4.Main Content
guidance
Brownian motion: facts and myths
Robert Brown is correctly referred to as having observed the jittering motion of small particles. But (i) he wasn't the first to record the observation, and (ii) he did NOT observe the motion of actual pollen grains.
How many text books etc continue to hand on these mistakes?
The title of Browns paper was "A Brief Account of Microscopical Observations ... on the Particles CONTAINED in the Pollen of Plants". Pollen itself is too large (and hence has too much mass) to be small enough to be buffeted significantly by water molecules etc. [The most recent reference pointing this out: Nature, 10 March 2005 p 137]
The first recorded observation of what we now call Brownian motion was made in 1785 by Jan Ingenhauz using charcoal dust. [Ref: Nature, 7 June 2001 p 641]
Having used particles derived from living matter, Brown had to try several other inanimate substances to convince himself that the motion he observed was not something to do with a 'life force', but a property of all microscopic matter. This 'systematic investigation' is what won for Brown the accolade of having the jittering motion named after him, work that Ingenhauz didn't need to do.
Today's research into nano technology now routinely fabricates nanoparticles. Controlling them suspended in liquids is quite a task. One method is to use a direct current controlled by a feedback system to cancel out the Brownian Motion. The position of the 20 nm polystyrene spheres is monitored by a fluorescence microscope and the voltage across the solution altered accordingly. So far nano-particles have been confined to within 1 micron. Alternatively, the path of the particle can be manipulated by suitable changes of the applied voltage. [Ref: Nature, 10 March 2005 p 156]
Even before the recent advent of nano-technology, Einstein's 1905 paper on Brownian Motion is his most cited paper (ie more than for Special Relativity or his work on photons). It is used by scientists working on such varied topics as aerosol particles ("pollution"), the properties of milk, paints, granular media (powders) and semiconductors. [Ref Nature, 20 January 2005 p 216].
Originally posted on CAPT (Connecting Advancing Physics Teachers) e-mail support by Rick Marshall. The Advancing Physics specification is OCR Physics B.
Updated 31 Mar 2005