Home > Press > Bridges of Water: "Keep Cool to Reduce Friction"
Study suggests that it may be possible to reduce the adhesion between surfaces
Bridges of Water: "Keep Cool to Reduce Friction," Suggests a New Study of Nanoscale Water Condensation
September 26, 2005
"Keep cool to reduce friction" might be the advice given designers of
nanoscale machinery by researchers who have just completed a study of
factors influencing the formation of "water bridges" - capillary
connections that can glue surfaces together, giving rise to friction
When surfaces touch in a humid environment, moisture forms water
bridges, or capillaries, between them. On familiar size scales, this
process - known as nucleation - helps hold sand castles and wet concrete
together, and is critical to the formation of clouds. But sometimes
these structures can be less helpful, causing friction sufficient to
slow or even stop nanoscale machinery - or in food processing, creating
large clusters of sugar, salt, baby cereals or coffee.
By studying the frictional forces acting on an atomic force microscope
(AFM) tip drawn across a glass surface, researchers at the Georgia
Institute of Technology have demonstrated for the first time that the
formation of these capillaries is thermally activated. Their study
suggests that it may be possible to reduce the adhesion between surfaces
by reducing temperatures and putting nanoscale surfaces into motion
before the water bridges have time to form.
"When you move very slowly, there is time for a capillary to form at
each tiny bump or asperity in the surface," explained Elisa Riedo, an
assistant professor in Georgia Tech's School of Physics. "But when you
move faster, you have fewer capillaries. If you go fast enough, the
capillaries do not have time to form."
Understanding the relationship between nucleation time and temperature
could be crucial to the designers of very small devices that must
operate in the presence of moisture, as well as to the food processing
industry. "Since formation of the capillaries affects friction and
adhesion between particles, if we understand this relationship, we can
understand how small particles and nano-surfaces glue together," she
A report on the research, which has been sponsored by the National
Science Foundation and the Petroleum Foundation, was published in the
journal Physical Review Letters on September 23rd.
Experimentally, Riedo and her postdoctoral collaborator Robert
Szoszkiewicz used an AFM with specially-crafted ball-shaped tips that
had diameters ranging from 40 to 100 nanometers. That provided a
multi-contact area of approximately 30 square nanometers.
While maintaining a constant humidity of about 40 percent, they moved
the tip across a slightly rough glass surface that had irregularities
approximately one nanometer high. While the tip was moving, they
recorded the resistance to motion - measured in piconewtons or
nanonewtons - while varying the temperature and velocity.
By charting their data, they saw evidence that the friction measured was
directly related to temperature, suggesting the growth of capillary
structures increases as temperature increases. "The more energetic the
water molecules are, the more likely it is that they will form
capillaries," said Szoszkiewicz. "We found that nucleation times grow
exponentially with the inverse of temperature."
The researchers found that the nucleation times of nanoscopic
capillaries increased from 0.7 milliseconds to 4.2 milliseconds when the
temperature decreased from 332 to 299 degrees Kelvin - which is
approximately room temperature.
"To form water bridges, molecules need to overcome an energy barrier.
The thermal energy can provide the energy they need, however, it takes
time for these bridges to form," Riedo noted. "The longer the surfaces
are together, the stronger the contact will be because more bridges can
When surfaces come close together, several processes can occur,
Szoszkiewicz said. After contact, moisture naturally adsorbed on the
surfaces - along with water molecules from the air - will concentrate
close to the true contact point because of diffusion. Some initial
water bridges will then form between contacting asperities.
When objects move close together but don't touch, a different process
occurs. Moisture adsorbed on each surface may coalesce, and because of
attractive forces, jump together, forming a water bridge. At a given
temperature, this nucleation process will differ for each surface
depending on its ability to adsorb moisture. Newly formed capillaries
then act as water sinks, attracting more water molecules because
pressure inside the capillary bridge is lower than the pressure outside
it. The process continues to a point at which an equilibrium capillary
bridge is formed.
"The question we considered was what would be the dominant phenomenon
and what would be the time scale for both phenomena," Szoszkiewicz said.
"We have experimentally demonstrated that with nano-rough surfaces,
nucleation will be dominant."
Beyond applications to atmospheric science, the food industry and
nanoscale sliding machinery, the findings suggest another way to control
ink flow in dip-pen nanolithography. In that process, ink flowing from
an AFM tip is used to write nanoscale patterns that could be useful in
such processes as semiconductor lithography.
"In this case, you might use the temperature dependence to increase the
velocity of the ink flow, decrease it, or make the flow improbable,"
said Riedo. "There are a lot of implications for the technology. Each
of the materials involved will have its own properties regarding
velocity and how rapidly it forms capillary bridges."
The researchers also measured the size of energy barrier required for
water molecules to nucleate. "This energy was predicted by theoretical
models using classical thermodynamics, and it matched really well with
our experiments," said Riedo.
The researchers hope the information they provide will help engineers
deal with capillary forces in a more efficient way. Because water is
ubiquitous, more information is needed about how it behaves at the
"Water is of crucial importance everywhere in our world - in biology,
earth sciences, atmospheric sciences and industrial processes," Riedo
noted. "From a fundamental point of view, it is difficult to do
theoretical models of water. But there is a huge interest in this from
both theoretical and technological standpoints."
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