Looking inside soot particles

By looking into early soot particles using a high magnification electron microscopes and a "computational microscope" we can start to construct a picture of how the molecules are arranged and better understand how to halt soot formation which is bad for human health and for our planet.

We have recently published two papers that have approached this question from two different directions; Kimberly Bowal leading the charge from the computational side and Dr Maria Botero from the experimental side. 

Together they provide an interesting picture of how mixtures of aromatic molecules are arranged in liquid soot, carbonise as they are heated in the flame and helps us better understand how to stop soot from forming or provides means of destroy them more readily.

Computational microscope

Using accurate mathematical descriptions of the interactions between atoms we can simulate the way molecules self-assemble and see inside a small soot particle. Previously our group has focused only on clusters composed of a single type of molecule. An example of a cluster of coronene is shown below.

Soot is not made up of a single molecule but many hundreds of different sized aromatic molecules. So in this paper, we wanted to study how mixtures of different sized aromatic molecules arrange themselves. There are quite a few challenges simulating such as system;

1. We can only simulate small isolated clusters otherwise the simulations will take years to compute
2. We can only simulate for a short duration of simulation time (nanoseconds) but we want to figure out what the most stable arrangement of molecules is over the timescale of soot formation (milliseconds).

The first challenge is problematic as clusters in the flame will be exchanging molecules with the gas phase. This would be too challenging to attempt with such large clusters. Instead we apply a rubber band to molecules that leave the cluster so they always rebound back to the cluster. 

The second challenge is overcome by simulating hundreds of each cluster in parallel all at slightly different temperatures we then exchange clusters that are more ordered with lower temperature clusters. In this way low energy orientations get cooled and quenched providing the most stable arrangement. This method is called replica exchange molecular dynamics and has been used to simulate transformation of cellulose into coals on geological timescales so it is able to really accelerate the whole simulation to flame timescales.

The low temperature structure is also not influenced by the rubber band (as it is a solid so no evaporation occurs on simulation timescales) and therefore we are able to find out given an initial configuration of different sized aromatic molecules what is the most stable cluster geometry. 

Here are the results from two of the low temperature clusters (below). You can see the large aromatics on the inside and the small aromatics on the outside. We found this pattern in all of our clusters studied. Big molecules in the middle small molecules on the outside. 

This is also consistent with theory which suggests that the molecules with the greatest interactions would reside in the centre. But what do we see experimentally?

Electron microscope

We used an electron microscope which is able to see the aromatic molecules. In order to convert the images into numbers we used a custom program Maria wrote to convert the dark fringes seen in the image below (a) into lines (b) we can then split these lines up and measure how long they are and how curved they are.

Here are some of the many images that Maria collected at each of these heights with the images already converting into lines. Just looking at these fingerprints you can start to see some patterns. The early soot particle contains short fringes (indicating small molecules) while the particles further downstream have long fringes around the outside and smaller fringes inside.

We also found that the fringes in the smallest particles are really quite curved. The fringes suggest that over 60% of the aroamtics at the lowest height above the burner contain pentagonal rings indicating curvature. You can look at a blog post on the impact of these curved aromatic which we published earlier this year.

We also found that at the lowest height you have longer fringes in the middle of the particle compared with the outside. From the computational studies this indicates these early soot cluster are liquid. 

But as you go through the flame this pattern inverts with long fringes around the outside and smaller molecules molecules in the middle. This suggests they are no longer liquid clusters but are now starting to chemically react in the high temperatures (carbonising) and forming longer aromatics when two fuse to become one. This makes sense of recent nanoindentation studies that show that soot that has gone through the entire flame is actually quite hard when you isolate a single soot sphere almost the same hardness as charcoal.

Hypotheses to test

What are planning to do next and how are we going to extend this work

• We have been talking with other collaborators on using nanoindentation to study early soot particles to confirm if they are indeed liquid. 
• If we can understand how the carbonisation process occurs we can limit this process. From imaging of soot as they are broken down it was found that the outer shell is the last part to burn away making it important to reduce this can be achieved by lowering the temperature and inhibiting the particles grow too large.
• We have also been considering clusters of curved molecules and seeing if the same trends of partitioning hold. They certainly look quite different (see below).

So through a combination of computational and experimental studies we have been able to understand how different sized aromatics partition inside soot particles with the arrangement of longer aromatic molecules switching from the inside to the outside of the soot spheres indicating that early on liquid clusters give way to carbonised clusters.