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Lubricated Textured Surfaces Boost Condenser Water Movement

Micro-textured and lubricated surface (Varanasi Lab, MIT)

On a surface patterned with tiny pillars (white squares), and with a coating of a lubricant liquid that fills the spaces between the pillars, dome-shaped droplets of water condense but remain free to move quickly across the surface. (Varanasi Lab, MIT)

Engineering researchers at Massachusetts Institute of Technology devised a technique for accelerating the movement of water off industrial surfaces like those in power plant and desalination condensers. The team from MIT’s Lab for Nanoengineered Surfaces, Interfaces, and Coatings published its findings earlier this month in the journal ACS Nano (paid subscription required).

In power plant steam turbines and desalination units that turn sea water into drinking water, condensers turn steam into water. The faster that the water can be removed from either of these industrial units, the more efficiently they operate.

The team led by engineering professor and lab director Kripa Varanasi tested the ability of microscale engineered surfaces to shed water. Previous research indicated that water had a tendency to condense within the textures of hydrophopic (water-shedding) surfaces, breaking the vapor pockets, and creating an attachment of water to the surface, the opposite of the intended outcome.

The MIT team found a combination of steps could vastly improve the hydrophobic capability of these surfaces. First, they created a a surface pattern with posts only 10 microns — 10 millions of a meter — across. They then added a small amount of lubricant held in place on that surface with capillary action by the tiny posts (pictured at top).

The researchers found the droplets of water condensing on this surface moves 10,000 times faster than on surfaces with the hydrophobic patterning alone. The increased speed that water droplets moves off the surface makes it possible for new droplets to form faster. This process can increase the efficiency of heat transfer in a power plant condenser, or the rate of water production in a desalination plant.

In addition, the amount of lubricant required is minimal, Varanasi’s team reports. The lubricant forms a thin coating, which is securely held in place by the posts. Any lubricant that is lost can be replaced from a small reservoir at the edge of the surface. First author and postdoctoral researcher Sushant Anand says it takes only about a quarter- to a half-teaspoon of lubricant to coat a square yard of the textured material, which also helps protect the underlying metal surface from corrosion.

Varanasi plans to conduct further research to quantify the extent of improvement possible with this the new technique in steam-turbine power plants. Because steam turbines are ubiquitous power plants generating electricity with fossil fuels, Varanasi notes that “even if it saves one percent, that’s huge” in its potential impact on global emissions of greenhouse gases.

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