Brownian Motion through #Foldscope

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.

Focusing setup with I-pad

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:

Observations

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!

4 Comments Add yours

  1. David says:

    A clearly explained demo, thanks. Was the highest power lens used as a bit more mag may be useful?

    Regards the original work by Robert Brown, he observed particles within pollen grains undergoing movement – pollen grains, even the smallest – are too large to show the motion. A number of online resources and some textbooks incorrectly state this.

    A classic material around the home that can show it well, is very, very dilute homogenised milk (supermarket whole milk is usually this grade, i.e. the cream doesn’t separate out). There’s a good range of fat globules in the right size range. Just a pin dipped in milk then dip pin in a drop of water is about right, to get same low density of globules that your video shows.

    Professor Brian J Ford, an authority on the history and use of the single lens microscope, recreated Robert Brown’s original experiment using Brown’s microscope. See link below.

    http://www.brianjford.com/wbbrowna.htm

  2. Manu Prakash says:

    These are like stars jiggling in the sky. It’s so much fun to drag the cursor back and forth and really see the net displacement over time. Can’t wait to see a diffusion coefficient calculation soon 🙂

    Secondly, the contrast is so good – one micron beads will also be visible.

    Also, imageJ has a bead tracker – the contrast is so good; it might be trivial to track and plot trajectory – that might be a future post 🙂

    Beautiful!!

    cheers
    manu

  3. laksiyer says:

    Wow.. thanks guys, love all the physics lessons and history. I love watching Brownian motion and here is a very low tech way. To a drop of water just added the minutest quantity of full fat milk (Just touch the milk with a toothpick tip and touch the drop).. The fat globules show an endless brownian motion.. If you use non-homogenized milk it is even better with fat globules of different sizes. Havent tried it in foldscope as I thought it might be best with dark field. Inspired to try it now.

  4. Manu Prakash says:

    @laksiyer: That would be wonderful to try – and also valuable since everyone has access to it. Can’t wait to see that Video. Use the paper stack trick to get steady videos, which is critical for something like this.

    cheers
    manu

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