Super Non-Stick Surfaces
Chris - With us today we've got Ulrich Steiner: he's from the Thin Films and Interfaces group at Cambridge University: sounds like the reverse of widescreen TV, Ulrich! You're actually here to talk about some exciting new materials you're working on - a fancy new form of Teflon® that's ultra slippery...
Ullrich - Well the material itself is actually not so special. It's just Teflon®. What's different is the way we make the coating. Normal Teflon® is a rather smooth coating and we know that materials don't stick to it. If one changes how the coating is put down, it becomes even more slippery - more hydrophobic.
Chris - Why is Teflon® slippery?
Ullrich - Materials don't stick to it and that is to do with the fact that it has very low 'surface energy'. Water and what ever else you put on will stick to other materials that have high surface energy. Teflon® doesn't like to be covered by any other material and that's why it doesn't stick to it.
Chris - One of these questions that go around on email lists is that: if Teflon® is so unsticky, how do you stick it to the pan?
Ullrich - That's a good one. You need to go through a number of different steps. The company in the US (DuPont) that makes Teflon® has a detailed protocol. You first need to make the material rough, so you sandblast it. Then you put a primer down to which the Teflon® will stick. I can't tell you what the primer is because DuPont will not tell me what it is. They have worked out how this goes.
Chris - So it's not just a flippant question, there is actually quite a lot of thought that has to go into solving that problem?
Ullrich - Oh yes absolutely, because Teflon® doesn't stick to things it also doesn't want to stick to the surface you want to put it down on. It's a technological challenge that people have worked out.
Chris - What's the step forward that you're exploring?
Ullrich - This is something that has been known for a long time from plants and animals. There are surfaces that are called superhydrophobic. These are surfaces that water doesn't stick to at all. So if you drop water on these surfaces it forms an almost perfect pearl-like sphere. For example, the leaves of the lotus plant have that property and that's why this effect is also called 'lotus effect'. You can also observe it in the garden. If you go out in the morning you'll see some of the drops are just sliding off the surface of these plants. This comes from the fact that these have a rough surface. They have a certain surface texture.
Chris - So they've got a sort of anatomic Teflon®? So what is the leaf doing, if you zoom in on the surface? What is it doing to repel water in that way and why should a rough surface, paradoxically, repel water like that?
Ullrich - The surface of the leaf has small wax structures, wax needles. The water sits on top of these needles and doesn't really touch the surface of the plant leaf itself. The plants and sometimes even butterflies, for example, have these surfaces to keep clean. It's thought that they do that in order to prevent being infected from spores and so on. So the step forward that we made was to try and copy nature. We thought to make these structures, not from natural materials, but from something that we are more used to and we chose to use Teflon®.
Chris - So how do you apply your stuff? You can apply it to anything, can you, and make it really slippery?
Ullrich - We essentially do exactly the same thing as others do when they coat a Teflon® frying pan. So we follow all the steps that the manufacturer of Teflon® tells us we have to do. There are two little tricks that we play which make the top coating porous. So we add small particles in and these particles burn away during the treatment of the surface: making the surface rough. The second thing is a certain spray technology. We spray it on in a way that makes the top surface rough.
Chris - So can we see it working? I saw you playing around with a spoon earlier so I know you have brought something to show us.
Ullrich - Yes, I have a spoon here but unfortunately you cannot see it through the radio, which is a pity! It's a spoon that I covered and I've dropped some water on it. You can see when I move it around the water just moves with it, it doesn't stick to it at all.
Chris - And if you tip that water off, it'll just fall?
Chris - Oh my god! I'll just describe this to people at home. You know when you tip water off a surface it grips and adopts some of the shape of the surface it's falling from? Well, I've just seen this come off as a perfect droplet from the spoon.
Helen - It's quite beautiful actually. It's like a glass bead, almost just sitting on the spoon, moving independently
Chris - So if you were to dip that in something really sticky, e.g. honey, what would happen?
Ullrich - Well, the honey would also just bead-up: make a droplet. And if you turn the spoon it will also just roll off.
Chris - Like water?
Ullrich - Like water, well, slower than water because honey is slower. But eventually it will completely leave the spoon. Nothing will stay behind on the spoon. Also if you put a powder on it and you then put some water onto it the water will just move the powder/dirt off the surface. I think I just demonstrated that before the show to one of the team.
Helen - Yeah we had chocolate cake! We cleaned the spoon of all the evidence!
Chris - Speaking of which, my wife bakes a cake each week and I was thinking we could have a Cake of the Week as well as Question of the Week. Cake of the Week this week is a rather wonderful fridge cake (chocolate and digestive biscuits). Very agreeable and it's got raisins in it too...just in case anyone's feeling peckish! What would be the applications of this technology, Ulrich?
Ullrich - We are still trying to work this out but evident things are, of course, surfaces that are easy to clean: in the household. It could also be for industrial applications, like machine parts that are not easily accessible. Then you could clean them by just spraying water over it.
Chris - What about clothing? Could you decorate clothing with a similar technique so that furniture, shoes etc need less care and washing?
Ullrich - That's not one of the things we're thinking of because I think fibres could be made more easily self-cleaning. Another thing to develop would be in biomedical technology, surfaces where dirt doesn't stick, are easy to clean and are more hygienic could be useful.
Chris - Does that include bacteria, because you're saying dirt can't stick? I'm thinking if bacteria can't stick this could be good for prostheses.
Ullrich - I know too little about this to say yes or no. I would think for implants it's probably not that good because they have rather complicated surface requirements. I was thinking more in terms of syringe needles and materials that you use in a medical environment and that would be easily cleanable.
Chris - There I was thinking they'd be so slippery they'd just slide in and they wouldn't hurt. But thank you very much: that's Ulrich Steiner, he's from the Thin Film and Interfaces group at Cambridge University with an amazing new application of Teflon® which is so slippery that water just flies off it like a beautiful pearly drop!