This color-enhanced image shows a droplet of water being deposited on a superhydrophobic surface, just before it separates from the dropper used to deposit it. Image courtesy of Kripa Varanasi |
People living in older buildings often
hear pounding noises in their plumbing or radiator pipes—it’s a well-known
effect called a water hammer, which can occur when a valve is suddenly opened
or closed in a pipe carrying water or steam, causing a pressure wave to travel
down the pipe with enough force that it can sometimes cause the pipes to burst.
Now, new research shows that a similar effect takes places on a tiny scale
whenever a droplet of water strikes a surface.
MIT’s Kripa Varanasi, co-author of a
report on the new finding published in the journal Physical Review Letters, says the phenomenon could help
engineers design more durable condensing surfaces, which are used in
desalination plants and steam-based power plants. Other co-authors include MIT
mechanical-engineering graduate students Hyuk-Min Kwon and Adam Paxson, and
associate professor Neelesh Patankar of Northwestern Univ.
Varanasi,
the d’Arbeloff Assistant Professor of Mechanical Engineering, says the effect
explains why blades used in power-plant turbines tend to degrade so rapidly and
need to be replaced frequently, and could lead to the design of more durable
turbines. Since about half of all electricity generated in the world comes from
steam turbines improving their longevity and efficiency could reduce the down
time and increase the overall output for these plants, and thus help curb the
world emissions of greenhouse gases.
There has been widespread interest in
the development of superhydrophobic (water-repelling) surfaces, Varanasi says,
which in some cases mimic textured surfaces found in nature, such as lotus
leaves and the skin of geckos. But most research conducted so far on how such
surfaces behave have been static tests: To see the way droplets of different
sizes spread out on such surfaces (called wetting) or how they bead up to form
larger droplets, the typical method is to add or subtract water slowly in a
stationary droplet. But this is not a realistic simulation of how droplets
react on surfaces, Varanasi
says.
“In any real application, things are
dynamic,” he says. And Varanasi’s
research shows the dynamics of moving droplets hitting a surface are quite
different from droplets formed in place.
Specifically, such droplets undergo a
rapid internal deceleration that produces strong pressures—a small-scale
version of the water-hammer effect. It is this tiny but intense burst of
pressure that accounts for the pitting and erosion found on power-plant turbine
blades, he says, which limits their useful lifetime.
“This is one of the biggest unsolved
problems” in power-plant design, he says. In addition to damaging the blades,
the formation and growth of water droplets mixed with the flow of steam saps
much of the power, accounting for up to 30% of the system losses in such
plants. Since some steam-based power plants, such as natural-gas combined-cycle
plants, can already have efficiencies of up to 85% in converting the fuel’s
energy to electricity, if these droplet losses could be eliminated it could
provide almost a 5-percent boost in power.
“This is a new finding, indeed,” says
David Quéré, director of research at the laboratory of physics and mechanics of
heterogeneous materials at ESPCI, Paris. He explains that “Superhydrophobic
materials, on which water can glide and roll in a unique fashion, have
interesting properties, provided water stays at the tops of the decorations we
find on them. (I like to call that the fakir effect, since water then sits at
the tops of a bed of micro-nails.)”
This research, Quéré says, explains why
droplets often fail to stay on top and instead get impaled on the “nails,” and
so the new findings are “interesting in the context of superhydrophobic
materials, because it helps to design materials able to resist this kind of
detrimental effect.”
Small-scale texturing of surfaces can
prevent the droplets from wetting the surfaces of turbine blades or other
devices, but the spacing and sizes of the surface patterns need to be studied
dynamically, using techniques such as those developed by Varanasi and his co-authors, he says.
Regularly spaced bumps or pillars on the surface can produce a water-shedding
effect, but only if the size and spacing of these features is just right. This
research showed that there seems to be a critical scale of texturing that is
effective, while sizes either larger or smaller than that fail to produce the
water-repelling effect. The analysis developed by this team should make it
possible to determine the most effective sizes and shapes of patterning for
producing superhydrophic surfaces on turbine blades and other devices.
The work is related to Varanasi’s research on how to prevent ice
formation on airplane wings, also using nano-texturing of surfaces, but the
potential applications of this latest research are much broader. In addition to
power-plant turbines, this could also affect the design of condensers in
desalination plants, and even the design of inkjet printers, whose operation is
based on depositing droplets of ink on a surface.
This work was funded by the MIT Energy
Initiative, the National Science Foundation, the Dupont-MIT Alliance, and the
Initiative for Sustainability and Energy at Northwestern. MIT’s Edgerton Center also provided high-speed video
equipment.
