“When water is confined between two competing surfaces, the result is neither simple wetting nor dewetting,” said Steve Granick, a professor of materials science, chemistry and physics at the UI and senior author of the Science paper. “Instead, the surface of the water thrashes about, trying to get away from the undesirable material.”
Why water beads on some surfaces but not on others has puzzled scientists and engineers for a long time. Water-repellent surfaces – such as raincoats, plant leaves and freshly waxed cars – are called hydrophobic, and studying how water behaves when forced into contact with something it doesn’t like has not been easy.
“The problem, of course, is that the water doesn’t want to be there,” said Granick, who also is a researcher at the Frederick Seitz Materials Research Laboratory on the UI campus. In the past, scientists who attempted to study this behavior by confining the water between two hydrophobic surfaces were unsuccessful because the water would immediately squirt out – before measurements could be taken.
Now, however, Granick and his colleagues – postdoctoral research associate Xueyan (Rebecca) Zhang and doctoral student Yingxi (Elaine) Zhu – have succeeded in both pinning down the water and its response at a hydrophobic surface. First they “glued” a drop of water to a hydrophilic (water-loving) surface. Then they squashed it against a water-hating surface.
Thus tricked, the water was available for study at what Granick described as a “Janus interface.” (In Roman mythology, Janus was the god of change and transitions, often portrayed with two faces gazing in opposite directions.) After squeezing the drop into a thin layer about 10 molecules thick in a modified surface forces apparatus, the researchers carefully measured its motions.
“While surface energetics encouraged the water to dewet the hydrophobic side of the interface, the hydrophilic side held the water in place, resulting in a fluctuating film of capillary waves,” Granick said. “These waves moved in one direction and then another, unable to escape contact with the hydrophobic surface.” Granick compared the capillary waves to their much bigger brethren that roll across the surface of a pond. “Unlike a pond, however, where the waves ripple against the air, at the Janus interface the waves ripple against a surface,” he said. “The undulating tips of the capillary waves briefly contacted the hydrophobic surface, then moved off and touched the surface at another point.”
The researchers’ findings may aid in understanding the structure of water films found near patchy hydrophilic-hydrophobic surfaces that are ubiquitous in nature.
“With proteins, for example, the side-chains of roughly half of the amino acids are hydrophilic, while the other half are hydrophobic,” Granick said. “The non-mixing of the two is a major mechanism steering protein folding and other self-assembly processes.”
The U.S. Department of Energy supported the research.
Journal
Science