How does a drinking straw work?

28 November 2010

Question

When you sip a beverage through a straw, what exactly is going on inside your mouth to create the suction strong enough to bring up the liquid?
I know it doesnt have anything to do with breathing, because i could breathe just fine when i used the straw (ha ha no i didn't inhale the pop). It just seems so natural, I never really considered just how it works...

Answer

Chris - It's something we all just take for granted, isn't it? You'd put a straw in and suck up a drink or something; but what's actually happening?

Well the answer is, by changing the shape of your mouth, you're actually getting the atmosphere to do the work for you. What you do, when the straw goes into your mouth, you close your lips around the straw, which seals off the outside world from the inside of your mouth; and you then change the shape of the inside of your mouth by lowering your tongue or drawing it towards the back of your mouth. This is a bit like pulling the plunger out of a syringe. You increase the volume inside your mouth, which means the pressure goes down; and it goes down so that the outside world has a higher pressure than the pressure in your mouth; this pushes on the drink surface, forcinging the drink up the straw and into your mouth.

The reason you can still breathe is because actually, you're not breathing through your mouth. You're breathing through your nose; the nas and pharynx (throat) connect at the back of your throat and that's not where this volume change is going on because you sealed off the back of your mouth with the back of your tongue.

So, you've got this closed cavity that you take drink into and then you push it down your throat in order to be able to swallow.

That's basically how a straw works, and it's not too dissimilar really to when you're getting oil out of the ground: when you've got an oil well, unless it's under high pressure, what you do is pump something more dense than the oil (like water or mud) down into the well where it sinks beneath the oil and pushes it up above it; the oil then gets displaced back up towards the surface.

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