Chaotic Mixing of Food Coloring in Water

Carlos Caicedo-Carvajal
Troy Shinbro
Rutgers University
Newark, New Jersey

an aliquot of food coloring is dropped into a thin pool of water
This snapshot shows chaotic stretching and folding that appears
when an aliquot of food coloring is dropped into a thin pool of water.

In 1855, James Thomson -- better known as Lord Kelvin's brother - wrote a paper describing "certain curious motions" on liquid surfaces. These "curious motions" were later studied by Italian scientist, Carlo Marangoni, and can be reproduced easily by carefully depositing a droplet of food coloring into a shallow (about 4 mm) dish of water.

One of several intriguing motions that can be seen is the pulsation of the coloring. This pulsation leads to stretching and folding of small quantities of food coloring that spread on the surface of the water, shown in this false color image. Stretching and folding is the hallmark of chaotic mixing", which is vital to the manufacture of pharmaceuticals, cosmetics, and fine chemicals.

Remarkably, after more than 150 years, the effects described by Thomson still remain mysterious - for example, why does the coloring pulsate? Why are the patterns seen sometimes chaotic, and at other times nearly uniform? And, as the reader can verify, why do fine regular striations appear on the surface after about a minute? This simple experiment illustrates that the days when a novice in her home can uncover new and unexplained physics are far from over.  

This work is funded by the National Science Foundation.

Reporters and Editors

Reporters may freely use this image. Credit: Carlos Caicedo-Carvajal & Troy Shinbrot (2010).