A New Supersurface Lets Water Bounce Like a Ball

There's waterproofing, as in a duck's back or a properly waxed cotton jacket, and then there's the world of superhydrophobes. This is where you find materials that are so intensely waterproof that a droplet of water will find itself resisted enough such that it will be just barely touching the surface, with the angle between said surface and the lower surface of the droplet exceeding 120 degrees. When you're trying to imagine that, keep in mind that 180 degrees is total flatness, and consider the image below of a water droplet on a superhydrophobic lotus leaf. The critical angle is between the two diagonal straight-lines and the point of contact between them.

In this image, that angle just a hair under 150 degrees, which is some intense superhydrophobia.

Get it? For the angle to be an honest full 180 degrees, the droplet would have to be not touching all. Within the natural world, the lotus leaf doesn't have much company for the above sort of dryness ability. Butterfly wings are also known for topping 140 degrees, and both organisms have helped guide research into advanced synthetic superhydrophobic surfaces.

A study out today from researchers at Brigham Young University is advertising a superhydrophic material so resistant to a jet of water that droplets will actually bounce off the material and roll down it, like little rubber balls. The material just does not get wet, in the precise scientific sense of leaving no water behind that's been absorbed or incorporated in the material, much like a rubber ball typically doesn't leave a whole lot of rubber behind on the concrete it's been bounced off of. The result is about the most pure version of "dryness" imaginable.

Hydrophobia in the current study is measured as a function of relative locations of the "hydraulic jump" that occurs when you direct a jet of water at a surface. To see a hydraulic jump in action, head to your sink and turn on the water. Where the stream hits the stainless steel of the sink, you'll see a circle of flatness where water is shooting outwards from the impact site. The jump is where that radiating water loses energy and bunches up in a relatively thick band of water. As hydrophobicity increases, you'll see the radius of the pre-jump circle increase, such that the water can travel laterally across the surface for longer without losing energy. If you were to take a variety of surfaces and put them under that faucet stream—the back of a duck, a piece of cardboard, a dirty dish—you could, in a very crude sense, do your own hydrophobicity measurements.

Here's what it looked like in the BYU experiments:

The four jumps shown above are for four different variations of a hydrophobic surface using different patterns of microscopic ridges or posts. The surfaces are not properly "flat," and flatness is actually given as the boundary between hydrophobic and superhydrophobic materials. It's only possible to reach an internal angle of 120 degrees with flat materials; going above, into the current realm of dryness, takes microengineering of surfaces. This microengineering is made possible through a process similar to a sort of photo film development that etches different patterns on wafers.

In the images with the circular jumps, you're seeing the effects of the designs using arrays of tiny Lego-like posts. When the posts are swapped out for tiny ridges, you start to see the elliptical jumps. The stretched out portion of the ellipse in these cases occurs parallel to the micro-ridges, with the stubbier part of the ellipse occuring when the jump is more perpendicular to the ridges.

If you look closely, you'll also note that jump radius and shape aren't the only differences being observed. There are an abundance of other scenarios possible for that inner circle of sideways-traveling water, "including thin film growth out to a hydraulic jump, thin film breakup into droplets or filaments, or a collapse of the downstream water depth at the point of impingement" (from the image's caption text) The flat ring isn't flat, in other words. The material is exerting a whole lot more water resistance than might allow most of the phenomena you'd see in your sink.

Here's an older video released by the BYU team:

You could probably make some pretty good guesses as to the applications of superhydrophobic materials. Some of them are indeed rather everyday: showers and solar panels that self-clean, or perhaps the hulls of ships or submarines made ultra-resistant to drag. Remember the since-banned LZR Racer swimsuits? That was just the beginning.

But the application highlighted in a BYU press release is in power plants that use steam to spin turbines. In a coal-fired or nuclear power plant, a thermal power plant, heat is used to turn water into steam, which creates pressure that spins the blades of a turbine. Once the steam passes through the turbine, it's condensed back into water, using pressure and temperature. The reclaimed water is then reused.

This part of the plant is called the surface condensor, and the BYU researchers, led by mechanical engineering professor Julie Crockett, suggest that in the future it might be fabracated from superhydrophobic materials. “If you have these surfaces, the fluid isn’t attracted to the condenser wall, and as soon as the steam starts condensing to a liquid, it just rolls right off,” Crockett said in a press release. “And so you can very, very quickly and efficiently condense a lot of gas.”