 Good afternoon, welcome to another episode of Likeable Science here on ThinkTech Hawaii. I'm your host, Ethan Allen. And Likeable Science is going to take an interesting direction today. We're moving down into the nanoscale all around the very tiny. Those regular viewers of Likeable Science know that I like water a whole bunch. I saw this very interesting work by Dr. Tuxing Long, and he's joining us by Zoom meeting today. Welcome, Tuxing. Hi, Ethan. How are you? I'm good. How are you? I'm very well. Thank you. And Tuxing is an assistant professor of mechanical engineering and biomedical engineering in the Department of Mechanical and Nuclear Engineering all at the Pennsylvania State University. He is a Wernley family early career professor of engineering and is an amazing guy. So a little bit ago, I ran into a news piece in a science news site called Slippery Rough Surfaces. And I was very intrigued by this because it showed these slippery rough surfaces condensing water out of the air. And as my regular viewers will know, I've worked in water issues around on the Pacific Islands. I'm very interested in how people can obtain fresh water in any way possible because these small isolated islands in the Pacific are surrounded by salt water but lack a lot of fresh water. So I exchanged emails and talked to Tuxing and he agreed to come on the show and tell us about his new development. This bio-inspired slippery rough surfaces getting water from air. So tell us how you sort of came up with this idea, Tuxing, if you would. Yeah, so to introduce my laboratory at Penn State, we study biological system. The name of my laboratory is called Laboratory for Nature-inspired Engineering. So we look into the natural world and got inspiration from them. Specifically, my group are interested in studying surface and interface like anything that is on the surface of a plant, insects, or even animals. And we use advanced manufacturing method to replicate the surfaces for new materials and devices and find exciting application for that. And this water harvesting surface that you just mentioned is just one of these examples. We constantly look into nature and one of the previous examples that we did was this picture plan. If you look into one of the figures, as you see just now, that picture plan has evolved from highly slippery surface such that when insects such as ants walk on the surface of the picture plan, it will just slide off from the surface and not digested by the plant. So by studying surfaces like picture plan, we can come up with super slippery surfaces that can be used for a number of different applications. It's really good to be able to make surfaces that have these various varying degrees of roughness, smoothness, hydrophobicity, hydrophilicity, the hating or loving of water and those characteristics really allow us to play a lot of interesting games with surfaces and water, right? Definitely, definitely. Yeah, like for example, just give you an example of a material that we developed in our lab earlier. If we go ahead to look at one of the movie number two and for that you will see that we have created a synthetic slippery surface that can repel all kinds of fluid such as crude oil in the movie number two. So the surface there is what's called slips and it's slipper than porous Teflon you're saying here, right? That's right, that's right. So in this movie, you will see that at the top panel, this is coated with slips which stands for slippery liquid infused porous or typical aluminum. You will see like as sticky as crude oil or even blood job, it doesn't stick on slips but it's staying on everywhere else. This is one of those products that we developed from the inspiration of the picture plan. Yeah, excellent. And we think of Teflon as being a very slippery, very slick surface. That's why it was developed, right? Not having to stick to it. And here you've got materials that are much more slippery than it. So that's pretty amazing. That's right, that's right, yeah. And yeah, it's interesting that by looking at nature, you find nature has typically developed some of these same kinds of materials already for its own use in different organisms, right? Because in the case of the picture plan, it wants the insects to slip down and fall into the digestive liquid inside the plant, right? That's right, that's right, that's how nature does it. And indeed we take the concept and move a step further and creating only can repel insects but also can repel a broad range of liquid as you just saw in the movies. It can repel crude oil, blood, and we even further develop it to material that it can repel ice or even bacteria for anti-biofiling coating. Okay, yeah, that's very valuable to have those multifunctional surfaces then. Definitely, definitely. Yeah, so to get into the issue of water harvesting though, how did this idea come up? Yeah, so the surface, the water harvesting surface we developed recently was also inspired partly by the slippery surface of the picture plan. And on top of that, we also look into different plant species, in this case the rice leaf. If you stick an electron microscope on the rice leaf surface, you will see there's different microscopic and endoscopic groove structures which helps to move water droplets in one direction. So by combining the slippery surface concept from the picture plan and the directional micro and endo groove structures of the rice leaf, we combine them together to create this synthetic surface, what we call the slippery rough surface, which can collect water from air through either a fog harvesting mechanism or surface condensation and then they can transport the water away from the surface very effectively so that we can directly transfer water from air to the surface and then to somewhere that can be collected. Right, because yeah, you want to be able to move that water around rather than have it stay in place where it collects, right? You need to be able to move it so it condenses and gets into bigger and bigger droplets and finally drops off or goes into a container and can gather. That's right, that's right. So that is really like the concept of how this slippery rough surface came up. We were thinking whether we can create a synthetic surface that combines multiple surface property of different plant species to create a multifunctional surface and in this case we put picture plan and the rice leaf together. Yeah, that's very ingenious, the smooth slickness of the picture plant and the larger scale structure of the rice leaf to guide the droplets so that once they form, they slip where you want them to slip, right? That's right, that's right, that's right, yeah. So in creating this, so this is the concept but in actually making them, there's some more details into the design. For example, we know that for different surfaces, for example, surface if it is hydrophilic, water