Definition: What is Brownian Motion?

Not all bumper cars are created equal. Somehow I always pick the one with a sticky gas pedal and become the object of torment for my opponents. This is essentially what happens to microscopic particles when you stick them in a jar of water. The microscopic particle is a massive bumper car stuck in the middle of the rink surrounded by a bunch of tiny super-charged bumper cars, water molecules. The water molecules are all crashing into the poor, defenseless particle at once, and this makes it move. If you were to watch the particle, it would look like it’s moving in random directions for no apparent reason. A botanist named Robert Brown (hence the name Brownian Motion) watched pollen grains doing this in water with a microscope almost 200 years ago.

Click on this image to watch "Dark Field Video Microscopy of 100 nm Gold Nanoparticles"

Click on this image to watch “Dark Field Video Microscopy of 100 nm Gold Nanoparticles” by Gregg Duncan

It turns out actually that this process isn’t entirely random. How fast particles jiggle around is dependent on a few things. As the particles get bigger, they will move slower because more water molecules have to gang up on it to push it around. We can also give the water molecules more energy by increasing the temperature. At higher temperatures, the water molecules will move around faster and bang into the particle with more force that will make the particle move more quickly. It’s also important what kind of fluid the particle is immersed in, as you may or may not know from experience, it’s a lot easier to swim through water then something more viscous, or “thicker” like molasses. So if you take these few things into consideration, we can predict how big the random jumps particles make pretty accurately.

So then how do we measure how fast they move? Well, you need some way to watch the particles move around and this is still done the same way Robert Brown did back in the day with a microscope. Then you can watch the particle for awhile and write down where it went over time. You’ll see that if given x amount of time, the particle on average moves about the same distance. In fact, if you plot the average distance the particle moves versus time, it’s a straight line. The slope of that line is what we use to figure out the diffusion coefficient of the particle, and this tells us how fast or slow the particles will move.

Luckily in my lab, we have fancy cameras attached to our microscopes to make movies of particles moving around. I have a video here I took recently of some gold particles I was messing around with. I used a technique called dark field microscopy, which is a nice way to image really, really tiny particles like these that can’t be seen in normal microscopes. We’ve also written particle tracking software that does all the number crunching for us to figure out how fast the particles are moving. It is important for us to know how fast particles are moving as they approach each other for instance, when we try to build crystals out of them or as they approach a surface, like a cancer cell.

Story by Gregg Duncan, a Ph.D. student in the Department of Chemical and Biomolecular Engineering with interests in nanoparticle-based biosensing and drug delivery.

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