Droplets speeding on surfaces
Researchers from Illinois Institute of Technology (IIT) in Chicago and Massachusetts Institute of Technology (MIT) in Cambridge have teamed up to explain the findings of scientists from Lehigh University in Bethlehem, Pennsylvania. The Lehigh researchers describe the speeding of water droplets (as fast as one meter per second) across specially treated surfaces in the January 26 issue of Science. Science asked researchers from IIT and MIT to explain the phenomenon, which could have applications in fabricating microchips and printing.
“The secret lies in manipulating the factors that control the surface tension of the droplet,” says Darsh Wasan, Ph.D., vice president and Motorola professor of Chemical Engineering at Illinois Institute of Technology. Wasan and Alex Nikolov, Ph.D., research professor of chemical and environmental engineering at IIT analyzed the results of the Lehigh observation.
“Water droplets spreading on surfaces are primarily driven by differences, or gradients in surface tension,” says Wasan. Droplets move towards the edge with higher surface tension, which is partially controlled by temperature. Cooler temperatures increase surface tension, and the drop is pulled towards the cooler temperature. If the surface temperature is uniform, the drop will spread out in all directions at a rate of only millimeters per minute, but will not completely flatten because the edge of the droplet encounters friction at the surface (known to fluid dynamics experts as contact angle hysterisis).
Temperature driven water droplet movement only accounts for speeds of around a half meter per second. The Lehigh researchers got their drops to move faster by reducing frictional forces at the liquid-solid interface. They did this by using a hydrophilic surface. The hydrophilic molecules in effect “wet” the surface. When a water droplet comes into contact with the hydrophilic molecules, it is pulled along by capillary action. Capillary action helps pull like molecules along by forces of adhesion. The water drop is attracted to the hydrophilic molecules and can speed across the surface without being slowed by friction.
Founded in 1890, IIT is a Ph.D.-granting technological university awarding degrees in the sciences, mathematics and engineering, as well as architecture, psychology, design, business and law. IIT’s interprofessional, technology-focused curriculum prepares the university’s 6,200 students for leadership roles in an increasingly complex and culturally diverse global workplace
