[Patrick Herd] was in Sweden recently and decided to help out a team of high school students in the International Young Physicist Tournament — The challenge? Chocolate Hysteresis.
Chocolate what? When chocolate melts, it doesn’t actually re-solidify at it’s melting point — in fact, it’s quite below that. The challenge here is figuring out a scientific way of measuring the time (and temperature) it takes to return to a solid state. This in itself is kind of tricky considering you have to accurately measure the temperature and be able to empirically tell if its solid or liquid.
The first scientific apparatus they came up with was the Chocolate Rig V1 – a very simple peltier heated and cooled calorimeter. They used an Arduino to control the temperature and a motor shield to power the peltier plate. It kind of worked but they discovered it was difficult to assess the physical state of the chocolate. This is when [Patrick] started doing some research and discovered rotary viscometry.
It works by determining the shear force in a fluid by rotating some kind of object in it, with sensor feedback measuring the torque. This was quite a bit more challenging to create than their first rig, but they pushed onward anyway.
The sensor they built to measure the shear force is quite ingenious. They are rotating the heated crucible with a shear probe stuck in the middle — string is wrapped around the probe and attached to a force gauge. When the chocolate is liquid it spins around the probe with little to no rotation — but as soon as it hardens up, the probe starts rotating with the chocolate which in turn pulls on the force gauge.
The entire project is quite fascinating, so if you’re into scientific experimentation, you should definitely give his whole blog post a good read!