Going with the flow

Society has many laws, science only a few. There are only a handful of principles such as Galileo's law of gravity or Newton's laws of motion that nature always obeys.

In Adrian Bejan's field, there are just two great laws of thermodynamics. So the 59-year-old professor of mechanical engineering at Duke is making no small claim when he says that he has discovered another.

He calls it the Constructal Law and says it predicts how everything -- from lightning bolts and rivers to plants, people and money -- flows through time and space. It explains why birds and airplanes can fly, how cities grow and epidemics spread. It unravels the mysteries of snowflakes and Egypt's pyramids, the evolution of written languages.

By applying its principles, we can build better houses and roads; derive deeper understandings of the challenges posed by illegal immigration and climate change. Through the constructal law, we can make the world a better place.

Bejan's revolutionary idea challenges the age-old assumption that nature is without design. Just as scientists say life developed thanks to a series of random mutations, they reject the notion that an organizing principle informs and shapes the movement of materials.

Where others saw disorder, Bejan found pattern. "One of the basic aims of design is to get the most amount of work done with the least amount of effort," he explained. "What we as engineers try to accomplish through our plans, nature achieves on its own. Everywhere you look, animate [people, trees] and inanimate [rivers, mud cracks] phenomena, if given freedom, organize themselves into patterns and shapes that help them get from here to there in an easier manner."

He pauses, as a smile spreads across his chiseled face. "Like the old saying, they go with the flow."

It is no accident, for example, that blood vessels and pulmonary airways have round cross-sections just like the pathways carved by earthworms and ants. This shape is the most efficient way for them to flow. A river's width is proportional to its depth because this, too, maximizes its flow.

Or consider lava. As it begins its journey from the volcano's belly, it organizes itself into a series of concentric circles. In the center is lava of high viscosity (less runny), on the outside is lava of low viscosity (runnier). The low viscosity lava that touches the ground helps it flow. No one, of course, is telling the lava to do this -- there is no lava flow manager saying you here, you there. It just does it, Bejan says, because of the natural tendency of moving matter to organize itself into patterns that increase the efficiency of its flow.

As it makes its way down the mountain, something else happens: The lava does not just ooze, which is an inefficient way to flow. Instead it forms a channel, from which treelike branches spread. This, it turns out, is the best way for it to flow.

In his book-lined office at Duke, Bejan displays an aerial view of flowing lava. Then he spreads out pictures of river basins and snowflakes, of the air passages in lungs and the coronary arteries of the heart. All display the same treelike design. "They look alike because they display this universal tendency of things that flow to seek great access," Bejan said. "As time goes on, so long as they are not overly constrained by outside forces, they will organize themselves into even more efficient design flows."

From hunch to law

Bejan did not shout "Eureka!" on that momentous day in 1995 when he discovered the constructal law. He was, after all, in a crowded conference room in France. But inspiration came in the proverbial flash when he heard a Nobel laureate proclaim that the treelike flow structures that characterize lightning bolts and river basins are "nondeterministic."