Slick Research Says Fluids Slip On Solids, Depending On Speed

How fluids behave on extremely smooth surfaces is important in such high-tech applications as moving materials through microfluidic devices and lubricating computer hard drives.

“We found that if surfaces are smooth enough, and if the liquid is moving fast enough, the liquid will slip over the surface like ice skates gliding over ice,” said Steve Granick, a professor of materials science at the UI and a researcher at the Frederick Seitz Materials Research Laboratory on campus.

Liquids may be attracted poorly to a solid surface – like beads of water on a freshly waxed car – or they may be attracted strongly – like cooking oil on an old iron skillet. A basic tenet of textbook fluid dynamics – called the “no-slip” boundary condition – says that a layer of fluid molecules flowing across a solid surface will be stuck in place, regardless of the strength of attraction.

“When standing in a shower, for example, the no-slip boundary condition says that the water molecules closest to your skin will actually stick to you and come to rest,” Granick said. “Molecules one layer away will move a little, those a little farther away will move a little faster, and so on, until the water is running freely off your body. This also explains why large dust particles can be blown off dirty eyeglasses, but smaller bits must be wiped off – a thin layer of air next to the lens doesn’t move.”

To explore the no-slip boundary condition, Granick and doctoral student Yingxi (Elaine) Zhu placed drops of liquid between molecularly smooth mica surfaces within a modified surface forces apparatus. Surface spacing was measured using optical interferometry and dynamic forces were measured using piezoelectric methods. The team’s findings were reported in the Aug. 27 issue of Physical Review Letters.

By first coating the mica with a smooth monolayer of octadecyltriethoxysiloxane, the researchers studied the behavior of two dissimilar fluids – tetradecane (an oil) and water. Each drop was squeezed until the fluid was only a few layers thick. Not only did none of the layers in either fluid “stick” to the surface (as textbooks claim they should), the amount of slip depended on the velocity of the fluid.

The researchers also saw the same effect when, instead of first modifying the solid surface, they added soap-like molecules to the flowing liquid. “The surfactant migrated to the surface where it formed a smooth coating that lessened the attraction of the liquid for that surface,” Granick said. “This means we can achieve the same lubrication goal without going through the complicated protocols of producing a perfect surface.”

This could be an easy and inexpensive way to save energy when transporting fluids through pipelines, and for reducing friction in engines and machinery, Granick said. “There will be many other applications down the road, when we know more about manipulating the no-slip boundary condition.”

The National Science Foundation and the U.S. Department of Energy supported the research.