersion medium.
Importance of Brownian Motion
The study of the Brownian motion was instrumental in determining the Avogadro's Number (NA) and is also extended to the other branches of science -biology, physics, mathematics, evolution, and finance (stock market analysis).
