The right tool for the job

I almost titled this post Daily Mail celebrates work of immigrants shocker but as they have written a pretty accurate article on work I am part of, that would be a bit ungrateful. Yesterday a paper came out in Physical Review Letters that I am really rather proud of, although I made only a small contribution to it. Most of the credit should go to Andrea Fortini, who discovered the effect the paper describes, and to Nacho Martin-Fabiani and Joe Keddie who did the experiments to show that it works in the real world too. Andrea found the effect in computer simulations. We also had help from collaborators in Lyon who made the particles Nacho used.

So it was a team effort, like a lot of science. But its origin was simple. Emulsion paints (and a bunch of other things) are basically very small particles suspended in water. When they dry the water evaporates pushing the particles together, and as these particles are soft they then coalesce, forming a film that is the dry coat of paint. By small I mean mostly less than a micrometre, or a thousandth of a millimetre, in diameter.

As the particles are so small, we can’t actually see how they move and pack together as the paint dries, and so how paint dries is very poorly understood, despite the millions of tons of paint that are used all over the world. Paint is relatively complex and basically with complex things, unless we can directly see what is going on, we are often a bit clueless as to what is actually happening.

If we can’t see what is going on, then one way round this to make a simple model and study that model in a computer simulation. In a computer simulation we control everything and can always see exactly what is going on. This is what Andrea did, and my recollection is that almost as soon as he studied mixtures of big and small particles he saw layering – which is the subject of the paper. Basically, we discovered that if you mix big and small particles together then during drying they separate out into a layer of small particles on top of a layer of big particles. This surprised us, as earlier work had predicted layering the other way around, with the big particles on top.

But the effect is real, and it is a nice example, I think, of applying the right technique, here computer simulation, to the right problem. I am bit surprised people have not applied computer simulation to this problem before, but I am not complaining, it is great to be part of team coming up with this cute result.