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.
Liquids may be attracted poorly to a solid surface or they may be attracted strongly. 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.’
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.
By first coating the mica with a smooth monolayer of octadecyltriethoxysiloxane, the researchers studied the behaviour 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 added.