Mixing is a problem that has been tackled by engineers, has fascinated physicists and has inspired philosophers for many years. In the chemical industry, engineers have to struggle with mixing, knowing that empirical rules do not necessarily help when it comes to designing new equipment. The problem of mixing also challenges atmospheric scientists who are trying to reliably predict the path of pollutants circulating around the planet.
Considerable progress has been made in understanding turbulent mixing over the last two decades. An important landmark was the discovery of "chaotic mixing" whereby laminar (or streamlined) flows can mix in a way that is similar to turbulent systems. However, recent experiments by David Rothstein, Emeric Henry and Jerry Gollub at Haverford College in the US have demonstrated that this picture of chaotic fluid mixing is oversimplified and requires substantial revision (Nature 1999 401 770). Moreover, their detailed analysis of the whole chaotic-mixing process shows that a new theoretical approach is required.
In the January issue of Physics World magazine, Joanne Deval of the University of California, Los Angeles, and Patrick Tabeling of the Ecole Normale Supérieure in Paris, write about this new insight into chaotic fluid mixing.