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First synthetic tree mimics transpiration

Cornell University researchers have created the world's first synthetic tree. So far, it's a very small 'tree' which stands in a palm-sized piece of hydrogel. This 'tree simulates the process of transpiration, the cohesive capillary action that allows trees to wick moisture upward to their highest branches.' Apparently, the scientists have proven that a long-standing theory saying that transpiration is not a biological process, but a physical one. What could be some possible applications for this synthetic trees? The researchers say that this 'may lead to new passive heat transfer technologies for cars or buildings, better methods for remediating soil and more effective ways to draw water out of partially dry ground.' But read more...
Written by Roland Piquepaille, Inactive on

Cornell University researchers have created the world's first synthetic tree. So far, it's a very small 'tree' which stands in a palm-sized piece of hydrogel. This 'tree simulates the process of transpiration, the cohesive capillary action that allows trees to wick moisture upward to their highest branches.' Apparently, the scientists have proven that a long-standing theory saying that transpiration is not a biological process, but a physical one. What could be some possible applications for this synthetic trees? The researchers say that this 'may lead to new passive heat transfer technologies for cars or buildings, better methods for remediating soil and more effective ways to draw water out of partially dry ground.' But read more...

World's first synthetic tree

You can see on the left two pictures illustrating the concept of this synthetic tree. On the top is a transparent sheet of hydrogel, 1 millimeter thick, "etched with 80 parallel channels of varying lengths arranged to form a circle and connected by a single channel." The bottom picture describes "an optical micrograph of a synthetic tree showing a 'trunk' channel entering from the left into a network of microchannels in the 'leaf,' or 'root,' network. The channels are approximately 100 micrometers wide, and total field of view is approximately 1.5 centimeters wide." (Credit: Cornell University) Here is a link to additional details provided by a figure published by Nature.

As you can see, "the synthetic tree doesn't look much like a tree at all. It consists of two circles side by side in the gel, patterned with evenly spaced microfluidic channels to mimic a tree's vascular system. In nature, trees use water in tubular tissues, called xylem, like ropes that pull more water out of the ground, delivering it to leaves." Abraham Stroock, an assistant professor of chemical and biomolecular engineering, and graduate student Tobias Wheeler "used pHEMA hydrogel, or polyhydroxyethyl methacrylate, to form the plant membranes. The hydrogel is a solid embedded with water and has nanometer-scale pores. The material acts as a wick by holding liquid in the pores, through which capillary action creates tension in the water."

Here is an additional quote from the Cornell Chronicle Online article. "Besides supporting the theory of transpiration as a physical, not biological, process, the synthetic tree also introduces a new way to study water under tension -- a subject interesting to physicists and chemists. Many questions about the metastable state of water could be answered using this new 'tree.' 'Water is the most studied substance on Earth, and yet there is a big metastable region in its phase diagram waiting to be characterized,' Stroock said."

In "Tiny synthetic tree pumps water," Heidi Ledford gives more information (Nature News, September 10, 2008). "Stroock and his colleague, Tobias Wheeler, say that they have overcome these technical hurdles by making water channels from a hydrogel based on poly(hydroxyethyl methacrylate) that contains tiny, homogeneous pores. Their system generates a continuous negative pressure that pulls in water from a vapour via the 'roots' and transports it as a liquid along hydrogel channels to the 'leaves', from which the water is evaporated. [...] Stroock thinks that the design could be modified to create devices that extract purified water from above the water table -- a feat achieved daily by plants, but that has proven a challenge for humans to replicate.

This research work has been published in Nature under the title "The transpiration of water at negative pressures in a synthetic tree" (Volume 455, Number 7210, Pages 208–212, September 11, 2008). Here are some excerpts from the abstract. "Plant scientists believe that transpiration—the motion of water from the soil, through a vascular plant, and into the air -- occurs by a passive, wicking mechanism. This mechanism is described by the cohesion-tension theory: loss of water by evaporation reduces the pressure of the liquid water within the leaf relative to atmospheric pressure; this reduced pressure pulls liquid water out of the soil and up the xylem to maintain hydration. [...] Here we present the design and operation of a microfluidic system formed in a synthetic hydrogel. This synthetic 'tree' captures the main attributes of transpiration in plants: transduction of subsaturation in the vapour phase of water into negative pressures in the liquid phase, stabilization and flow of liquid water at large negative pressures."

Finally, here are some quotes from Nature Editor's summary, "Transpiration: the pulling power of a 'synthetic tree'." "Evaporation of water from the leaves of plants pulls water up from the roots via a passive wick-like action. This 'transpirational pull' generates pressures up to a hundredfold greater than in synthetic wicks. A team from Cornell University has now developed a microfluidic system in a synthetic hydrogel that captures the main attributes -- and pulling power -- of transpiration in plants. [...] This process validates the cohesion-tension theory of transpiration, and the synthetic tree should also be a useful platform for the study the properties of metastable liquids and a starting point from which to design new technologies for the management of water in chemical processes, heat transfer, and environmental engineering."

Sources: Anne Ju, Cornell Chronicle Online, September 10, 2008; and various websites

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