Surf against surface: tortured water ripples at contact

January 24, 2002

CHAMPAIGN, Ill. -- Water trapped against a surface it doesn't like will ripple in frustration as it seeks to escape, say researchers at the University of Illinois who will report their findings in the Jan. 25 issue of the journal Science.

"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.

University of Illinois at Urbana-Champaign

Related Physics Articles from Brightsurf:

Helium, a little atom for big physics
Helium is the simplest multi-body atom. Its energy levels can be calculated with extremely high precision only relying on a few fundamental physical constants and the quantum electrodynamics (QED) theory.

Hyperbolic metamaterials exhibit 2T physics
According to Igor Smolyaninov of the University of Maryland, ''One of the more unusual applications of metamaterials was a theoretical proposal to construct a physical system that would exhibit two-time physics behavior on small scales.''

Challenges and opportunities for women in physics
Women in the United States hold fewer than 25% of bachelor's degrees, 20% of doctoral degrees and 19% of faculty positions in physics.

Indeterminist physics for an open world
Classical physics is characterized by the equations describing the world.

Leptons help in tracking new physics
Electrons with 'colleagues' -- other leptons - are one of many products of collisions observed in the LHCb experiment at the Large Hadron Collider.

Has physics ever been deterministic?
Researchers from the Austrian Academy of Sciences, the University of Vienna and the University of Geneva, have proposed a new interpretation of classical physics without real numbers.

Twisted physics
A new study in the journal Nature shows that superconductivity in bilayer graphene can be turned on or off with a small voltage change, increasing its usefulness for electronic devices.

Physics vs. asthma
A research team from the MIPT Center for Molecular Mechanisms of Aging and Age-Related Diseases has collaborated with colleagues from the U.S., Canada, France, and Germany to determine the spatial structure of the CysLT1 receptor.

2D topological physics from shaking a 1D wire
Published in Physical Review X, this new study propose a realistic scheme to observe a 'cold-atomic quantum Hall effect.'

Helping physics teachers who don't know physics
A shortage of high school physics teachers has led to teachers with little-to-no training taking over physics classrooms, reports show.

Read More: Physics News and Physics Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to