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 wed 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 nations 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 inte
Contact: William Johnson