# Do bubbles in Guinness go down?

## The filming

Our initial attempts to video the bubbles in Guinness with a regular video camera were all in vain.  The bubbles are just too small, and too fast to see clearly.  We borrowed a high-speed digital camera (Kodak HS4540), which is capable of taking up to 4,500 full frames in one second.  Below is a picture of the setup.  The camera takes the pictures and passes the images to the electronics box where they are stored.  These can then be recorded to VCR, or transferred as digital images to the computer.

## How small? How fast are they?

Most of the footage you can see on this site was taken at 750 frames per second, and is replayed at 25 frames per second - effectively slowing the bubbles down 30 times.

We used a zoom lens at around 6 times magnification to see the bubbles, giving a field of view (size of the whole picture) of about 3 mm or 1/8" - about the same as the width of two pennies.

What can we say about the size of the bubbles?  We can measure them from the individual frames: see the picture opposite.  We find that they are about 40 to 120 micrometers (1 to 4 thousandths of an inch) in diameter.
That makes them roughly the size of human hairs (a human hair is roughly 100 micrometers wide).

And how fast are they going?  The time between the two frames below is 20 thousandths of a second.  The same single bubble has been circled in yellow at both times.

So the bubble travels 0.422 mm (0.0166") in 0.02 s.  At that speed it could travel 2.11 cm (0.831") in 1 second, about 0.076 km (0.047 miles) per hour, or about a mile a day!

Hold on a second!  You said they were fast - right? 0.047 miles per hour doesn't sound fast to me!

Yes, that's true.  But we are mostly used to hearing about speeds of things that are much bigger - like cars and planes.  To put it into perspective, let's see how our bubble stacks up against a 100 m athelete.  How wide (from front to back) is a 100 m runner?  Let's say 40 cm (16"). Our bubble is only 0.011 cm wide (0.0043") - four thousand times smaller.

Our athelete does 100 m in 9.78 s, that is, 10.2 m in 1 s.  Compared to his width, that's 26 body widths in 1 second. By comparison, our bubble only does 2.11 cm in 1 second.  But that's 192 times its own diameter in 1 second - more than 7 times better than our 100 m champion!

## Acknowledgements

We thank the EPSRC Engineering Instrument Pool for the loan of the Kodak hi-speed digital camera equipment.

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(c) 2004 Alexander & Zare