Question: If the particles are so small, how do you find them?

Sean Collins answered on 5 May 2020: last edited 5 May 2020 5:11 pm

I often work with powders, where the collection of particles can be seen and handled (so I know where they are). That’s a bit like taking a handful of sand and then putting it under a microscope to be able to see the details of what’s happening at the surface or inside a single grain of sand. We can see individual grains of sand, but the particles I work with are even smaller.

To be able to see the particles, and to find individual particles, I use electron beams. Atoms, some of the basic building blocks of molecules and solids, are usually made up of a nucleus at the centre of the atom. Electrons are located around the nucleus usually, but we can pull them out of the atoms of metals and turn them into a so-called ‘beam’. It might be useful to think about this beam as a particle beam, but physics actually tells us that the beam will behave like a wave as well as behaving like a series of particles.

We accelerate (speed up) the electrons to about half to two-thirds the speed of light (a sort of cosmic speed limit, so it’s a useful constant to refer to). Basically, the electrons are moving incredibly fast (meaning, they have really high energy). The high speed means they can be focused very sharply. I use magnets to focus the beam of electrons, and then I am able to see particles that are invisible to the human eye – I can even see individual atoms!

Sometimes finding the particles can be a bit trickier. For example, there are many particles in the air that we don’t see with our eyes (especially in cities when there is pollution but also from natural sources like spray from sea water that sometimes releases salt into the air). To find these particles, I would usually need to have a filter that catches or traps the particles (a specially designed filter that allows gas molecules through but that catches dust). Then I could stick the filter into my microscope to look at the particles.

Sometimes I also work with particles that are suspended in a liquid. There are ways to measure the composition (what chemicals are in the liquid), so I can tell what’s in the liquid. Either if I put particles into it myself or if I know that the chemicals in the liquid cannot be dissolved in the liquid, then I can be pretty sure there are particles in the liquid! Then I might take some of the liquid and put a few drops onto a very thin piece of carbon. In this case, I can just let the liquid evaporate and it will leave the particles behind. Then, with my electron microscope I can zoom into look at the particles.

It can sometimes take a long time to find particles, though. I usually have to move the piece of carbon (prepared on something called a ‘sample grid’ because the carbon is supported on a grid of copper that is made to be the right size for my microscope) until I see some particles in the microscope.

I can ‘zoom out’ on my sample to look for particles quickly over a larger area as long as I can still pick up signals from the particles that are very small. Otherwise, I need to zoom into where I can see them and then go through a search pattern. Imagine you have to look at a piece of paper through a tube (like a loo roll or kitchen roll cardboard tube) – the only way to see more of the paper is to move the tube or to move the paper. It’s much easier to move the sample grid in the case of my microscope. Moving the sample (or the paper) systematically allows for exploring and finding a particle or section of the sample to study more.