A few weeks back Riley did a tremolo pedal as part of his school science project for 2017. This is the rest of his project - a tremolo build with sloshing liquid. He did a bunch of testing to work out the best liquid to use, and a bunch of designing and building to make the thing work. It has two modes - upright and horizontal, which amazingly produce a sine wave and square wave tremolo respectively.
It also has a bit of randomness to the sine wave, which sounds really cool. The waves don't always come at the same period, and come in groups sometimes:
A quick demo of it working:
A longer (5 minute) video of science: