Natural selection has given rise to some incredible things. The diversity of life on Earth is an ongoing testament to that, showcasing that life can sustain itself pretty much anywhere so long as there’s water present. What’s incredibly interesting to see is how parts of nature take on properties of things you wouldn’t necessarily think they would, like the planthopper with gears in its legs. It seems the more we investigate life here on Earth the more weird and wonderful behaviour we come across and none seems to be more stranger than the hive mentality of fire ants giving rise to a substance that’s neither liquid nor solid:
The research paper that this comes from is quite interesting as they performed a whole bunch of materials tests on the fire ants to see what the properties of the giant ball were like. Interestingly the fire ants, whether they’re alive or dead, exhibit properties of non-Newtonian fluids, specifically shear thinning (like when paint doesn’t drip off a brush). However the characteristics of the live fire ant ball don’t directly classify it as either a solid or a liquid although a similar non-live sample acted much more like a solid. That interesting property is most likely due to the way the ants rearrange themselves in response to stress but the actual mechanism of how they do that, especially in large numbers, is still something of a mystery.
It seems that this behaviour likely arose out of a particular selection pressure, namely flooding. The fire ants can bind themselves together in a ball or mat to form a raft that will float on water thanks to the large surface area relative to the fire ants weight. It’s the same principle that allows water skimmers and other insects to seemingly float on top of water, using the surface tension to provide them with buoyancy. The material properties that fire ant ball carries with it are likely a side effect of that adaptation, although there might be other pressure that led to it as well.
I’d totally go out and try this for myself but I value my hands far too much.
There’s a really interesting experiment you can do in the comfort of your own home that demonstrates an effect I’m about to show you. All you need is a frying pan and some water. Heat up the frying pan until its good and hot and then flick droplets of water onto the pan. Curiously the droplets won’t instantly burst into little puffs of steam, instead they’ll skitter around on the surface of the pan in apparent defiance of the blazing surface that’s underneath it. This effect happens when any kind of liquid comes into contact with a surface past a certain temperature but I hadn’t really considered what would happen if you put the surface in the liquid:
The phenomenon at work here is called the Leidenfrost Effect. It’s a pretty cool reaction whereby an initial layer of vapour formed by a liquid hitting a sufficiently hot surface forms a protective barrier which is what allows those water droplets I described earlier to skitter around rather than turning into steam. It’s clearly visible in the video at the start where a pocket of water vapour forms around the outside of the red hot sphere. It eventually collapses as the vapour isn’t a perfect insulator but it does manage to stay quite hot for a lot longer than you’d expect.
One thing I can’t figure out a good explanation for those is the incredible sounds that are produced. The rapid generation of steam could possibly explain part of it as some of the sounds are similar to what you hear from say a steam wand on a coffee machine but most of them have a definite metallic twang to them. It’s quite possible that all of the noises are coming from the ball itself as it cools down much like some cars which make a distinct “tink” noise when turned off (the noise comes from the exhaust pipe cooling down). I wasn’t able to track down a name or reliable explanation for this effect however so if you’ve got one I’m all ears 😉