Couette’s cell

A few years ago I walked into the office of soft matter chemist Susan Perkin and noticed a strange device on her filing cabinet. It consisted of a clear cylinder of Perspex. Inside, there was a clear drum with a handle on top; the space between the two was filled with glycerine. What was it, I asked. Sue took a syringe with food colouring and carefully injected a stream of blue liquid into the glycerine, carefully writing a letter C. She then turned the handle clockwise. As she did so the blue liquid began to spread, the letter blurring until it was unreadable. Then, without a word, Perkin stopped and turned the handle the other way, and as she did so the blue dye gathered itself and the letter was magically restored. My jaw hit the floor; it was the closest thing to a time-machine I’d ever seen. But the device is no toy – it’s a crucial instrument invented by Maurice Couette, one of the fathers of modern rheology.