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Georgia Tech physicists have
discovered that water behaves differently when its compressed in
nano-sized channel. In these small spaces water behaves much like
a solid, exhibiting high viscosity and organizing itself into
layers.
Image © Georgia Tech |
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.
A theoretical physicist, Landman conducted the
first-ever computer simulations of these forces for tip-confined water
films and found good correspondence between his team's theoretical
predictions and the experiments.
So why did Riedo and Landman's results differ from
their peers? According to Landman, most previous studies on confined
water were limited by technology at the time and could not directly
measure the behavior in the last two nanometers. Instead they had to
measure other properties and infer the forces acting in films of one
nanometer thickness or less.
"If you want force, it is preferable to measure it,"
he 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."
Riedo and Landman conducted their experiments in
several different environments. They found that the layering effect
was more pronounced when water was placed on top of hydrophilic
surfaces that allow 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.
At the same time, Riedo's team was measuring the
vertical force exerted on the tip by the confined water film, they
also measured the film viscosity by measuring the lateral force. They
found that when water was placed on a hydrophilic surface, the
viscosity began to increase dramatically as the thickness of the
confined film reached the 1.5 nanometer range. As they continued to
compress the water and measure the lateral forces, the viscosity
increased by a factor of 1,000 to 10,000.
On hydrophobic surfaces, they did not see such an
increase in viscosity. The results of the molecular dynamics
simulations support these findings, showing a dramatically decreased
mobility for sub-nanometer thick water films under hydrophilic
confinement.
"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."
Understanding the properties of water at this scale
could also be important for biological and pharmaceutical research,
especially in understanding processes that depend on hydrated ionic
transport through nanoscale channels and pores.
Riedo and Landman's next steps are to introduce
impurities in the water to study how that affects its properties. |
