Water, truly one of the most weird substances known, gets ever weirder at the nanoscale. The Georgia Tech scientists’ research on how the water is organized at the atomic level might have implications for the development of diagnostic devices such as lab-on-a-chip technologies:
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; when it becomes ice, it becomes less dense. For this reason, many scientists reasoned 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. 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 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,” said Riedo, assistant professor in Georgia Tech’s School of Physics. “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.”
As the tip compresses the water even more, the repulsive force oscillates, indicating that the water molecules are 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.
Press release: Water Flows Like Molasses on the Nanoscale …