The fluid contained in the clear plastic flow visualization bottle is called rheoscopic fluid, and is commercially produced just for demonstrations like this one. This fluid allows clear visualization of the flow patterns in a moving liquid. In it are suspended many small flakes of a reflective solid, analogous to fish scales. When the fluid is flowing, the flakes are aligned by the shear. They reflect light like little mirrors, revealing the flow patterns.

This fluid should not be removed from the bottle. It can be colored with a very small amount of food coloring (a drop on a toothpick) if desired, for greater effect. If the bottle has been sitting for more than a day or so, the flakes will have settled out. A few shakes will get them back into suspension.

It is also interesting to place a glass marble in the fluid, and observe the patterns produced as it sinks, or is rolled around the bottom of the bottle.