David Terraso
Institute Communications and Public Affairs
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 water freezes, it becomes less dense.
For this reason, 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 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. She
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, an assistant professor
in the 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 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
its liquidity in the horizontal direction where it can
flow out — resembling some phases of liquid crystals,”
said Uzi Landman, a Regents’ professor in the
School of Physics.
Riedo and Landman conducted their experiments in several
different environments. It was 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, 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 — by a factor of 1,000 to 10,000.
Hydrophobic surfaces did not experience such increase
in viscosity.
“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.”
Riedo and Landman’s next steps are to introduce
impurities in the water to study how that affects its
properties.
|
