A VIDEO starring a pair of white sneakers and a bottle of Hershey’s chocolate syrup is not the sort of thing you would necessarily expect to become a YouTube hit. Especially not if it is a local news report about a nanotechnology company.
Yet has generated 4 million hits and counting. Rather than staining, the gloopy chocolate glides off, leaving the shoes as pristine as ever. The secret? A spray that makes the fabric repel almost anything that comes into contact with it.
If the claims made of this and other, similar “omniphobic” materials stand up, it’s not just stained trainers that could be consigned to the dustbin of history. The tent contents soaked through during a wet weekend’s wild camping, the smartphone screen blighted by greasy fingerprints, the ketchup stubbornly stuck in the bottle – all could succumb to omniphobia’s repulsive charms. If the most expansive suggestions are to be believed, the technology might also herald the approach of faster planes, greener ships and safer medical implants. So are the claims solid science – or as slippery as the materials themselves?
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The hunt for “phobic” materials began with those to repel that most ubiquitous of liquid irritants: water. As early as the 6th century AD, the Mayans were daubing their garments with a milky liquid from rubber trees – latex – to make them waterproof, according to John Loadman, . It wasn’t until the 1820s, however, that the Scot Charles Macintosh rediscovered these tricks, painting a fabric sheet with a sticky, noxious liquid of rubber dissolved in naptha to make his eponymous raincoat.
It was a serendipitous discovery over a century later that produced probably the most prevalent hydrophobic stuff around today. In 1938, Roy Plunkett at the DuPont laboratories in Deepwater, New Jersey, was looking for better chlorofluorocarbon refrigerants . Polytetrafluoroethylene is now better known under its brand names Teflon, as a coating for non-stick frying pans, and Gore-Tex, as a breathable, waterproof fabric.
Coming unstuck
Even so, reliably phobic materials remain few and far between. That’s partly because it is not simple to divine exactly what repels a drop of rain when it lands on a jacket, or a soupçon of oil poured into a frying pan. It has to do with the relative strength of the cohesive forces between molecules within the liquid droplets and the forces that make those molecules adhere to the surface they land on. If the adhesive forces are much stronger, a liquid droplet will rapidly lose its shape, spread into a flat puddle with a “contact angle” close to zero and seep into the surface (see diagram). But if cohesive forces dominate, the droplet will bead up, forming a sphere with a large contact angle – in the ideal case, 180 degrees – that easily rolls or can be shaken off. A phobic surface is one that creates contact angles of greater than 90 degrees.
Water’s problem is that it gets over-friendly with most surfaces. Water molecules have an uneven distribution of charge that tends to create adhesive electrostatic forces between them and surfaces that also have an uneven charge distribution, among them glass and fabrics such as wool and cotton. That causes the droplets to readily spread out and, in the case of fabrics, seep in.
Latex repels water because its tangled layers of tightly bonded hydrocarbon chains are difficult for water molecules to penetrate. They also have little free charge floating around. Polytetrafluoroethylene is similar: its central backbone of carbon is surrounded by fluorine atoms, which grip their electrons tighter than any other element does. With little charge left to grab on to, water’s contact angle with Teflon is about 110 degrees.
But such chemical insights can only take you so far. The inspiration for something better came from a slightly unexpected quarter. Plants have many reasons to want to keep water off their leaves – to keep pathogens and dust away, for example, and to ease the exchange of carbon dioxide and oxygen with the air during photosynthesis. In 1997, botanists Christoph Neinhuis and from the University of Bonn in Germany used scanning electron microscopes to find out how the structure of the leaves of 200 plant species affects how they do this ().
