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Science

When I pour a fizzy drink, why do bubbles stick to the straw?

I was sat with friends and enjoying Christmas festivities, when this text came through:

“Why do bubbles stick to the straw in fizzy drink?”

My niece had come up with the question after watching bubbles dance around in a glass of soft drink. For a seven year old, this was a pretty good question. My sister (who also has scientific inclinations) was unable to give her a convincing answer, and knowing that I’m a sucker for trying to solve anything remotely scientific, she turfed it to me…

I looked into it and gave her my best answer. I also discovered that the science of bubbles is pretty interesting (no really). So if you’ve ever wondered about how and why bubbles form in a glass of soda/champagne/beer then read on!

Enter the fascinating & bubbilicious world of fizzy drinks…(with some stuff even seven year olds will enjoy)!

Where do bubbles in a drink come from?

I think that watching bubbles in a glass of champagne has a mesmerising effect, a bit like watching the movement of flames in an open fire. But where do those bubbles come from? Why does shaking a bottle of coke before opening it cause an explosion of sugary drink?

Sodastream: Yesteryear's kitchen gadget

Invisible to our eyes, all water contains dissolved gases. Take river water, if it didn’t contain oxygen fish wouldn’t be able to ‘breathe’. Most liquids only contain tiny amounts of gas, but you can squeeze more gas into a liquid in a process called ‘supersaturation’. Anyone remember Sodastream?

Those devices were all the rage in the 1980’s, and they let you carbonate juices to make your own fizzy pop by squirting compressed gas (carbon dioxide) into your chosen beverage. We had one when I was a kid, and I loved it; although if you kept pushing the ‘gas’ button it started to make a funny noise that sounded like something would explode!

If you don’t happen to have a Sodastream, you could always just try to get hold of a cylinder of carbon dioxide (although I doubt Tesco’s stock them) and try this novel way of ‘supersaturating’ water (just don’t try this at home kids)…

The Life of a Bubble

Carbonated drinks are bottled under high pressure. Opening the bottle releases the pressure that is keeping the carbon dioxide gas dissolved, and with a tell-tale ‘psssst!’ bubbles are brought into existence.

Pssst! The sound that betrays many a school child trying to secretly drink a bottle of pop. That noise is the sound is pressurised gas escaping.

But bubbles can’t form just anywhere: These delicate things are rather choosey about where they call home! They prefer to start their microscopic life on any uneven surface. Tiny scratches, irregularities or bits of dust and dirt are good enough!

Anywhere a bubble likes to form is called a 'nucleation site': A finger has lots of tiny bumps and ridges and is ideal for bubbles to form and stick to.

In the few seconds after opening a bottle, bubbles start to grow at these ‘nucleation sites’. When they grow too big, they will float off toward the surface. The bigger the scratch or bump is then the longer the bubble will be able to ‘hold on ‘; just like how a climber can hold onto an irregular surface for longer than a smooth surface. A bubble will rise to the surface (and pop), unless it hits into another ‘sticky’ nucleation site.

Drinking straws look smooth, but actually they are covered in tiny imperfections. Like 'Velcro', each microscopic bump offers a place for a bubble to stick to.

You can test all this for yourself with two wine glasses, some fiz (e.g. champagne or fizzy drink) and some sugar. Sprinkle sugar into one glass and then pour drink into both:

Sweet Champagne: The glass with a dusting of sugar inside has more 'nucleation sites' and will fizz more!

But what about shaking before opening?

Tiny bubbles on the inside of a shaken bottle


Get a bottle of fizzy drink and give it a good shake! I dare you…

If you’ve done this then look closely at the inside the bottle (don’t open it – d’uh)! You’ll see that your mixing has caused a few bubbles to form on the inside of the bottle. There will dozens more microscopic bubbles that you can’t see. And each one of these bubbles acts as ‘nucleation site’, so when you open the bottle and literally millions of bubbles will explode into existence around each!
And we all know what happens next…

Kitchen towels at the ready!

