Water flows like molasses on the nanoscale

Malaysia Sun (ANI) Thursday 26th April, 2007

Washington, Apr 26 : Georgia Tech physicists have discovered that water loses its fluidity and displays high viscosity when compressed in nano sized channels.

The team discovered that water displays the same level of viscosity as molasses when confined to channels less than two nanometres wide.

As such, determining the properties of water on the nanoscale may prove important for biological and pharmaceutical research as well as nanotechnology, the researchers write in their study in the March 15 issue of the journal Physical Review.

In its bulk liquid form, water is a disordered medium that flows very readily. When most substances are compressed into a solid, their density increases. But water is different; as when it freezes to become ice, it becomes less dense.

For this reason, many scientists believed that when water is compressed (as it is in a nanometer-sized channel), it should maintain its liquid properties and shouldn't exhibit properties that are akin to a solid.

Several earlier studies came to that very conclusion - that water confined in a nano-space behaves just like water does in the macro world, and consequently, a number of scientists considered the case to be closed.

But when Georgia Tech experimental physicist Elisa Riedo and her team directly measured the force of pure water in a nanometer-sized channel, they found evidence suggesting that water was organized into layers.

Riedo conducted these measurements by recording the force placed on a silicon tip of an atomic force microscope as it compressed water. The team found that water was confined in a nanoscale thin film on top of a solid surface.

"Since water usually has a low viscosity, the force you would expect to feel as you compress it should be very small. But when we did the experiment, we found that when the distance between the tip and the surface is about one nanometer, we feel a repulsive force by the water that is much stronger than what we would expect," said Riedo, assistant professor in Georgia Tech's School of Physics.

Findings revealed that as the tip compressed the water even more, the repulsive force oscillated, indicating that the water molecules were forming layers.

"As the tip continues to increase its pressure on a layer, the layer collapses and the water flows out horizontally. In effect, the confined water film behaves effectively like a solid in the vertical direction by forming layers parallel to the confining tip and surface, while maintaining it's liquidity in the horizontal direction where it can flow out - resembling some phases of liquid crystals," said Uzi Landman, director of the Center for Computational Materials Science, Regents' and Institute professor, and Callaway Chair of Physics at Georgia Tech.

Profs. Reido and Landman further found that the layering effect was more pronounced when water was placed on top of hydrophilic surfaces that allowed water to wet the solid surface, such as glass.

When the water was confined by hydrophobic surfaces where water tends to bead up, like graphite, the effect was still present, but less pronounced, New Scientist quoted the study as saying.