Author: Falishia Sloan
Institution: Eastern Virginia Medical School
Date: May 2007
Researchers at the Georgia Institute of Technology have discovered the current view of water is not as clear as we thought. It appears water at the nanoscale actually behaves like molasses, with a thick, viscous consistency. This new discovery changes some previously held views on the properties of waters, and creates new ones that were previously unreachable.
The group, along with associate professor and experimental physicist Elisa Riedo, measured the force of pure water in a nano-meter sized channel and found clues that suggested water at the nano-level is organized into layers. Under normal conditions, water is already unique in many ways with respect to its molecular structure. Water is known for flowing freely, but it is also well known for its fantastic cohesion properties (ability of its molecules to "stick" to each other), its high boiling point, and for having a greater density as a liquid than as a solid, which is the complete opposite for many other substances. Because of this, many scientists assumed that water compressed at the nanoscale would exhibit the same properties. Elisa Riedo and her team, however, have found otherwise.
"Since water usually has a low viscosity, the force you would expect to feel as you compress it should be very small," Riedo reasoned. "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."
In her experiment, Riedo recorded the force placed on a silicon tip of an atomic fore microscope as it compressed water. The water was confined on top of a solid surface in a nanoscale thin film. As the tip compressed the water, the repulsive force moved to and fro, indicative of the water making layers. As pressure is increased by the tip on the layer, the layer collapses and the water flows out horizontally.
Director of the Center for Computational Materials Science, Regents' and Institute Professor, and Callaway Chair of Physics at Georgia Tech Uzi Landman said, "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."
While measuring the vertical force exerted by the confined film of water on the tip, the team also measured the lateral force to measure the film viscocity. They discovered the viscosity was greater on a hydrophilic, or water-loving and compatible, surface. As they continued to compress the water, the viscosity reached the 1,000 to 10,000 range. The viscocity increased greatly as the confined film reached a thickness in the 1.5 nanometer range. In contrast, not much of an increase in viscosity was observed on hydrophobic surfaces.
"Water is a wonderful lubricant," said Riedo, "but it flows too easily for many applications. At the one nanometer scale, water is a viscous fluid and could be a much better lubricant."
The team's research is a testament to one of the growing fields of science,nanotechnology. Many scientists previously thought the properties of water to be consistent at all levels, and, according to Landman, the team's results differed due to their new ability to study the last two nanometers.
"If you want force, it is preferable to measure it," Landman said. "This is the first experiment to directly measure the force and it's the first simulation done of these forces. The fact that we have direct measurements married with theoretical results is rather conclusive."
The ability to study water at this level presents a new range of possibilities of treating various diseases at the nano-level.
Written by Falishia Sloan