We are gonna, in principle, make the most boring Sixty Symbols video ’cause we’re gonna talk about watching paint dry. At a macroscopic level, watching paint dry is, as you know, an incredibly boring thing to do. At a microscopic level, I wanna try and get across over the next few minutes, that not only is it interesting, it can be fascinating. So we got latex white paint. So what this is, it’s 1 µm sized – 1/1000 of a millimeter sized – spherical particles of polymer in water. So that’s the simplest possible paint you can use. And for physicists, that’s what we really like doing. We really like and… throw away all the difficult chemistry and really, let’s focus on what the physics is here. So I’m gonna take some of the paint, and to make it more straight-forward and easy to see what’s going on, I’m gonna take this paint and what I’ve done is, I’ve diluted it down slightly, I’ve added some… just, basically water. Increase the magnification.
Now. You might be able to see these little things moving around, those are the individual particles. Let’s try and focus in, and you can see they’re already collecting at the edge. We should be able to go even… increase the magnification even more… So, each one of these is one of these individual µm-scale… µm-diameter particles, and you can see that they’re moving across, you can see them moving around in the solution and they’re joining on here, they’re aggregating at the edge; they’re falling out of solution. Now as we move this in and out of focus, we can see now, there’s our second layer building up at the edge and you can see these little… they’re joining. As the droplet dries, what’s happening is that this, what we call the wetting front, or the contact line between the solvent, the substrate and air, is moving. The droplets are drying. And so we’re gonna see some very interesting things happen. But it’s moving now as the droplet’s… basically the droplet’s shrinking, and we’re looking at that shrinking process. Brady: So that’s moving in real time?
Prof. Moriarty: And that’s moving in real time. That’s absolutely moving in real time, yeah. And you’ll see it starts to become pinned- let’s try and focus on that. Here’s another pinning event. Where you can see… and then it’ll sudddenly let go. And these pinning events are very, very important where the edge of the droplet wants to move back but sometimes there’s an inhomogen near the… there’s a piece of dirt or there’s a defect at the surface which pins it and stops it moving. And sometimes that can… and in many cases what stops it moving is when the particles themselves pile up at the edge. [I] must point out that the colors we see here are false colors – these aren’t actually the colors of the solution. It’s the way we’re doing the experiment; we’re playing around with polarization to improve the contrast so these really aren’t… The colors aren’t true colors as such. And in fact, you can map the color that you see onto the thickness of the film. But this is… it’s exactly this type of process that, really, state of the art nanoscience – in some aspects of nanoscience is trying to exploit this process with nanoparticles, so that you can lay down regular lines of nanoparticles – much, much smaller particles than this – on surfaces. When we have a droplet of the paint on a surface, as we have here, and when you’re painting it at a wall you got many- there are many, many thousands of droplets in all sort of thin films of particles and solvent, but the key thing here – and again, here’s a nice de-wetting event, or de-pinning event. You can see, what’s happening is, it’s piling up. Let me… Sorry, Brady, to keep interrupting. So it almost looks organic. This is purely… it almost looks like it’s living, I think. But it’s purely anorganic, it’s just water and particles. But with fluid dynamics it does remarkably interesting things. Let me chase the front again. And this is a very, very small droplet – again, more pinning events. In this case, it’s leaving particles behind. You see the particles leaking out and being left behind? Ah, that’s a really nice one. That’s really nice, see them just dropping out, and you can see the solvent thinning down. So, in terms of painting, this IS the painting. This is, in essence, the painting process where you want to – you have particles in a solvent, you want to get those particles, you spread them on the wall and then you want the solvent to evaporate. And here we go. So… Oh, it’s moving around, that’s very interesting. So you can see here, now it’s stopped. We’ve got a lot of solvent motion still over here. And you can see the layers, the various layers of the paint particles and you can see here, it’s all piling up. So as I’m changing the focus, we’re just going up, the drop, you can see it moving around. That’s possibly because I’ve been moving the focus quite a bit. And now the last part… ah.
So that’s the last part of the drying. All we had was that. What’s making this white is the way that the particles scatter the light. So we started off with that suspension of particles in water and so, all we’ve got left behind are those latex particles. And if we wanna look at that droplet, how it’s dried… we are looking at a dried drop of paint. What excites me about this is that a process that seems so mundane, boring, uninteresting when you look at it in the right way, it is absolutely fascinating. And that, I guess, is what physics is all about. It’s taking all the stuff that happens around us in the real world and trying to understand it, trying to interpret it, and to a certain extent, trying to control it. And I guess that’s… the fundamental reason I’m most excited about that is, you got the most boring thing or what many people would consider the most boring thing, when you look at it in the right way and you think about the physics, it becomes unbelievably exciting.