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Super Condensing Surfaces
November 9, 2012
Autumn is half over, so
winter is not that far away. It's officially winter only after the
winter solstice (sometimes called the
December solstice). It occurs this year on December 21 at 11:12
UTC (6:12 AM
EST), but we usually see
snow in my area before that date. That day is the shortest day of the year for the denizens of the
northern hemisphere. Since our
calendar, because of
leap years, is just an approximation of what's happening
astronomically, the winter solstice can happen on dates from December 20-23.
As we learned in
elementary school, we have
seasons because the
Earth's rotational axis is tilted with respect to its
orbital plane. At the winter solstice, this 23.5° tilt puts north
latitudes of 66.5° and greater (the
Arctic Circle) in day-long darkness, and south latitudes of 66.5° and greater (the
Antarctic Circle) in day-long daylight.
Degrees of angle are interesting, but it's
degrees of temperature that most concern me at this time of year.
New Jersey summers are very nice, as the
tomatoes will attest, but the winters can be somewhat difficult.
Ice and
snow are major problems for people living in these higher
latitudes, especially for a society that spends an inordinate fraction of its wealth on
automobiles,
automotive fuel,
insurance and
roads.
In a recent article (
Icing-Resistant Surfaces, August 8, 2012, written in sunnier times, I wrote about research on the development of icing-resistant surfaces by a team of
scientists at
Harvard's School of Engineering and Applied Sciences.[1-2] Although
nanoscale-modified surfaces are
superhydrophobic (water repellent), since the
contact angle of droplets is very large (>150°), they are surprisingly not icing resistant.
The Harvard solution was to infuse
superhydrophobic surfaces with a
water-immiscible liquid. This method, which they call SLIPS, for "Slippery Liquid Infused Porous Surfaces," presents a
molecularly flat liquid interface to oncoming water. The nanostructured surface holds the liquid in place, and water droplets, frost and ice can't adhere to the liquid, so they slide off.[1]
Now, a crosstown
Boston team from
MIT's Department of Mechanical Engineering has applied similarly lubricated, nanotextured surfaces to improve
water condensation in
condenser systems used for a variety of applications.[3-6] Condensers are ubiquitous. About eighty percent of
powerplants use condensers to turn
steam back to
liquid water after it's done its job of powering
turbine-generators. They're also a principal component of
desalination plants, condensing fresh water from heated
brack.[3]
A typical steam condenser system.
Steam condenses on the interior pipes.
Illustration by Milton Beychok (modified), via Wikimedia Commons)
This improved performance arises from an enhanced water-shedding of the treated condenser surface, which is composed of 10
micrometer posts and a
lubricant coating. The
capillary action of the regions between the posts holds the lubricant to the surface.[3] Water droplets as large as 10 μm condensing on this type of surface are 10,000 times more mobile than those condensing on
hydrophobic patterned surfaces without the lubricant treatment.[3-4] This droplet motion assists in droplet removal from the surface, so that new droplets can condense (see photograph).[3-4]
Still from a YouTube video)
The required quantity of lubricant is small, since the surface layer is very thin, and low
vapor pressure oils can be used in the
high temperature environment of a condenser.[3] Just a
milliliter can coat a
square meter of surface, and the lubricant can be multifunctional, serving as an
anti-corrosion coating.[3]
Kripa Varanasi, an
associate professor at MIT, and coauthor of the study, points out that just a 1%
efficiency increase resulting from this type of coating can lead to a huge
environmental advantage by reducing emission of
greenhouse gases.[3]
An important part of this research effort was a new
scanning electron microscopy technique developed by Varanasi,
Konrad Rykaczewski, an MIT research scientist presently with the
National Institute of Standards and Technology (NIST),
Gaithersburg, Maryland,
John Henry Scott and
Marlon Walker of NIST, and Trevan Landin of
FEI Company.[5-6]
Specimens in a
scanning electron microscopes (SEMs) need to be under a reasonable
vacuum, since SEM is an
electron imaging technique. The novel imaging process works by flash freezing a droplet-coated surface using
liquid nitrogen. Says Konrad Rykaczewski, who was involved in the imaging study, "The freezing rate is so fast (about 20,000 degrees
Celsius per
second) that water and other liquids do not
crystallize, and their geometry is preserved."[3]
The MIT condenser surfaces research was supported by the
National Science Foundation and other organizations. The imaging research was also supported by NIST.[3] A paper describing these super-condensing surface appears in
ACS Nano.[4]
References:
- Michael Patrick Rutter and Twig Mowatt, "A new spin on antifreeze - Researchers create ultra slippery anti-ice and anti-frost surfaces," Harvard School of Engineering and Applied Sciences Press Release, June 11, 2012.
- Philseok Kim, Tak-Sing Wong, Jack Alvarenga, Michael J. Kreder, Wilmer E. Adorno-Martinez and Joanna Aizenberg, "Liquid-Infused Nanostructured Surfaces with Extreme Anti-Ice and Anti-Frost Performance," ACS Nano, vol. 6, no. 8 (August 28, 2012), pp. 6569-6577.
- David L. Chandler, "A better way to shed water," MIT Press Release, October 22, 2012.
- Sushant Anand, Adam T. Paxson, Rajeev Dhiman, J. David Smith and Kripa K. Varanasi, "Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces," ACS Nano, Article ASAP, DOI: 10.1021/nn303867y, October 2, 2012.
- Konrad Rykaczewski, Trevan Landin, Marlon L. Walker, John Henry J. Scott and Kripa K. Varanasi, "Direct Imaging of Complex Nano- to Microscale Interfaces Involving Solid, Liquid, and Gas Phases," ACS Nano, Article ASAP, DOI: 10.1021/nn304250e, September 28, 2012.
- Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces, YouTube Video, October 22, 2012.
Permanent Link to this article
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