These are dipping ducks or drinking birds or whatever you like to call them. We made a film about them about six years ago you can still watch that. But today I’d like to look at them in a bit more detail, and in particular, we’d like to use a thermal-imaging camera. First let’s have a look at how they’re made. As you can see, when the felt head’s removed it’s essentially just two glass bulbs connected by a tube. But it’s important to note the tube extends down here and that the liquid inside is something like dichloromethane, a liquid that evaporates easily. It’s normally clear but colored here so we can see it better. Because the liquid is so volatile, it means we have quite a bit of vapor in this area. And importantly that vapor is sealed here in the upper chamber because, as you can see, the tube down here is below the liquid level. Now with the felt head on — and that’s also important — we wet the bird’s beak and water spreads through the felt quickly encompassing the whole head. This water then starts evaporating, cooling the head. It’s a bit like the bird’s sweating to keep cool and that’s when things get interesting. Vapor in the upper chamber starts to condense as it cools, and that means the pressure’s dropping. Something called the ideal gas law would show with the temperature and amount of gas falling, the pressure falls too. Actually because of the gas-liquid phase transitions, this is more accurately described by something called the Clausius-Clapeyron relation, but that’s not quite so easy to pronounce. You can look into that more later. Anyway, the important thing is the higher pressure down here where it’s not cool, starts forcing liquid up the tube. We can see it and eventually due to the toy’s design the bird becomes top-heavy and tips over. And now because of the clever placement of the tube, it’s now above the liquid level. The seal between the head and the tail is momentarily broken. Warm gas from below can now force it’s way up the tube and the liquid drains back down the bottom. Importantly, the pressure is equalized and because the liquid’s drained back down, the toy tips back to the upright position. This can go for a long time but even longer if we have a glass of water placed here because the head stays wet for as long as we choose. Now of course this isn’t any kind of perpetual motion. Heat is constantly being extracted to keep the water evaporating. But speaking of heat, let’s try this another way. Instead of cooling the head, let’s use a hair dryer to warm the tail. Now first the blow-dryer actually moves the toy into its own position but that’s to be expected. But here now, instead of lowering the pressure up top, we’re increasing the temperature and amount of gas in the bottom bulb, thereby increasing the pressure down below. And again liquid is forced up the neck and sure enough, this works too. Here’s a comparison: you can see the hot one’s almost too hot. The neck’s filled with liquid almost all the time. Having done this, I then decided well, maybe we should try a few other things. First, using this tea light as a source of heat under the bird. Look how quickly the liquid rises. And the bird stays sort of permanently tipped. Trying to force it to the vertical shows the problem: there’s just too much heat. Sort of endless loop of liquid and hot gases being forced up the tube by the high pressure below. Here you can see just how much the lower section’s being heated. So next I tried putting the tealight here hoping for a gentler application of heat to that bottom bulb. Just a little bit of liquid creeping up, but never quite enough to tip the bird. And it was thermal imaging that revealed the problem. You can see the tea light is actually applying tremendous heat to the top bulb. The gas pressure up top is increasing, pressing down on the liquid: exactly what you don’t want. Here’s something else and I really didn’t know what to expect this time. How will the drinking bird work if you use really hot or freezing cold water? Before watching, do you have any guesses? Well, after an initial dunk and letting the water spread, the cool-headed bird pretty much works properly, what you’d expect. But the hot-headed bird, that’s more interesting. The heat in the head must be keeping the gas pressure in the top bulb too high, preventing the liquid from being forced up the neck of course. But watch closely. Evaporative cooling starts to kick in. The temperature drops. The pressure drops. The higher pressure below now does its job, forcing liquid up the neck. The bird starts dipping as it over balances. It’s gradual and actually the mere act of dipping puts more heat into the head, stalling the process – you can see it. But eventually the back part of the head becomes cold enough and it’s business as usual. In fact, though the beak’s been constantly replenished with hot water, as you can see, the back part of the head stays cold enough. And soon, you can see the two birds dunking pretty much similar rates. If you’d like to go back and watch our original video with Professor Phil Moriarty about these dipping birds, I’ll include a link on the screen and in the description. And thanks to Professor Moriarty for helping me out with this video. And lastly, also a big thanks to Google’s Making & Science team. If you’d like to see more films with various YouTubers they’ve supported, there are also links on the screen and in the description.