Lehigh Univeristy scientists, as reported in the Jan. 26, 2001, issue of Science, have succeeded in making droplets of water speed at rates of centimeters, even a meter or more, per second.
Manoj Chaudury, professor of chemical engineering, says that by passing saturated steam over a hydrophobic surface possessing a surface tension gradient, droplets of water can be induced to move at the faster rates.
"This phenomenon results from the combination of the surface gradient with the fast condensation," says Chaudhury. "We had no idea when we started working that we'd discover this new effect. We were greatly surprised to see the speeds at which these drops moved."
Chaudhury first presented his ideas on surface tension gradients at Lehigh at a chemical engineering department seminar in 1994, just before his appointment to the faculty.
In attendance was John C. Chen, the Carl R. Anderson Professor of chemical engnineering and now dean of the P.C. Rossin College of Engineering and Applied Science.
Chen, an expert in heat transfer and multiphase flow problems, suggested that Chaudhury apply the gradient effect to heat-transfer problems. The two researchers began a collaboration that culminated in their co-authorship of the current Science article, which is titled "Fast Drop Movements Resulting from the Phase Change on a Gradient Surface." (The earlier article was given the more accessible title of "How to Make Water Run Uphill.")
Ten years ago, Chaudhury found a new way to make droplets of water "creep" against their natural instincts. Writing in 1992 in Science, the nation's leading science journal, Chaudhury said he had coaxed a microliter of water to "run uphill" on a surface of polished silicon at about 1 mm per second by varying the degree of hydrophobicity (water resistance) on the surface.
The change in surface properties, Chaudhury said, created an imbalance of surface tension forces, or a gradient of low to high interfacial energy, helping to propel the water upward on a tilted horizontal plane. Today, the creeping droplets that defied gravity a millimeter at a time have acquired the speed of an Olympic sprinter. The current article was also co-authored by Susan Daniel, who earned a B.S. in chemical engineering from Lehigh in 1999 and is now a Ph.D. candidate in chemical engineering here. Daniel began working on the project with Chaudhury through the National Science Foundation-funded Research Experience for Undergraduates program and continued through Lehigh's Presidential Scholarship program, which offers a tuition-free fifth year of study to undergraduates graduating with a cumulative GPA of 3.5 or better.
Whereas before, Chaudhury could make the water drops move in only one way - from point to point - the increased energy created by the condensation now makes it possible to make the drops move radially, or out from the center of a surface, as well as up and down parallel channels, or columns. These movements can be viewed in real time by visiting Chaudhury's web site at www.lehigh.edu/~mkc4/movie1.mov.
Chaudhury says the new phenomenon can be potentially applied to heat transfer problems, especially those involving systems operating in zero or micro-gravity. In these systems, he says, a surface tension gradient, performing the function that gravity normally would, could pump water radially from a horizontal surface, preventing liquid build-up and thus improving the efficiency of heat transfer.
Another potential application, he says, is to microfluidic devices, especially the microchips equipped with microfabricated miniature fuel cells that, scientists envision, may one day serve as power sources in laptop computers, military uniforms with coolants and other portable devices.
"A chemical reactor on a microchip would run on a very small amount of liquid," says Chaudhury. "That liquid would have to be pumped from one end of the reactor to another. It would be very difficult to use external sources to do this, but surface energy could act as a pump."
His latest article in Science is Chaudhury's fifth contribution to the journal. Like the other research projects he wrote about, his current project utilizes very ordinary equipment.
A boiler boils water to make steam, which is transported through a steel tube (covered with cloth and aluminum foil) to a silicon surface coating a large copper block. The device also contains a pressure gauge and thermocouples to measure the temperature inside the copper block. As the droplets move across the silicon surface, Chaudhury and his colleagues view them through an optical microscope.