This is a really neat discovery. As we all know, ice is a big killer and safety hazard, especially on airplanes. This new material prevents supercooled droplets from freezing, sticking, and accumulating.- Anthony
From Harvard: Breaking the ice before it begins
Nanostructured materials repel water droplets before they have a chance to freeze
Cambridge, Mass., November 12, 2010 – Engineers from Harvard University have designed and demonstrated ice-free nanostructured materials that literally repel water droplets before they even have the chance to freeze.
The finding, reported online in ACS Nano on November 9th, could lead to a new way to keep airplane wings, buildings, powerlines, and even entire highways free of ice during the worst winter weather. Moreover, integrating anti-ice technology right into a material is more efficient and sustainable than conventional solutions like chemical sprays, salt, and heating.
A team led by Joanna Aizenberg, Amy Smith Berylson Professor of Materials Science at the Harvard School of Engineering and Applied Sciences (SEAS) and a Core Member of the Wyss Institute for Biologically Inspired Engineering at Harvard, focused on preventing rather than fighting ice buildup.
“We wanted to take a completely different tact and design materials that inherently prevent ice formation by repelling the water droplets,” says Aizenberg. “From past studies, we also realized that the formation of ice is not a static event. The crucial approach was to investigate the entire dynamic process of how droplets impact and freeze on a supercooled surface.”
For initial inspiration, the researchers turned to some elegant solutions seen in nature. For example, mosquitos can defog their eyes, and water striders can keep their legs dry thanks to an array of tiny bristles that repel droplets by reducing the surface area each one encounters.
“Freezing starts with droplets colliding with a surface,” explains Aizenberg. “But very little is known about what happens when droplets hit surfaces at low temperatures.”
To gain a detailed understanding of the process, the researchers watched high-speed videos of supercooled droplets hitting surfaces that were modeled after those found in nature. They saw that when a cold droplet hits the nanostructured surface, it first spreads out, but then the process runs in reverse: the droplet retracts to a spherical shape and bounces back off the surface before ever having a chance to freeze.
By contrast, on a smooth surface without the structured properties, a droplet remains spread out and eventually freezes.
“We fabricated surfaces with various geometries and feature sizes—bristles, blades, and interconnected patterns such as honeycombs and bricks—to test and understand parameters critical for optimization,” says Lidiya Mishchenko, a graduate student in Aizenberg’s lab and first author of the paper.
The use of such precisely engineered materials enabled the researchers to model the dynamic behavior of impacting droplets at an amazing level of detail, leading them to create a better design for ice-preventing materials.
Another important benefit of testing a wide variety of structures, Mishchenko adds, was that it allowed the team to optimize for pressure-stability. They discovered that the structures composed of interconnected patterns were ideally suited for stable, liquid-repelling surfaces that can withstand high-impact droplet collisions, such as those encountered in driving rain or by planes in flight.
The nanostructured materials prevent the formation of ice even down to temperatures as low as to degrees Celsius. Below that, due to the reduced contact area that prevents the droplets from fully wetting the surface, any ice that forms does not adhere well and is much easier to remove than the stubborn sheets that can form on flat surfaces.
“We see this approach as a radical and much needed shift in anti-ice technologies,” says Aizenberg. “The concept of friction-free surfaces that deflect supercooled water droplets before ice nucleation can even occur is more than just a theory or a proof-of-principle experiments. We have begun to test this promising technology in real-world settings to provide a comprehensive framework for optimizing these robust ice-free surfaces for a wide range of applications, each of which may have a specific set of performance requirements.”
In comparison with traditional ice prevention or removal methods like salting or heating, the nanostructured materials approach is efficient, non-toxic, and environmentally friendly. Further, when chemicals are used to de-ice a plane, for example, they can be washed away into the environment and their disposal must be carefully monitored. Similarly, salt on roads can lead to corrosion and run-off problems in local water sources.
The researchers anticipate that with their improved understanding of the ice forming process, a new type of coating integrated directly into a variety of materials could soon be developed and commercialized.
In addition to Aizenberg, who is also the Susan S. and Kenneth L. Wallach Professor at the Radcliffe Institute for Advanced Study and a Professor of Chemistry and Chemical Biology at Harvard, and Mishchenko, the co-authors of the paper included Benjamin Hatton and Vaibhav Bahadur, both at SEAS and Wyss, and Ashley Taylor and Tom Krupenkin, both at the University of Wisconsin-Madison.
The researchers acknowledge L. Stirling and A. Grinthal for their valuable contribution and funding from DARPA (Award Number HR0011-08-C-0114); the Wyss Institute for Biologically Inspired Engineering at Harvard University; and the U.S. Department of Homeland Security (DHS) Scholarship and Fellowship Program.
