Bombardier spray cans

31 May 2009

Interview with

Andy McIntosh, University of Leeds

Chris - And it's not just popcorn that goes pop! Bombardier beetles are so-called because they quite literally bombard their predators with explosions from their rear-ends. They can actually expel boiling liquid over distances of a few feet. Now researchers have worked out how they do it and by copying their strategy they hope to build better aerosols and more efficient engines...one of the scientists doing that work is Andy McIntosh. 

Andy - These insects, bombardier beetles, are found mainly in the hot parts of the world. Although I believe they have been discovered on the south coast of England. They can be as large as two centimetres but usually they're a bit smaller than that. They have in their backsides the most amazing combustion chamber, Chris, which is about one millimetre long or smaller. They mix two chemicals: hydroquinone and hydrogen peroxide. Even before they come into the chamber (and nobody quite knows why) they don't react in the very small tube that comes in. In the chamber there is a mixture now of catalysts: peroxidase and catalase. These cause the  reaction to go much faster than it normally would which then heats up the water which is also there. It tries to expand but it can't initially expand and it can't vaporise therefore and boil properly until an exhaust valve, which is just a bit of cuticle - which won't really give way until there is a pressure rise sufficient to make it give way - when that goes then it basically suffers a vapour explosion. This all happens in 1/400th or even 1/500th of a second.

Bombardier BeetleChris - This is like you unscrewing the cap of a radiator of a car engine that's hot and the water still boiling as the pressure goes off. This is happening in the back end of this insect?

Andy - All the time when it's doing this, yes. It will do it for a few seconds. You're getting a huge number of blasts. If it goes on for ten seconds then obviously if it's doing it 400 times a second you can work out how many blasts it's doing.

Chris - Why would it do this?

Andy - Basically this beetle is being preyed on. Creatures like birds, spiders, ants are trying to get at this creature and they usually don't win. Very rarely a spider might be able to wrap it round in its web and eventually eat it but that's rare. They're usually stunned by this horrible mixture which is hot. Although they won't be killed by it they're be so stunned they won't be able to do anything for a few minutes and the beetle runs away.

Chris - It's an intriguing adaptation. It's amazing to think the beetle doesn't burn itself in the process. There must be some kind of adaptation that protects it. When you zoom in on the apparatus this beetle uses to do this what does that tell you about how it works?  

Andy - As I mentioned earlier there is an exhaust valve that doesn't give way until there's a particular pressure. Also you should note that there is an inlet valve which is, as the explosion takes place there is an expansion which causes the whole chamber to begin to pinch the inlet valve. As it explodes that stops more stuff coming in. What we didn't realise was that there was an exhaust valve which was made out of a bit of a cuticle which would only stretch under a particular pressure. We reckon it's about 1.2 or 1.3 bars. We're not really quite sure. Nobody's been able to get an instrument small enough to measure it.

Chris - The one thing that strikes me, when you hear a story like this is there are so many applications for what amounts to a spray gun. Could you copy this?

Andy - Yes. Just be aware that people are already using this pulse combustion idea for engines. There is such a thing as a pulse combustor engine. And indeed it was used in the V1: the German's didn't realise that was the same was what was happening with the beetle. Without realising it people have been using this type of idea already. What is unique about this is that it gives a facility for, if we can copy it, for actually getting a spray of liquid and controlling the droplet sizes if we know how to get the valve system working properly. Rather than relying on a passive system which is what the beetle does we have introduced an active system whereby we control when the inlet of water comes in and the outlet of steam and water takes place. We're able to actually use the idea of the beetle, not on the chemistry but on the physics of this valve system, to actually get some very unique spray gun (as you call it) applications.

Chris - Engine have to atomise fuel to get the fuel into the cylinder quickly and in a very widely distributed and with a very uniform particle size. That means the burn would be better straight away.

Andy - Absolutely. That's one of our major interested areas that we're developing. We know that we can, without using a huge amount of pressure to atomise the fuel which is what usually happens. We know that we can do this with much lower pressures, albeit we're using a vapour explosion idea from the beetle. We are using some pressure but nowhere near the pressures that people use at the moment. Also there's aerosols and other applications that these can be used for, maybe other pharmaceuticals as well.

Chris - I was going to say therapeutically because there are instances where you want to get very uniform, very fine droplet nebulised mixtures, for example. These will then be carried on the air deep into someone's lungs. Could you do something like that with this?

Andy - Yes. We think we can although we think that's a little bit of a long way off at the moment. We're pretty sure that just by using water, not fuel obviously (as in the fuel injector idea) but just using water we think we can use that as a spray which can produce steam and water which actually cools down pretty quickly and can take a drug which is in solution into a targeted area in the patient. We're not quite sure yet whether this will work out but we're certainly looking into that.

Chris - Leeds University's Andy McIntosh, who's working with the organisation Swedish Biomimetics 3000 to copy the bombardier beetle's spray technique.  

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