SPLAT! Everyone knows the sound of a blob of mayonnaise hitting a clean kitchen floor. But why does it make such a disgusting splodge on your tiles? And why, as soon as you try to wipe it up with a cloth, does it smear out, instead of being easily absorbed? What’s wrong with the stuff?
Of course, mayonnaise isn’t the only liquid that has the knack of making a mess. Almost all of them do – even water. It might seem pretty tame when it forms a neat puddle on the floor, but water makes a mess of glass. Rain not only obscures your view as droplets run down your living room window, it also leaves a trail of grime.
But a determined band of researchers spend their time trying to get the better of splats and splodges. Their vision is surprisingly grand – to create a world where fluids collect into neat little droplets or bounce free of surfaces like balls off a tennis court. In other words, a world without mess.
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The rewards for cracking the splatter problem are huge. For window manufacturers, creating an unsplattable surface is a first step to marketing windows that never need cleaning. So it is no surprise that glass manufacturers employ the most cutting-edge splatter science. In 1998, the world’s largest glass producer, Saint-Gobain in Paris, approached materials researcher David Quéré at France’s national research organisation, the CNRS, with one simple question: could he create a splatter-proof window?
Quéré knew the main force that counteracts a fluid’s intrinsic ability to spread out and make a mess is its surface tension – the net result of its attraction towards itself and its repulsion from other substances. Surface tension can work wonders. It allows skating insects to skitter over the surface of a pond. And it is responsible for the distinctive shape of droplets. Molecules in a liquid tend to be more attracted to others in the liquid than to air molecules, and a spherical shape allows as few as possible to be next to the air.
The strength of surface tension varies from fluid to fluid. Most liquids have a surface tension in air of around 25 millinewtons per metre, while that of water is about 72, almost three times as much. That’s partly why mayonnaise makes more of a splat. But surface tension also depends on how strongly a fluid is attracted to the substances it is touching. Air doesn’t attract water much, but a material like glass attracts it very strongly.
One way to think about surface tension is in terms of the contact angle. This is the angle between the droplet’s edge where it meets the surface on which it sits and the surface (see Diagram). If this angle is small, then the droplets are very flat and the fluid tends to spread all over the surface. If it’s more than 90 degrees, then the droplets are globular. And that’s the best way to prevent them from wetting, streaking or spotting the surface.
One way to keep droplets spherical, Quéré reasoned, would be to place them on a surface that includes a lot of air. Then there is little attraction to disturb the droplets’ globular shape. His idea was to create “fakir droplets” – beads of water that would sit undisturbed on a pinprick surface, like a Hindu fakir lying on a bed of nails without damaging his skin. By leaving microscopic air spaces on a surface to cushion the droplets, it might be possible to create droplet contact angles of almost 180 degrees. Under those conditions, the droplets would stay spherical and simply roll away or bounce off the surface without smearing out.
Quéré had uncovered a secret that nature already knows. Feathers are made of filaments that trap air, forcing water droplets to bead and roll off. The same trick keeps ducks buoyant and stops them getting soaked when they dive. And in plants, many leaves are covered not only with water-repellent waxy compounds but also with microscopic bumps and needles that trap air between the leaf surface and water droplets, ensuring the droplets roll off before they can become breeding ponds for fungi or plant-eating microbes. Even aphids are in on the secret. Their anal glands secrete waxy needles that coat the surface of their faeces. Instead of splatting onto the nest floor, the excrement is deposited in neat, liquid-tight packets that the aphids simply roll out of the door.
Forest of spikes
Using soft lithography, a high-resolution patterning technique that allows materials to be etched with detailed patterns, researchers at Saint-Gobain have made glass surfaces covered with a forest of microscopic spikes. They are 100 nanometres high, just a few nanometres wide, flat on their tops, and no more than 200 nanometres apart. This last dimension is crucial. Being less than the wavelength of visible light means the glass texture can’t be resolved by the human eye, so it looks smooth instead of flecked. The spikes are coated with a fluorinated carbon compound that is hydrophobic, adding to the water-repellent effect. “There is no adhesive bonding,” says Hervé Arribart, Saint-Gobain’s scientific director.
But texturing glass also makes it more brittle, a problem that Saint-Gobain is still struggling with. In the meantime, Pittsburgh Plate Glass (PPG) in Pennsylvania has beaten Saint-Gobain to market with its own splatter-proof glass. Back in 2000, PPG researchers reasoned that there might be another, completely counterintuitive way to prevent water from splattering. When a drop of water hits a surface, it shatters outward, but its high surface tension helps to create smaller droplets. Each droplet snaps together with considerable force and balls up into a roughly spherical shape. Each can then become a vehicle for more “mess” when it rolls down a windowpane.