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“The finding, reported online in ACS Nano on November 9th, could lead to a new way to keep airplane wings…”
The word “could” in the above sentence is the smoking gun. I’m a materials engineer (and current PhD candidate) with over 20 years in the business. I don’t know how many times I’ve seen articles on nanotechnology making similar claims yet most fail to pan out, at least in the short-term. My own belief is that these press releases are designed to promote the research with the sole intent of ensuring continued funding.
Maybe there’s a “Nanogate” waiting in the wings…
Well actually there is nothing new about this idea or technology; BUT I would agree that anti-icing is a new application that could be of great value.
For $100 you can buy a nice brand new 3-M (Scientific Anglers) fly fishing line, which has a nano-structured surface that makes it extremely hydrophobic. Their rade name for it is the Sharkskin Line.
3-M has used this technology for eons. One of their most famous uses of it was its use by Dennis Connor, on his America’s Cup Yacht in the 1987 America’s Cup Challenger series in Fremantle Australia. The 12 Metre formula that was then being used allowed a designer to exchange hull waterline length for maximum sail area. A longer hull had a higher hull speed; but then you would have less sails to power it. That wouldn’t matter if the wind speeds were high. On the other hand with low wind swpeeds that would favor a shorter hull and a bunch of sail.
The Wind history (obligatory climate content) showed that by the time the actual AC races started; the “Fremantle Doctor” would show up, and wind speeds would be high; but they were low when the Challenger preliminary races started. Connor opted for the longest hull length for his “Stars and Stripes” Yacht. That made him somewhat slow in the preliminaries, and he nearly didn’t make it into the final rounds. 3-M supplied him with some of their sharkskin tape, to coat the hull with, and that gave him enough extra speed to eventually defeat the New Zealand team and win the Challenger series. Then in the AC races themselves, his boat was clearly superior in speed with the high winds, and he easily beat the Autralian Defenders. The Sharkskin was then banned from use in the cup races.
But sailors know to sand their hulls with the right grade of sandpaper to roughen the surface to the correct pit size.
The basic functioning principle is the surface tension and internal pressure of bubbles.
A water droplet or an air bubble in water must have an excess internal pressure over the outside ambient pressure; and the amount of that internal pressure excess is given by 2T/r, where T is thew surface Tension in Newtons per metre, and r is the radius of the droplet or bubble in metres. So the smaller the bubble, the higher the internal pressure.
If you make pits on the surface that are roughly hemispherical in shape (exact shape doesn’t matter), then water has to form a surface of shorter radius than the pits, in order to get down into the pit and actually wet the surface. Instead the bubble sits up on the rim with a larger radius, so it is like a water strider sitting on the surface.
With the sharkskin fly line, 3-M is able to create a higher floating line, since the pits literally lift the line off the water surface, which simply refuses to curve into a smallenough radius to wet the line surface at the bottom of the pits. The internal excess pressure is literally lifting the line off the surface, so lowering its apparent density.
So you put such a pitted surface on a plane wing, and water droplets driven onto the surface at some velocity are going to be repelled from the surface by those pits.
The sharkskin fly line is quite amazing; when you pick it up off the water surface into the air, it literally rains underneath the line, as water streams of fthe surface. It also makes an annoying buzz, going through the guides on the rod, and it peels the skin off your stripping fingers; but it sure does float better than any other fly line. The patter they use is basically a diamond grid; and they literally have to emboss it onto the circumference of the lien from end to end, one line at a time; there is no bulk process for putting it on the plastic coating; which is why you pay $100 for a line that normally would be $65.
“”””” Dave says:
November 15, 2010 at 10:09 am
“The finding, reported online in ACS Nano on November 9th, could lead to a new way to keep airplane wings…”
The word “could” in the above sentence is the smoking gun. I’m a materials engineer (and current PhD candidate) with over 20 years in the business. “””””
Dave; may I suggest you try to get a PhD in Ice Cream Making instead of Materials Engineering.
If you are doing a PhD in ME and you aren’t familiar with the internal excess pressure of bubbles; then you need a change of career; or at least of your PhD advisor. Don’t forget that with soap bubbles the internal pressure is 4T/r because there are two surfaces to a soap film. It should be a trivial derivation for you to prove Dave; I would hint that the principle of virtual work leads to a very easy derivation.
George,
First of all, I said nothing about doing my dissertation in nanotechnology. Materials engineering is a very broad field of which nano is just a very small piece (pun intended). But I’ve been watching the area for a long time and have seen time and time again where such discovery announcements have been made only to never again see the light of day. As I intimated in my original post, the same PR tactics used in AGW research exists in other fields.
Care to go get an ice cream cone?