One of the most proficient water repellers is the lotus, a symbol of purity in many cultures owing to its ability to emerge from the muddiest of waters unsoiled. Its leaves are covered in a wax that, like rubber, consists of hydrocarbons with little chemical affinity for water. But investigations revealed that wasn’t their only secret. The wax is also made up of millions of tiny bumps. When water droplets hit the leaf, they are stranded atop pockets of air trapped between the bumps. That limits their scope for electrostatic interactions with the surface, and they maintain a near-spherical shape, with a high contact angle of 162 degrees – and simply roll off the leaf.
The lotus leaf was dubbed superhydrophobic, defined as a material that produces a contact angle greater than 150 degrees, or requires a 5 degree tilt or less to get droplets to roll off. “It led to an explosion of studies,” says , a mechanical engineer at the University of Wisconsin-Milwaukee.
The recipe for recreating the lotus effect in the lab turned out to be relatively simple: take a chemically extremely inert material such as polytetrafluoroethylene and roughen its surface by etching it or blasting it with sand. “You want it to be the world’s worst paint job,” says , a mechanical engineer at the Massachusetts Institute of Technology.
“Recreating superhydrophobia in the lab turns out to be relatively simple: you just need the world’s worst paint job”
Dissolute liquids
Thanks to such lotus-inspired innovations we now have super-non-stick frying pans that are not only coated with Teflon, but have microscopically roughened surfaces. There are also hydrophobic and to create water and dirt-repelling surfaces with contact angles of 140 degrees or more.
But this was only ever going to be a first step. “It was a natural progression to go from water to other liquids,” says at the Max Planck Institute for Polymer Research in Mainz, Germany. “From blood to red wine – it’s not enough to just repel water.” Oil is a particular bugbear: not only does grease blight kitchen surfaces and ruin clothes, but where it gets trapped it can attract dirt and sustain colonies of bacteria. Hardest hit are difficult-to-clean surfaces on devices such as medical implants and electronic chips. Oil can also attack materials such as rubber, commonly used for seals in vehicle engines.
But if water is over-friendly as a liquid, oil is positively dissolute. Its molecules tend to be only weakly attracted to each other, so it doesn’t take a large adhesive force to overcome a droplet’s cohesion when it hits a surface. Rather than bead up and fall off, it spreads out and seeps in – as many a soiled party dress can testify.
Overcoming this behaviour required a finer-grained approach. In 2007, Nosonovsky showed that if you got the architecture of a surface’s roughness just right, creating caves, nooks and crannies that bend back in on themselves, you could create repulsive surface forces equating to huge contact angles for all sorts of liquids ().
It was a simple but game-changing insight. “Not all roughness is created equal,” says McKinley. In 2008, he and his team put it into practice, etching mushroom shaped protrusions into silicon dioxide surfaces that they called “microhoodoos”, after similarly shaped hoodoo rock formations. These surfaces could make oil and water ball up into spheres and roll off, with contact angles for both of over 150 degrees. McKinley’s team dubbed them omniphobic ().
Since then, many groups have built on and modified their approach, creating ever more complex repellent topographies. “It is a beautiful demonstration of the behaviour of liquids,” says , a chemist at Harvard University. But there was a problem. The micro- or nanoscale structures are easily damaged as well as being eye-wateringly expensive. For commercially useful products, a different approach was needed.
Here, too, nature supplied the inspiration. It came from a plant with anything but the lotus’s pure image: the carnivorous pitcher plant. These insect-eaters capture their prey when it lands on the lip of a cylindrical trap whose walls are covered in tiny bumps. In the plant’s humid growing conditions, a layer of water seeps in between these bumps, forming a continuous slippery film. Water and oil famously do not mix, and when an insect lands on the lip, oil on its feet is repelled, causing it to hydroplane into the plant’s stomach.
“When an insect lands on the lip of the trap, oil on its feet is repelled and it hydroplanes into the plant’s stomach”
Last year, Aizenberg and her team were able to recreate the pitcher plant effect using a bumpy porous material infused with a commercial lubricant that, like polytetrafluoroethylene, contains lots of antisocial fluorine and carbon atoms disinclined to interact with anything. The resulting slippery liquid-infused porous surfaces, or SLIPs, don’t win any prizes for contact angle. On the measure of tilt angle, however, they are firmly superomniphobic: all substances tested hydroplane off when the surface is tipped by less than 5 degrees (). “It hates everything, or rather everything we want to get rid of hates our surface,” says Aizenberg. So far .
