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:

1. 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.
2. 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.
3. David L. Chandler, "A better way to shed water," MIT Press Release, October 22, 2012.
4. 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.
5. 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.
6. Enhanced Condensation on Lubricant-Impregnated Nanotextured Surfaces, YouTube Video, October 22, 2012.