Scaling of columnar joints
19 January 2009
Colloidal corn starch
Work by Post doctoral researcher Dr Lucas Goehring has been featured on the cover of PNAS, the Proceedings of the National Academy of Sciences of the United States of America.
Cooling lava often breaks into remarkable columnar shapes, that display an uncanny degree of order. These regular columns are most famous from sites such as The Giant's Causeway in Northern Ireland, and Fingal's Cave in Scotland, where local legends attribute them to the work of giants. Lucas and his colleagues studied these patterns both in the field, and in the lab, and observed columns measuring less than a millimeter in diameter, to those wider than a meter. They found the key to understanding this pattern in dishes of colloidal corn starch, which cracks into small columns when dried. By studying the flow of moisture and the interplay of stress and strain in a peitri-dish of starch, they developed a theoretical model of columnar jointing in a porous medium. By exploiting a mathematical analogy between the transport of heat and moisture, they were able to directly apply this model to the cooling of lava. Finally, they successfully tested this model rigorously against measurements we gathered, around the world, of columnar lavas.
Their discovery answers a fundamental, and ancient question about these patterns: what decides the scale of columnar joints? Using their results, anyone can now easily deduce the cooling rate that produced specific examples of these amazing structures. The results can also guide interpretations of similar patterns in dried mud, cracked paint, and frozen soils.
This work was done while Lucas was a graduate student at the University of Toronto. The theoretical development was developed in close collaboration with L. Mahadevan, a professor at the School of Engineering and Applied Sciences, Harvard University. Lucas is now a research associate working with Dr Alex Routh, and a research fellow of Wolfson College.