If you spend your day staring at the froth in a beer glass, you鈥檙e liable to get a bad reputation. Such are the occupational hazards for Robert MacPherson and David Srolovitz, who are using mathematics to describe how the frothy networks of gas-filled bubbles that constitute a beer head change over time.
The mathematics of how beer bubbles behave is similar to that behind how grains in metals grow. The granular structures in metals coarsen as their boundaries move, says MacPherson, of the Institute for Advanced Study in Princeton. Similarly, the bubbles in a beer head 鈥� separated by liquid walls moving under surface tension 鈥� merge and coarsen the foamy structure until the head is no more.
Two-dimensional grain boundary mathematics was developed by the computing pioneer John von Neumann in the 1950s, but now MacPherson and Srolovitz, who is at Yeshiva University in New York, have adapted it to describe how 3D grain boundaries 鈥� such as bubble walls in a beer head 鈥� change over time (Nature, vol 446, p 1053).
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They discovered that the rate of change of domain volumes 鈥� the rate of beer head decay 鈥� depends on the width of the domains rather than their number of adjacent domains. By carefully controlling surface tension properties, these widths can be adjusted to order, allowing brewers a new level of head control.
