On The Boil: New Nano Technique Significantly Boosts Boiling Efficiency
ScienceDaily (June 27, 2008) — Whoever penned
the old adage “a watched pot never boils” surely never tried to heat up
water in a pot lined with copper nanorods.
A new study from researchers at Rensselaer Polytechnic Institute
shows that by adding an invisible layer of the nanomaterials to the
bottom of a metal vessel, an order of magnitude less energy is required
to bring water to boil. This increase in efficiency could have a big
impact on cooling computer chips, improving heat transfer systems, and
reducing costs for industrial boiling applications.
“Like so many other nanotechnology and nanomaterials breakthroughs,
our discovery was completely unexpected,” said Nikhil A. Koratkar,
associate professor in the Department of Mechanical, Aerospace, and
Nuclear Engineering at Rensselaer, who led the project. “The increased
boiling efficiency seems to be the result of an interesting interplay
between the nanoscale and microscale surfaces of the treated metal. The
potential applications for this discovery are vast and exciting, and
we’re eager to continue our investigations into this phenomenon.”
Bringing water to a boil, and the related phase change that
transforms the liquid into vapor, requires an interface between the
water and air. In the example of a pot of water, two such interfaces
exist: at the top where the water meets air, and at the bottom where
the water meets tiny pockets of air trapped in the microscale texture
and imperfections on the surface of the pot. Even though most of the
water inside of the pot has reached 100 degrees Celsius and is at
boiling temperature, it cannot boil because it is surrounded by other
water molecules and there is no interface — i.e., no air — present to
facilitate a phase change.
Bubbles are typically formed when air is trapped inside a microscale
cavity on the metal surface of a vessel, and vapor pressure forces the
bubble to the top of the vessel. As this bubble nucleation takes place,
water floods the microscale cavity, which in turn prevents any further
nucleation from occurring at that specific site.
Koratkar and his team found that by depositing a layer of copper
nanorods on the surface of a copper vessel, the nanoscale pockets of
air trapped within the forest of nanorods “feed” nanobubbles into the
microscale cavities of the vessel surface and help to prevent them from
getting flooded with water. This synergistic coupling effect promotes
robust boiling and stable bubble nucleation, with large numbers of
tiny, frequently occurring bubbles.
“By themselves, the nanoscale and microscale textures are not able
to facilitate good boiling, as the nanoscale pockets are simply too
small and the microscale cavities are quickly flooded by water and
therefore single-use,” Koratkar said. “But working together, the
multiscale effect allows for significantly improved boiling. We
observed a 30-fold increase in active bubble nucleation site density —
a fancy term for the number of bubbles created — on the surface treated
with copper nanotubes, over the nontreated surface.”
Boiling is ultimately a vehicle for heat transfer, in that it moves
energy from a heat source to the bottom of a vessel and into the
contained liquid, which then boils, and turns into vapor that
eventually releases the heat into the atmosphere. This new discovery
allows this process to become significantly more efficient, which could
translate into considerable efficiency gains and cost savings if
incorporated into a wide range of industrial equipment that relies on
boiling to create heat or steam.
“If you can boil water using 30 times less energy, that’s 30 times less energy you have to pay for,” he said.
The team’s discovery could also revolutionize the process of cooling
computer chips. As the physical size of chips has shrunk significantly
over the past two decades, it has become increasingly critical to
develop ways to cool hot spots and transfer lingering heat away from
the chip. This challenge has grown more prevalent in recent years, and
threatens to bottleneck the semiconductor industry’s ability to develop
smaller and more powerful chips.
Boiling is a potential heat transfer technique that can be used to
cool chips, Koratkar said, so depositing copper nanorods onto the
copper interconnects of chips could lead to new innovations in heat
transfer and dissipation for semiconductors.
“Since computer interconnects are already made of copper, it should
be easy and inexpensive to treat those components with a layer of
copper nanorods,” Koratkar said, noting that his group plans to further
pursue this possibility.
Along with Koratkar, co-authors of the paper include Rensselaer MANE
Associate Professor Yoav Peles; Rensselaer mechanical engineering
graduate student Zuankai Wang; Rensselaer Center for Integrated
Electronics Research Associate Pei-I Wang; University of Colorado at
Boulder Chancellor and former Rensselaer Provost G.P. “Bud” Peterson;
and UC-Boulder Assistant Research Professor Chen Li.
The research was funded by the National Science Foundation.