Comment and Analysis
“MY FAVOURITE SCARF NOW SMELT LIKE THE START OF A TEENAGE LAD’S NIGHT OUT”
Helen Czerski on aerosols
One day last week, I was sitting outside with a glass thermometer, an aerosol spray can and a film crew, regretting having asked the director to acquire the aerosol can. For reasons incomprehensible to me, he’d picked the cheapest, smelliest men’s deodorant that he could find, and my favourite scarf now smelt like the start of a teenage lad’s night out.
But when I held the bulb of the thermometer directly in front of the escaping plume of spray, the red liquid dropped like a stone. It reached -20°C within five seconds, and the bulb completely frosted up. Even though this demo had been my idea, I hadn’t tried it before, and was astonished at the speed of the temperature drop. Physics demos sometimes have a reputation for ‘not working’ (which is almost always more to do with the experimenter and the setup than physics itself), but this one was jaw-droppingly effective. And so it should be, because that temperature drop is an integral part of how an aerosol works. You can’t have one without the other.
We know that aerosols have liquid in them, because we can hear it when the can is shaken. This liquid is a mixture of whatever you’re buying (hairspray, air freshener, paint, etc) and a propellant. In my deodorant can, the propellant was a mixture of butane and propane, both of which are gases if you release them to the atmosphere. But they become liquids when you put them under a bit of pressure. Inside a typical aerosol can, the pressure is three or four times higher than the atmosphere, so most of the butane/propane mixture is liquid. Once you empty some of the liquid out of the can by spraying it, a little of the rest of the liquid evaporates to become gas and fills the gap, and so the high pressure stays exactly the same. That’s why the spray pressure doesn’t change as the can empties.
The cold comes from the second stage. As I held down the button on top of the can (prompting the director to step sideways to avoid being tainted by the scents of his youth), the high pressure inside forced the liquid up a tube towards the nozzle and outwards to meet the big wide world. On reaching the low-pressure air, the propellant instantly evaporated to become a gas, shattering its deodorant sidekick into millions of liquid droplets. So just at the point that the spray left the nozzle, it was a jet of pressurised gas carrying a cargo of tiny liquid scent capsules.
The final act in this drama is that once the gas isn’t confined any more, it expands by shoving outwards on the air molecules around it. That has a heavy energy cost, but it has to happen, so the gas molecules themselves get cooler because they’ve given away energy.
The lowest temperature you can reach with a typical aerosol can is around -25°C, way below the normal temperatures of our world. It’s only a tiny volume of gas, but it could still damage your skin if you sprayed continuously at a very short distance.
The physics of gases is beautifully elegant, but because most gases are invisible, it’s usually hard to see that elegance in the world around us. However, the physical gas laws have nothing to say about the odours that the gases carry with them, and they may be less enticing.
I’m off to give my scarf a wash!
Dr Helen Czerski is a physicist and BBC presenter