Wednesday, August 19, 2015

Liftoff: Hydrophobic Fibers Fling Condensation From Their Strands

I recently moved to the DC area in the middle of the summer where, on a bad day, being outdoors is a lot like being inside a rice cooker; sometimes I wish I could stop sweating, because evaporative cooling doesn’t really work when the air is already practically saturated with moisture. As such, the dehumidifier has become my new best friend. This miracle of modern technology that keeps the mildew out of my apartment works by blowing air over a refrigerated mesh of wires, where the water condenses and falls into a bucket, sometimes at the surprising rate of a few liters per day.

As someone who relies so heavily on this device, I was excited to learn about a development that may make this and similar technologies much more effective by applying the nanoscale science of hydrophobic surface coatings; it looks like Research from Duke University has made a huge advance toward tackling one of the rate-limiting steps in the efficiency of liquid condensers: droplet removal.

Ordinarily, water droplets adhere to the condensing surface until they’re massive enough to be pulled away by gravity and fall into the collector. This works well enough, but it means droplets can stay on the wires for a while before falling, which keeps new droplets from forming as efficiently. Additionally, a lot of energy goes into cooling water that’s going to fall into a bucket and warm back up to room temperature before too long.

But the Duke team's research, published in Physical Review Letters, shows that droplets condensing on a superhydrophobic wire will leap off, seemingly of their own volition, as they combine with one another into larger drops.

Image Courtesy Fangjie Liu
How does it happen? Think of it this way:

A droplet on a hydrophobic surface will take a shape as close to a sphere as possible, seeking the configuration with the least possible surface area (and, correspondingly, the least possible surface tension energy). As a result, a system composed of two droplets next to each other on a wire is in a higher-energy state than a single droplet with their combined volume, because the two droplets have more surface area between them than the single drop would. If any part of those two droplets touch, they’ll coalesce into one. Since the combined surface tension energies of the individual droplets is greater than that of the conglomerate, the system suddenly has an excess of mechanical energy. If this energy is great enough to overcome the adhesion that kept them on the wire, the newly-formed conglomerate drop will be flung off, making room on the wire for new ones to form.
Image courtesy Kungang Zhang
This may seem trivial at first, but consider the potential for applications in systems like the Warka Water Tower, which aims to bring potable water to Ethiopian villages. For people who live a six hour walk from the nearest source of drinking water, such a “drop in the bucket” could make a world of difference.

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