Trees Infused With Glowing Nanoparticles Could Replace Streetlights
Taiwanese researchers have come up with the elegant idea of replacing streetlights with trees, by implanting their leaves with gold nanoparticles. This causes the leaves to give off a red glow, lighting the road for passersby without the need for electric power. This ingenious triple threat of an idea could simultaneously reduce carbon emissions, cut electricity costs and reduce light pollution, without sacrificing the safety that streetlights bring.
http://www.popsci.com/technology/article/2010-11/trees-could-one-day-serve-streetlights-thanks-gold-nanoparticles
“”” Dave says:
November 15, 2010 at 12:00 pm
George,
First of all, I said nothing about doing my dissertation in nanotechnology. Materials engineering is a very broad field of which nano is just a very small piece (pun intended). But I’ve been watching the area for a long time and have seen time and time again where such discovery announcements have been made only to never again see the light of day. As I intimated in my original post, the same PR tactics used in AGW research exists in other fields.
Care to go get an ice cream cone? “””””
Well Dave, I do hope you didn’t take any offense; because none was intended, and I hoped you would see the tongue in cheek; well Ic an be a cheeky guy. And I do like your ice cream cone suggestion; I’m actually out to lunch as soon as I post this.. No I didn’t imply you were working in nano-technology; and that certainly has many sides to it.
But the 3-M sharksin fly line works like a dream; but there’s as many people hate it as love it.
In the event that you aren’t familiar with the excess pressure in bubbles due to surface tension; I could rattle off the derivation here in a few minutes; but only after I get back from getting my ice cream cone lunch.
George
PS in the virtual work approach you calculate the work done by the excess pressure during a small increment in radius; and you equate that to the work done by the increase in surface area agaisnt the surface tension; QED.
Well I might as well do it for all those who maybe don’t quite understand the idea; since it actually is quite important in climate and may pay an important role in cloud formation.
Surface tension, T is a constant force in Newtons per metre that acts in a liquid surface trying to pull the surface into a minimum area lowest energy state. It is easiest to demonstrate with the “parallel wire” approach where you have a wire bent around to form an end loop and two parallel wired spaced (w) apart. Then you have a sliding cross wire such as a needle lying across the wires to form a closed area (A). So you dip this into a soapy water solution to form a film covering the area.
The surface tension will try to pull the needle down to the loop to reduce the film area, and the pulling force will be 2 T.w, Newtons where the 2 comes from the fact that the soap film has two surfaces pulling on the needle. Unlike a rubber balloon skin which would have a force proportional to the stretch; the surface tension force is absolutely constant and independent of the film area. Now it is of course highly dependent on conditions and Temperature, which will change the value of the surface tension.
So if you let the film pull the needle a distance (l), then the area will be reduced by l.w, and the work done will be T.w.l or simply T times the change in surface area; well I forgot the factor of 2 so it is 2T x delta (A). Well I fooled myself here because since the film has two sides, the actual change in area is 2 w.l, so the work done by a surface is indeed T delta (A).
So now we have our water droplet of radius (r), so it has a surface area of 4pi.r^2. If we let the droplet grow to r +dr then the surface area becomes 4pi(r^2 +2r.dr) and we ignore the miniscule (dr)^2.
So the increase in surface area is 8pi.r.dr. The work done agaisnt the surface tension will then be (8pi.r.dr).T
Now the surface tension pulled the droplet into a minimum area sphere, and was only stopped from crushing it to nothing by the internal pressure (p) that results from the surface tension . This pressure acts over the entire surface area of 4pi.r^2 to give a total outward force 4pi.R^2.p, and in moving the surface by (dr) the internal pressure would do work 4pi.r^2.p.dr
So we equate the two measures of the (virtual) work done; 4pi.r^2.p.dr = 8pi.r.dr.T
Some third grade algebra gives:- r.p = 2.T or: p =2T/r
So this holds true for a water droplet, or for an air bubble (or water vapor bubble) inside a water that may be in the process of boiling. The water vapor pressure would have to be equal to the ambient pressure in the water (in order to boil) PLUS the excess pressure of 2T/r.
So we have the odd problem that the vapor bubble has to start off at zero radius; but that requires an infinite excess internal pressure; so the Temperature must rise above the true boiling point, trying to generate enougfh vapor pressure for a bubble to exist.
Well dust and microbes, and even cosmetic rays can come along and form nucleation sited having a non zero radius that allows the vapor bubble to form at less than infinite temperature. Well the lesson from this; is that you DO NOT want to NUKE a cup of very clean water in your microwave. With the very uniform heating and no nucleation sites; the water will superheat; and if you bring it out and a dust particle falls into it; the whole damn thing can explode in your face.