So PPG researchers wondered what would happen if they were able to break the structural integrity of water droplets, making them spread out all over the surface. Rather than increasing surface tension to prevent splatter, they would decrease it to the point where splatter would disappear.
To do that, they coated glass with a film of titanium dioxide less than 50 nanometres thick. When ultraviolet light hits titanium dioxide, it knocks an electron out of the metal, leaving behind a “hole”. The electron-hole pair generates a small electric field that ionises hydrogen and oxygen in the moisture in the air to form negatively charged hydroxyl groups. Because water molecules have an uneven distribution of charge, they are attracted by the negatively charged hydroxyl ions and gravitate toward them like iron filings to a magnet. In fact, hydroxyl groups are so attractive to water that water droplets landing on them collapse to a contact angle of about 5 degrees – so flat that the droplets merge.
The effect is stunning. After hitting the material, instead of splattering into droplets, water slaps on and creates a sheet. If the material is vertical, the outer layers of water simply run off in sheets. “The effect is quite dramatic,” says Caroline Harris, a senior researcher on the project at PPG.
Those electron-hole pairs have another plus: they oxidise any organic matter that comes into contact with them. So bird droppings, dead insects and other types of dirt break down and much of it is carried away as rain flows off the surface. PPG markets the self-cleaning glass as SunClean glass for windows, because it is ultraviolet light from the sun that triggers the cleaning effect. The company is interested in further developing the coating to create spectacles that do not need polishing and ships’ hulls that stick so well to the water that barnacles can’t get a grip.
Changing the properties of surfaces is only one way to control splatter. Another is to change the fluid itself – where that’s an option. This was the challenge faced by Quéré’s colleague Vance Bergeron. Working with the global chemical firm Rhodia, based in Paris, he needed to figure out how to make herbicides and insecticides stay where they were sprayed. The same bumps and waxy water repellents that ensure that moisture does not cling to the surface of leaves also repel agricultural chemicals. So when chemicals are sprayed on crops, as much as 80 per cent ends up on the ground, polluting soil, or blowing onto nearby fields or harming wildlife.
Bergeron’s first thought was to make the chemicals thicker, so that, like mayonnaise, they would splatter onto the plants and stay there. “Spraying the liquids would be sort of like slapping dough onto a surface,” he says. The droplets would not rebound like water because the polymers would be inelastic. They would stay where they were put. “The polymers would dissipate all the rebound energy and the liquid would just lie there,” he says. It didn’t work. “It wasn’t a realistic solution,” Bergeron says, “because we couldn’t pump it and we couldn’t spray it. You can’t spray things that are the consistency of honey or dough.”
That’s what led him back to surface tension. One way to lower a fluid’s surface tension so it will spread out on a leaf is to add a surfactant like washing-up liquid that makes it more slippery. But surfactants proved to be another dead end. “That created a different problem when we sprayed,” says Bergeron, “For the same reason that surfactants lower the surface tension, they create very small droplets that tended to float away on the air.”
Then Louis Vovelle, Rhodia’s vice-president of research and development, hit on the idea of using a different kind of thickener. The original thickeners were stiff polymers that stuck to the plants’ leaves as well as to each other. But some sticky molecules can still be flexible enough to slide past each other, meaning the surface tension should be low enough to be sprayed.
A quick test proved the concept, but the polymer they used was not environmentally friendly. “We needed a polymer that Rhodia sold and also was bio-friendly,” Bergeron recalls. “We were very lucky because the company is a leader in guar polymers.” Guar polymers are stiff, long-chain molecules extracted from the guar plant. They’re harmless and commonly used to thicken yogurt and other foods, without making those things as sticky as dough or molasses.
“The result,” says Bergeron, “is as if we were able to gently lay the droplet in place.” Instead of 80 per cent of the compound now dripping onto the ground or blowing onto neighbouring land, as much as 70 per cent stays on the leaf. Rhodia began selling the guar polymer additives to agrochemical makers around the world in 2001.
It’s early days, but manipulating surface tension is already leading to greener agrochemicals and cleaner office blocks. It is, perhaps, only a matter of time before you’re dolloping self-spreading mayonnaise on your self-dressing lettuce. And it’s all thanks to the science of splats.