Fun Stuff to Try with Kids (big and small):

Mixing coke with mints or make your own lemonade

A can-tapping experiment for slightly older kids

Read More about bubbles:

A Video of how soft drinks are made (from Youtube)

Learn about how champagne is made (from Youtube)

All about Nucleation Sites (Wikipedia article)

An excellent book “How to Fossilise Your Hamster” by Mick O’Hare has more fun things to do with bubbles!

About Stuart Farrimond

I love writing about science and health subjects. Strange, because I also teach the same things. I trained as a medical doctor before turning my hand to other things. Shortlisted for The Guardian/Observer for Science Writer of the Year 2011 and editor for Guru Magazine I also like to grow large pumpkins...

Discussion

5 thoughts on “When I pour a fizzy drink, why do bubbles stick to the straw?

  1. Hey Stu,

    I stumbled upon your article whilst wondering (and looking on the internet for an answer!) about something else bubble related.

    I would like to know how fizzy drinks keep thier bubbles? I know they will go flat eventually as all the bubbles or CO2 finally escape, but why don’t all the bubbles escape at the same time?

    I hope you can find an answer!

    Cheers,

    John.

    Posted by John Lyons | May 24, 2011, 3:47 pm
    • Hi John,
      Great question! I spent last night pondering that one and think I have the answer.
      – Why when you open the freezer door doesn’t all the heat rush in instantly? Why when you spray a fragrance in a room does it take time to spread out?
      Much like in these scenarios, the answer that it takes time. Gases, like temperature take time to move (it’s called ‘diffusion’). The speed at which gas or temperature moves from a place of high concentration to lower concentration depends on the difference between the two. So, for example, opening the freezer door will result in heat rushing in quickly to begin with and then slower as the temperatures get closer to room temperature. (Chemists and physicists call this difference a ‘gradient’). Likewise, with our bottle of fizzy drink when we first open the bottle there is a large difference between the pressure of CO2 in the drink and the pressure of CO2 in the air — and so the CO2 rushes out very quickly. Over time, and as the drink loses its ‘fizz’ and the CO2 concentration ‘gradient’ between the drink and the air is less, so the CO2 takes longer to come out – the bubbles get smaller, and less frequent. You can think of it as the difference (or ‘gradient’) being like the pushing force that propels the gas to ‘want’ to escape. In the same way that when you open a refrigerator door it won’t loose it’s temperature as quickly as when you open a freezer door.
      Even more science-stuff about what an ‘equilibrium’ is is at Wikipedia.
      I agree, it is quite weird – you would almost expect it to come out as one huge bubble in an instant… but then opening fizzy drinks would be very problematic…
      Phew! Does that make sense?

      Posted by Stuart Farrimond | May 25, 2011, 10:13 am
  2. Excuse me, but I was wondering if you could help me with some homework.

    This might sound stupid but…..

    Why does carbon dioxide stick to a raisins rough surface???

    Thank you, all answers are appreciated.

    Posted by Lisa | March 31, 2012, 8:46 am
    • The bubbles that you see inside a drink are dissolved carbon dioxide ‘coming out’ of solution. The dissolved carbon dioxide forms bubbles, and these bubbles tend to form on surfaces – the bumpier the better. Raisins are very irregular surfaces and so bubbles tend to form and stick to them (see the above description of ‘nucleation sites’).

      Posted by Stuart Farrimond | April 10, 2012, 4:31 pm
  3. I have COPD and noticed my breathing is worse after drinking a couple of glasses of champaign. Do you think the bubbles could have any connection to the breathing condition.
    Thank you

    Posted by Dee Jowers | June 6, 2012, 5:05 pm

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About Dr Stu

Hospital doctor turned lecturer and science communicator, I love trying to answer life's questions - whether it is how our body works or the best way to dunk a biscuit.... Read more...

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