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September 10th, 2008

Growing smart materials idea comes from trees

Published on September 10th, 2008 | By: pwray@ceramics.org
Adrian Bejan, J.A. Jones Professor of Mechanical Engineering at Duke University's Pratt School of Engineering.

Adrian Bejan, J.A. Jones Professor of Mechanical Engineering at Duke University’s Pratt School of Engineering.

What is the best way to move substances from one place to another – a key consideration when developing complex new “smart” materials? Duke University researchers believe they have found the answer to this question in the canopy of trees. Adrian Bejan, a professor of mechanical engineering at Duke’s Pratt School of Engineering, is the developer and chief promoter of this concept. He says the image of two tree canopies touching top-to-top can guide efforts to efficiently control the flow of liquids in new materials, including next-generation aircraft and rocket “skins” that can self-repair when damaged or self-cool when overheated.

“Examples of this branching design tendency are everywhere in nature, from the channels making up river deltas to the architecture of the human lung, where cascading pathways of air tubes deliver oxygen to tissues.”

According to Bejan, the development of efficient and effective methods for controlling flow is an increasingly important ingredient in smart-material design and the creation of nanodevices. The purpose of his particular research is to create materials that act like human skin by delivering liquid healing agents through a network similar to blood vessels. He says these kinds of materials require efficient delivery systems. While working with Sylvie Lorente, professor of civil engineering at the University of Toulouse in France, Bejan found that the laws of constructal theory , which he first described in 1996, could guide the creation of these novel “smart” materials.

The foundation of Bejan’s constructal theory is the principle that flow systems evolve to minimize imperfections, reducing friction or other forms of resistance, so that the least amount of useful energy is lost. He says this theory applies to virtually everything that moves.

“We examined a flow system that looks more like the canopy-to-canopy model and found it to be more efficient than models in use now that are made up of parallel flow channels. We believe that this strategy will allow for the design of progressively more complex vascular flow systems.”

Bejan found that – not only is flow maximized by these branching larger-to-smaller-to-larger systems – but also that, to maintain this gain in efficiency, the tree vasculature needs to become more complex as flow increases. He says this is an important insight because, as new “smart” components become smaller, the efficiency of the flow systems will need to increase.

“Constructal design concepts serve the vascularization needs of these new ‘smart’ structures ideally, because trees have evolved a natural architecture for maximally delivering water throughout the tree volume,” Bejan says. “If a single stream is to touch a structure at every point, then that stream must serve that structure much like a tree, or much in way the bronchial tree supplies air to the total lung volume.”

Bejan points out that this constructal law has previously been used to explain traffic flows, the cooling of small-scale electronics and river currents. He also says the theory can explain basic characteristics of locomotion for every creature – whether they run, swim or fly. This simple physics principle also supports other essential features of global circulation and climate, including the boundaries between different climate zones, average wind speed and the average temperature difference between night and day, he says.


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