September 1, 2010

The Scientist: Abraham Stroock

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Does water only exist in atmospheric or in elevated positive pressure?

Prof. Abraham Stroock, chemical and biomolecular engineering, noticed that plants utilize water in negative pressure to move it from the ground to their highest leaves, and so, he began studying water in order to understand its fundamentals.

When water is under negative pressure, it is in metastable state with respect to vapor phase. For example, water is in stable liquid phase at atmospheric pressure from its melting point to its boiling point. However, with careful controls, the liquid state can be sustained below its melting point or above its boiling point. In this case, the liquid is in metastable state with respect to its thermodynamically favorable stable phases, such as solid or vapor. Metastable liquid phase in negative pressure is achieved by applying tension, instead of heat. Water in negative pressure is also referred as water in stretched state or water under tension.

“A beautiful thing you learn from plants is that you can use liquid water, a very familiar substance, in a very exotic thermodynamic state,” said Stroock.

Negative pressure is an unfamiliar concept, but plants use it all the time when they transpire water from the ground soil to the top through their xylems (the primary conduits of water transport). Leaves are porous membranes that separate the air from the water in the xylem. Because the relative humidity of the air is lower than the humidity of xylem water, the pressure in the liquid water drops in order to achieve equilibrium across the leaves, lowering to a negative pressure.

In Stroock laboratory, this process was demonstrated in a synthetic environment by using hydrogel membranes and microfluidic channels.

Using the same technology he employed while creating his synthetic tree, Stroock and his group members studied the cavitation pressure of water, which is the point in negative pressure at which the liquid water turns into vapor spontaneously.  His group and collaborators found that the cavitation pressure measured in their experiments was much less in its magnitude than they had predicted theoretically.

His group members have pursued the causes to this phenomenon by experimentally studying the thermodynamic and dynamic parameters of water in negative pressure.

“There are a few concepts I find attractive; one of them is thermodynamics, which is a magical and mysterious topic for me,” said Stroock. “By pursuing ideas inspired by micro-physiology of plants, we created a new way of going deeper in understanding the thermodynamics of water in this regime and how the features of phase diagram connect to the molecular structure of water.”

Water, despite being the most common and nearly ubiquitous substance on earth, remains one of the most mysterious substances in the scientific community.

Stroock’s research focuses represent some of the core subjects of chemical engineering, such as thermodynamics and transport phenomena. However, his group is consisted of students with backgrounds from various disciplines of engineering and science, including the horticultural, chemical and biomolecular engineering, and the electrical and computer engineering departments. In addition to emphasizing the fundamentals of chemical engineering, his research displays the application of them.

Students in the Stroock group are currently working on two applications born from his “synthetic tree” experiment.

The first is a pressure sensor that may directly probe the tension of water in xylems of plants. The pressure sensor may impact agriculture and horticulture in different aspects. For example, this tool may improve grape harvests. The seasonal amount of tension that water experiences in the xylem could affect the quality of grapes, hence the quality of wine.

Seocnd, the group works to develop heat pipes that may mimic plants’ ability to transfer energy efficiently by moving water in negative pressure. The heat pipes, when operating in negative pressure, can create very large pressure differences and enhance energy transfer greatly.

“These studies could provide new scientific tools for scientists to explore water and to develop new technologies that would benefit by operating water in this regime,” explained Stroock.

As a graduate student, Stroock studied transport phenomena in microfluidics and the behavior of soft materials. Unlike many professors, he did not have a post-doctoral position before starting at Cornell as a professor.

“I choose topics that appeal to me personally,” he related. “I encourage finishing graduate students and post docs to look back to the deep curiosity they had in their childhood, rather than being restricted to the ideas of the moment. Because every project you take will require enormous efforts, and you will need personal commitments.”

Original Author: Eugene Choi