Com­ment and Anal­y­sis

“MY FAVOURITE SCARF NOW SMELT LIKE THE START OF A TEENAGE LAD’S NIGHT OUT”

BBC Earth (Asia) - - Contents -

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

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