So put the coffe grinds in before you nuke the water or else. That’s why the kettle will pop and bump on the stove as it tries to boil internal to the liquid; and having to generate an overpressure to sustain bubbles.
Now I haven’t quite figured out the way in which this surface tension excess pressure mechanism affects the formation of rain drops; but I’m sure that it does; and of course The Cosmic ray theory of cloud formation is related to this. As is the role of aerosols in the formation of clouds.
And I know that it works to get fly lines to shed water; and maybe ice too. Some fly fisherfolk I know are actually dumb enough to fish in icing conditions, where their guides get filled up with ice, and the ice sticks to the fly line.
An interesting anecdote; I happened to be present at the Fishing Tackle trade show held in Denver; a couple of years back, when 3-M, Scientific Anglers introduced their “Sharkskin” line, and had a big booth demonstration of the thing, and they were handing out samples of the material, that went from smooth to rough so you could feel the surface.
I happened to walk up to their booth just as one of the technical experts from their biggest fly line competitor was walking away. The competitor (another very fine fly line manufacturer) was giving a private seminar to all of its reps and sales people lauding the virtues of their newest super slick and smooth fly line technology, that they were introducing at the same show; and evidently the chap had been at the 3-M booth telling them they were all wet (pun intended). So they handed me the Sharkskin sample; and a very famous fishing figure and fly casting expert; who however has only a high school education; asked me; say George, how does this thing work ? Well I could feel the rough surface but I hadn’t even looked at the photomicrographs of the actual surface showing the diamond embossing; that they had on a computer in another part of the booth. So I took over the flip pad and marker pen, and launched into basically the above explanation of why this should work, according to the laws of Physics. Well it was faster to do on the flip chart than to write the above.
The fly guru said wow; even I can understand that. Then the guy next to him, with a 3-M badge piped up. He said I just spent an hour here trying to explain this to the guy from (XYZ-company) who just left here. Why don’t you stay here and explain it, because I like your explanation a lot better than mine. Turns out he was the chap who invented the stuff. His explanation involved all sorts of contact angles and other complexities (all of which was correct).
The lab R&D people at 3-M incude a lot of very high power researchers; they truly are a high tech company. I always pull the leg of the fly line people; by asking them how they ever got permission to put out a 3-M product that doesn’t have a sticky gum on the back of it.
Applying this to the de-icing problem is quite interesting, and I will be fascinated to see what comes of it.
But Dave’s caution may be appropriate; it might be a more theoretical solution that is hard to implement in practice. It would be nice thoguh because the leading edge rubber boot de-icer is a bit of a complex and somewhat Rube-Goldberg solution.
THE FRAUENHOFER INSTITUTE IN GERMANY DEVELOPS ICE FREE WINDSHIELDS FOR CARS
http://www.autoevolution.com/news/volkswagen-preparing-ice-free-windshields-26128.html
I wonder if they looked at the skin friction with these materials, when used as aircraft skin. at 600mph, any increase in friction coefficient will stop this dead in it’s tracks.
“Well dust and microbes, and even cosmetic rays can come along…”
What do Mary Kay and Maybelline have to do with this? 😉
Looks to me like a good coat of wax would totally bollix up any of those nano structured surfaces.
From my own personal experience WRT ice, a smoothly rough ice surface is far easier to slip and fall on than a perfectly flat ice surface. When formed under the right conditions, ice can have an evenly bumpy surface that’s all smooth. Walking on it is like trying to walk on a concrete floor covered with oiled bearing balls. Without active surface deformation traction aids (metal spikes) the smooth ice lumps allow your shoes to easily slide in any direction.
In contrast, a dead flat ice surface provides maximum contact area with shoe soles, higher than any typical dry and solid surface. (A dry concrete surface that’s not polished has billions of little pits and lumps in the area covered by an average shoe sole. The roughness and lack of lubrication provides the traction that keeps your feet under you.) Of course pressure melting will still make the flat ice slick but with higher sticktion than the smooth/bumpy ice.
Now… take the smooth/bumpy ice texture down to nano scale so the lumps and the spaces between them meet the criteria for all that math posted higher up. 🙂 My bet is water will shed off because it won’t be able to fully wet the surface yet where the entire surface is smooth with no sharp edges, there won’t be anywhere for really fine dust to catch and slapping on a coat of wax or Rain-X (followed by appropriate buffing off of excess) shouldn’t significantly degrade the water shedding, if it does at all.
Mail the big $ check to…
Greg E.;
Ever heard of “curling”, the sport? Getting just the right “pebble” on the ice is key to proper slide and curl of the rocks. YCLIU
George E.;
About 3-M – makers of Scotch brand. I claim copyright on the following advert slogan which I’m going to license to them: “Tape it and stick it!”
You read it here first. 😉