Have you ever noticed bubbles in a glass of a carbonated drink, and how they rise? Have you ever noticed how they seem to be really close together near the bottom, but further apart near the top?

Well I have, and today I did some science using some Kopparberg Mixed Fruit Cider.  Nom.

If you have two fluids, the lighter fluid will float upon the more dense fluid. In the case of my pint, the two fluids were a ~94% water mixture (delicious cider) and carbon dioxide. Clearly, bubbles will float in a pint.

With increased depth in a fluid there is also increased pressure. This is why air pressure is very low very high up in the sky, and why many submarines will get crushed below a certain depth in the oceans. With higher pressure, too, one finds that the fluid is more viscous and harder to move through.

In the case of a carbonated drink, the dissolved carbon dioxide is released due to agitation from microscopic impurities (this is why the bubbles seem to come from the same place; if there were no impurities, there’d be no fizz).

Let’s assume that the carbon dioxide is agitated and released at a constant rate from the vicinity of the impurity. Considering the cider at the bottom of the glass will be at a higher pressure (and therefore higher viscosity) than cider near the top, it follows that the bubbles will move through the cider quite slowly. As such, the bubbles will be placed closer together, if they’re created at the same time.

As pressure decreases closer to the top of the cider, the carbon dioxide bubbles can move faster through the less viscous liquid, and the spacing in between them will be greater. Because the cider doesn’t suddenly jump from a high pressure to a low pressure (it’s a gradual change) we can expect that the spacing between the bubbles gets bigger gradually from bottom to top.

So here is my science. I took a photo of my cider that was displaying a stream of bubbles. You can clearly see that the bubble spacing changes as a function of depth of the cider (or as a function of pressure or viscosity, if you like).


Using Adobe Photoshop, I measured the location of each bubble in the photo between the two blue lines. So, for each bubble I then had a pixel number (its location in the photo).

Here’s what I got.

This curve is exponential. While it shows that the spacing between bubbles grows exponentially, it may also show that pressure and viscosity vary exponentially, as both pressure and viscosity are directly related to depth (I think, I’ll have to whip out the ol’ physics book to check this one out. Also, as the bubble size changes, this graph might not give a direct measurement of pressure or viscosity. Stay tuned.).

Basically, the x-axis (along the bottom) shows the bubble numbers, 1 being the bottom bubble, and 32 being the top. The y-axis (on the side) shows the bubbles’ positions as a pixel number. As you can see the positions change greatly from one bubble to the next: the higher bubbles (at the right hand side of the graph) move much faster than the ones near the bottom (at the left hand side).

So there ya have it. I am a total nerd.

Further experiments:

  • Compare bubbles in alcoholic drinks to those in non-alcoholic drinks (the ethanol affects density).
  • Measure the growth of bubble size as a function of depth/pressure.