I first heard about Brownian motion in school. My teachers taught me how Robert Brown, the English Botanist had observed Pollen grains dancing about when suspended in water and viewed under a microscope. As I went on to college I learnt about the equations describing this phenomenon. For my Masters degree I even did research involving simulating Brownian-motion type motion on a computer.
Only recently did I see it with my own eyes.
Preparation and Setup
The foldscope setup is simple. My friend Matt and I made a simple flow cell using two pieces of double sided tape stuck with a gap between them on a glass slide. On this we stuck a cover slip such that there was a gap of around 10 micron between the glass slide and cover slip in the region between the two pieces of tape. We used a solution of 2 and 6 micron polystyrene beads diluted around 1/1000th by volume in deionized water. This suspension was pipetted slowly onto the slide where capillary action automatically pulls the suspension into the gap.
To get video from foldscope it was coupled via the provided magnetic couplers to an Ipad camera. To a get a good static focus we used a trick where the battery compartment of the foldscope (which points down when using with a mobile camera or Ipad) is lifted up by placing sheets of paper one by one till the focus is just right.
With all that in place and a bit of tweaking around with the focusing, we shot the video that you see below:
One can clearly observe the beads dancing around in the fluid. It was exciting to see something and know that this was evidence of molecular reality! Brownian motion occurs due to a micron-sized particles suspended in a fluid getting random kicks due to the molecules in the fluid colliding with them incessantly. These molecules are, of course, constantly moving around even when the fluid is at rest due to thermal motion. Since the particle is not too big one can see the effects of these random kicks and the particle appears to jiggle around in the fluid. If one watches closely it is also clear that the 2 micron beads jiggle around more than the 6 micron beads. What is also clear from the video is that particles diffuse and have a net displacement with time. The theoretical framework which explains Brownian motion tells us that the square of this displacement scales linearly with time and that the governing equation is a diffusion equation.
History of Brownian motion
Observing bits of pollen suspended in water under a microscope, the English Botanist Robert Brown noticed in 1828 that the pollen exhibited an incessant, irregular “swarming” motion — since called “Brownian motion”. Fast forward to the beginning of the 20th century and Brownian motion had a crucial role in establishing that molecules and their thermal motion were indeed real and not merely theoretical concepts, when this was not a widely accepted view. None other that Albert Einstein predicted that the random motions of molecules in a liquid impacting a larger microscopically observable particle would cause random motions of the particle which would be available under a microscope. This theoretical prediction corresponded with observed Brownian motion which eventually led to the existence of atoms and molecules being firmly established. It is interesting to note that before turning his attention to Special Relativity and the nature of our Cosmos, Einstein played a fundamental role in our understanding of our Microcosmos through his 1905 paper on Brownian motion.
Stay tuned for more microscale flow phenomena seen using Foldscope!