Aizenberg’s team can make even a smooth surface such as a metal sheet omniphobic by coating it in a nanostructured spray followed by a lubricant. SLIPs also self-heal – any scratch or defect in the porous material is almost immediately filled by the lubricant spontaneously wicking into the crack. And unlike lotus-inspired materials, air-filled surfaces can work in temperatures of up to 250 °C, in conditions of driving rain and at pressures equivalent to being submerged 7 kilometres underwater.
Sticking points
Sounds great. But not so fast, warns , a physicist at the École Polytechnique near Paris, France, who developed a similar liquid-infused surface in 2005 (). He points out that the cost issue hasn’t gone away: the sort of fluorinated oils needed to cover omniphobic surfaces cost thousands of dollars per litre. They are also environmentally hazardous if not disposed of correctly.
Vollmer, meanwhile, questions whether these materials are really omniphobic. Although they can repel many substances, they tend to fall down with widely used liquids that have lower cohesivity than oil, among them solvents such as ethanol, acetone or benzene that are commonly found in clean environments such as labs. “Calling something omniphobic comes with such a huge expectation,” she says. “I think it will be the aim, but rarely the reality.”
She prefers to call her own nanotextured materials that repel oil and water superamphiphobic. Created by burning soot particles off a silica surface, they are cheap to produce and readily replicate the nooks and crannies needed to repel both oil and water (). “I like it because all I need to take to conferences is three matches and a glass slide to show people how it works,” she says. But silica is fragile, and although Vollmer is in the process of applying for patents, the materials are unlikely to be commercially available anytime soon.
Aizenberg counters doubts about her own material by saying that they have tested SLIPs with liquids including pentane, a hydrocarbon that has a lower surface tension than any of the solvents Vollmer is worried about. “We know the concept will work but now we need to scale it up,” Aizenberg says. Her team is currently testing the approach on the aluminium sheets used to build aeroplanes, in the hope of producing surfaces that can reduce drag and prevent ice build-up, a common cause of flight delays in cold regions (). Together with McKinley’s group, they are also starting a project with the US Office of Naval Research to develop surfaces that will reduce fuel-consuming friction exacerbated by algae and molluscs sticking onto the outsides of ships and submarines.
Nosonovsky agrees that such applications have great potential, but points out that there are still a few sticking points before they become reality. For a start, they would require the surface to constantly replace its lubricant, because every droplet rolling off the surface takes a portion of the surface with it.
But what about those of us seeking help with life’s littler staining crises? Despite all the promise of omniphobic materials, none has yet made it onto the shelves. Earlier this year, a group led by mechanical engineer at MIT announced they had produced that solves the problem of constant slapping and shaking to free the intransigent liquid, but they are currently seeking commercial partners.
Hence the interest in those chocolate-repelling trainers. The manufacturers of the spray that was star of the YouTube video, Ross Nanotechnology of Lancaster, Pennsylvania, say it will be available in the US by the end of the year, and internationally a little later. Called , it is not being marketed as omniphobic – it cannot repel solvents such as acetone or ethanol, says Andy Jones, president of the company. Although he is reluctant to divulge any trade secrets, he will say that the aerosol is made of nanoparticles that self-assemble when sprayed on to a surface, and gives a contact angle of 165 degrees for water and around 150 degrees for oil.
So that’s one claim at least we should soon be able to test for ourselves. Time will tell whether all the promise of omniphobic and similarly antisocial materials can be made to stick. But with the attractions of a self-cleaning future they offer, it seems unlikely we will let them slip entirely from our grasp.
