Most people, at some point in their lives, must have pushed a take-away coffee cup across a table and watched as patterns form on the liquid surface. Sometimes these patterns seem to stand still, we’d say that they form ‘resonances’. On even rarer occasions, on dragging your cup across the surface, you may have seen coffee droplets jump out of the coffee and then dance on the coffee surface for a couple of seconds as the liquid vibrates.
Today’s Daily Grind investigates these ‘floating droplets’ with an experiment in time for Christmas: Decorate your coffee with coffee baubles.
To make these droplets form on your coffee in a controllable way you will need a few bits of equipment:
- A couple of loud-speakers with the woofers exposed
- Some sort of liquid soap (washing up liquid, hand soap, soap for hand washing clothes etc)
- Some water (or coffee but you will do horrible things to it)
- A shallow dish (I used the bottom of an old yoghurt pot)
- A “dropper”, a pipette or syringe would be ideal, a straw will probably work.
You can do this completely systematically, in which case you’ll also need a signal generator to provide a fixed frequency output to the speakers (I used “ScorpionZZZ’s Lab, Signal Generator Lite for iPhone). Or you can just go straight to the fun bit which is to make these droplets dance to music. It’s Christmas so it’s entirely up to you!
Balance your speakers on a flat surface and put the shallow dish so that it sits in good contact with the woofer. Because my dish was ever so slightly larger than the vibrating bit of the speaker, I ‘coupled’ the speaker to the dish with a roll of sellotape. Mix 10ml of soap with 100ml of water (this does not have to be exact but you may want to investigate just how much/little soap you can get away with). If you are using coffee rather than water, you will need to mix 10ml soap with 100ml coffee.
Pour about half the soapy-water into the dish and then turn the speakers on. If you are using a signal generator, watch what happens as you sweep the frequency from 10-200 Hz. Now, either choose a frequency which shows a nice resonance pattern on the water, or start playing the music through the speakers. Music with a good beat will work well (I watched drops dance to Tiesto, Blondie, and Josh Woodward’s “coffee”).
Drip a drop of the remaining soapy-water onto the resonating surface. A video of my playing with these droplets can be seen above. Although not all the drops will float, it is fairly easy to start to form patterns of flowers or rows of droplets and then it’s worth just playing. How big a droplet can be made to float without collapsing? How many minutes can you get a drop to last before it sinks? What happens if you combine a drop of black (soapy) coffee with a drop of milky (soapy) coffee?
Disclaimers & Credits:
No coffee was wasted in the making of this video. A very good coffee from Roasting House was thoroughly enjoyed before the remnants were diluted and mixed with soap.
Inspiration & experimental details taken from Jearl Walker’s great article “The Amateur Scientist” in Scientific American, p. 151 (1978).