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Wednesday, September 17, 2008

Synthetic Tree hauls Water


Branching out: Microscopic channels etched into a thin sheet of hydrogel mimic the capillaries in the root and leaf system of a tree. Another channel represents the tree trunk.
Credit: Abraham Stroock and Tobias Wheeler

A tree can transport water an amazing distance--from its roots, through a trunk up to 85 meters tall, and finally to its leaves, where the water evaporates. Now, scientists at Cornell University have created a microfluidic system to mimic that process. Their "synthetic tree" opens up a new way to move liquids over long distances without using mechanical pumps.

Abraham Stroock, an assistant professor of chemical and biological engineering at Cornell, and graduate student Tobias Wheeler created the synthetic tree out of a thin sheet of hydrogel, a material more commonly used to make contact lenses. They etched two networks of parallel channels into the hydrogel to represent the capillaries in a tree's root system as well as the ones in its leaves. They connected the two networks with a single channel representing the trunk of the tree.

In a real tree, evaporation from the leaves is what pulls water up through the plant--a process known as transpiration. This evaporation occurs because plants need to take in carbon dioxide to perform photosynthesis. "When they open their cells up for all this CO2 diffusion, the water is diffusing out much faster," says N. Michele Holbrook, a professor of biology and forestry at Harvard University. "All this water that's coming up the tree is because it's trying to get CO2. Ninety-nine percent of that water is going right through the tree."

Stroock and Wheeler found that their system accurately mimics this transpiration process, pulling water through at strengths several times greater than those inside a real tree. The researchers' findings appeared last week in the journal Nature.

Furthermore, because the water in a tree is under negative pressure--as if it were being sucked up through a straw--the water is in a metastable state, meaning it is between a liquid and a vapor. So the synthetic tree could also serve as a model system for studying liquids in this state. "Metastable liquids, though they are important in fundamental issues of science, tend to be curiosities, as opposed to main components of technological applications," says Pablo Debenedetti, a professor of chemical engineering at Princeton University. "In the case of liquid under negative pressure, it would tend to boil and become a vapor to relieve the negative pressure. But trees have managed to handle water in a metastable state very efficiently, so that's why this work is so nice."

Choosing a hydrogel for the material was key to making the system work, Stroock says. His team knew that a porous solid generates the capillary action in plants to pull the water through the channels, and that a smaller pore size translates into larger negative pressures. What's more, the team knew that the pore size can be no greater than 10 nanometers or else "that pore will fail to hold on to the liquid, and the whole plant will dry out through that pore," Stroock says. "The characteristic of a gel that's important is, it's a porous solid, but the mixture of the solid phase and the liquid phase is down at the molecular scale. It's like getting subnanometer-scale pores."

Stroock envisions that the synthetic tree system could be used to move liquids passively without needing mechanical pumps. In heat-transfer applications, it could cool small devices, like laptop computers, or larger ones, like vehicles, or even buildings. It could also be part of an soil remediation system, Stroock says. Instead of needing to flood soil with water to flush out contaminants, a synthetic tree could pull the contaminated water out.

"This paper is more proof of principle, but by clever selection of materials and micromachining, it shows you can handle liquids under tension in a stable and reproducible way," says Debenedetti

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