Coating helps hot metal hang onto protective vapor layer that prevents explosive boiling
Image: Patankar - 11418
-
Gravity's Engines
We’ve long understood black holes to be the points at which the universe as we know it comes to an end. Often billions of times more massive than the Sun, they...
Read More »
By Katharine Sanderson of Nature magazine
One trick to test whether a frying pan is hot enough is to sprinkle water on it. If the surface is sufficiently above the boiling point of water, droplets will skip across the pan. Those jittery beads of water are held up from the hot pan by a cushion of steam. The vapour cushion collapses as the surface falls below the 'Leidenfrost temperature', causing furious bubbling and spitting when the water droplet hits the surface and boils explosively.
The Leidenfrost effect lies behind the discovery, published today in Nature, that water can be made to boil without any bubbling if a surface is specially treated so that the vapour cushion does not break down. The key is to make the surface very water-repellent, according to Ivan Vakarelski, an engineer at the Clean Combustion Research Center at the King Abdullah University of Science and Technology in Thuwal, Saudi Arabia, and his colleagues. The effect might be used to carefully control how metals are cooled and heated, or to reduce drag on ships.
The rough with the smooth
Vakarelski’s team covered metal spheres with a commercially available coating that made the surface rough and strongly water-repellent, and heated these superhydrophobic spheres to 400 °C (any higher and the coating would deteriorate). For comparison, they heated a set of smooth, water-attracting spheres to 700 °C.
Each hot sphere was dropped into room-temperature water, where a layer of water vapour formed around it. The vapour layers around the water-loving spheres quickly collapsed, leading to explosive bubbling. The coated spheres, by contrast, kept their vapour layers as they cooled down, with no bubbling or explosive boiling.
“We thought we could improve the transition” from the Leidenfrost regime to bubbling, says Vakarelski, “but we are not only lowering the transition, we are completely avoiding it”.
“It was really dramatic,” says Neelesh Patankar, a theoretical mechanical engineer at Northwestern University in Evanston, Illinois, and a co-author of the paper. “As the temperature goes down, this vapour phase nicely settles down.”
The implications of the work could be far-reaching, says Vincent Craig, an applied mathematician at the Australian National University in Canberra, who studies the physics of surface forces and bubbles. “They’ve shown that by keeping the surface rough you can keep that vapour layer at low temperature.” The effect could be used to reduce drag on surfaces such as the tiny channels in microfluidic devices, he suggests.
Hot stuff
In a related experiment, the team dipped metal rods with the same water-loving and water-hating surfaces into water. Heaters inside the rods warmed them, while probes monitored their temperature. The water-loving rods could only ever reach 106 °C, because the water was always coming into contact with the metal and cooling it. But the coated spheres got up to 250 °C, because they were constantly protected from the cool water by the vapour layer.
The next step, says Patankar, is to try to get the vapour layer to form at temperatures much lower than the boiling point of water. Water can exist as either liquid or vapour at room temperature, but it requires energy to stay in the vapour state. Patankar thinks that a surface could be designed that would make the vapour state more stable. A coating could then be used to form a vapour layer round a ship’s hull to reduce drag or discourage organisms such as algae or barnacles from attaching themselves to the ship, he suggests. “It will be mind-blowing,” says Patankar. “Who thinks of getting a vapour without heating?”
This article is reproduced with permission from the magazine Nature. The article was first published on September 12, 2012.
See what we're tweeting about
8 Comments
Add Comment"The next step, says Patankar, is to try to get the vapour layer to form at temperatures much lower than the boiling point of water." The problem is that pressure will not allow vapour below the boiling point, since air pressure will squash vapour, so that it condenses. At boiling temperature, vapour pressure can hold water away since vapour pressure balances atmospheric pressure. To form a film of vapour at room temperature, air would need to be forced thru tiny holes in the hull of a ship to keep water away.
Reply | Report Abuse | Link to thisI believe you mean water pressure. They want to get a vapor layer to form under water, which would indeed be a very good trick without heating, but they might be onto something.
Reply | Report Abuse | Link to thisI agree, that will be huge if they can create the vapor layer without heating. Vacuum through cavitation? Air forced through tiny holes would not be viable since it would quickly get clogged from contamination and corrosion. Corrosion at other points would expand holes and make the system fail. Doesn't take very long with sea water. Fun to conjecture but in all seriousness, I wish them luck on this.
Reply | Report Abuse | Link to thisI wonder if this can some how be applied to aircraft, especially hyper velocity ones...just a thought...
Reply | Report Abuse | Link to thisHeat is relatively easy to create, imagine a missile that could full submerge using this technology, then resurface and go airborne. This might even be the key to making the scramjet technology practical...
Reply | Report Abuse | Link to thisYou'd need to use a lighter gas for that, like hydrogen. Actually, plasma would be the most efficient. There's actually already a field of study for this, called plasma aerodynamics, also known as a electroaerodynamics.
Reply | Report Abuse | Link to thisTake a look at NASA's latest project announcement on magnetoshell aerodynamics, which could provide great advantages for re-entry aerobraking and maneuvering.
I really don't think that this technique will be very useful for there are a lot of means to protect and remove the scale, which is also cheap.
Reply | Report Abuse | Link to this兴许有些个劳民伤财吧。
basically, it HAS been applied to aircraft. I used to 'rough-up', in a 30 degree crosshatch, the wings on a cassutte sport racer and got measurably more speed. Jon Sharp suggested this to me back in the mid 1980's. This was supposed to keep the boundary layer flowing air - against - air, instead of dragging on the wing surface.
Reply | Report Abuse | Link to this