The 'mixing' needs to be as pure laminar flow as possible, and that's generally between two cylinders that are rotating at different speeds. The speed of the fl

The 'mixing' needs to be as pure laminar flow as possible, and that's generally between two cylinders that are rotating at different speeds. The speed of the fluid across the space should be a linear function of the relative distance from each wall. Any variation of that is going to reduce the reversability. So what they're doing is very carefully not mixing, but shearing. The fluid here is behaving like a very soft chunk of rubber, not like any fluid that you're used to dealing with. If you've got a spoon, there's going to be some turbulence as the flow goes around the edges and the flow detaches from the surface. Turbulence absolutely destroys the reversibility.