likes to attract on the surface and on the other hand if a surface that is hydrophobic as the naming price, water doesn't like it. So in the parts where we developed slips, it was mostly based on hydrophobic surface chemistry, meaning that like we designed a surface such that it can repel most of the liquid. But now we want to do it in a different way. We actually want water to attract on the surface. So the very first step that we did is to modify the surface chemistry of the original slips. So if you look at movie number four, in one of the movie number four, you will see that just previously, my group has developed this coating. On the one hand you can create hydrophobic slips, but on the other hand you can make a flips hydrophilic as well so that like water in air would like to attract onto the surface of slips. So that is really the difference. So in the video you can see on the top side is the hydrophobic slips where the water just falls up and slide up and on the lower end the water tries to threaten out and then slip away. And based on this surface chemistry, then we saw that even at the micro scale it can attract water more effectively than a hydrophobic surface. As you can see in this microscopic video which is taken by environmental scanning of actual micro scale. And further we need roughless because roughless helps us to attract water from air more effectively because of the larger surface area. And not only that because the groove structure helps to carry water away more effectively through capillary action than just barely flat surface. As you can see earlier in the video that with our micro groove slippery surface it can absorb water from air and transport it effectively as compared to just a flat slippery surface. Right and it's very interesting that both the hydrophobic condition which makes water beat up on top of things and sort of stay away from a surface and the hydrophilic condition where water is actually drawn flat down onto the surface. Both can be used for the same purpose. It's very ingenious that you figured out that you could sort of play this game either way. There may be advantages and disadvantages under certain circumstances to doing one versus the other right? Definitely, definitely. Like in cases you want to repel water you want a more hydrophobic surface but in case you want to attract water you want a surface that is thick and slimy. Okay, yeah. That makes sense. Right. This is intriguing stuff because again you're looking at nature. You're developing surfaces that both the surface structure itself has certain kinds of properties related to water and then you structure the surface so it can actually both have more surface area and also can guide the water going to where you want it. So you're really balancing a lot of different sort of different constraints and different almost levels of control over your materials. This is truly material science at the cutting edge I think. Definitely, there will be one of these materials, how surface chemistry so that it can harvest water at its maximum weight. So there will be a lot of engineering design consideration into making this material. Yes, exactly. And other things, how much materials are you actually using to do this? How available? How expensive are those materials? Will they hold up under outdoor conditions where they may be used? Will they stand up over time? A lot of different factors you have to think about to make this a usable material, right? We demonstrate a concept. We show the proof of concept that slippery rough surface can collect water from air surfaces which I will talk about in a minute. And in terms of the material cost right now, since this is a proof of concept, we use silicon to make this material which is a relatively expensive material at this point to make the surfaces. But our technology can be applied to other low cost material. For example, if you want to fabricate this onto material like aluminum, so it's quite straightforward to translate that concept into these surfaces depending on what application that you're interested in. And if it's used for outdoor application, this concept can also be applied. You can kind of formulate the surface coating such that it can be used for outdoor conditions and things like that. Yeah, this is sort of the amazing juggling act that you're doing all the time is figuring out the different structural levels from the very low, very tiny nano scale up to almost micro scale, the properties of the surface, the materials you're going to use, the processes. Now, it's all amazing. And we're going to look into this in even more depth when we come back. Right now, we're going to take a brief break. I'm talking with Toxin Long from the Pennsylvania State University. And I'm Ethan Allen, your host of Legible Science, and we'll be back in one minute. And the guests that we have are very, very well informed. Just think we have the upcoming negotiation between President Trump and Kim Jong-un, the possibility of Xi Jinping, the leader of China remaining in power forever. We'll see you then. Aloha. I am Howard Wigg. I am the proud host of Cold Green for Think Tech Hawaii. I appear every other Monday at three, and I have really, really exciting guests on the exciting topic of energy efficiency. Hope to see you there. And you're back here with us on Legible Science here on Think Tech Hawaii. I'm your host, Ethan Allen. And with me today joining me via Skype is Dr. Toxin Long from the Pennsylvania State University. And we're talking about bio-inspired slippery rough surfaces and getting water from air. We talked in the first part of the show about how he looked at the world nature and particularly how various plants, the pitcher plants have a very slippery surface that water slides off of, that insects can slip down and get trapped by these pitcher plants. And it was inspired also by the micro structure on the rice leaves that have little fine little grooves actually guide condensed water either into or off of the leaves depending upon the need and how he's combined these into some making of material that condenses water very effectively and sort of runs it off effectively. And now to sort of move ahead on this Toxin, so you begin to have to sort of compare this to among different sort of different combinations of your constraints, right? So take it away here. Tell us how you did this. We've got some movies I know coming up for you. Of course like when we develop this material we need to do a benchmarking. We need to compare with other existing state of the art technology. So with that we take two particular surface for comparison. The first surface is called a superhydrophobic surface and superhydrophobic surface is inspired by a plant species called a lotus leaf. So for those of you who have played around with lotus leaf before if you put water on it, water doesn't stick on it. Indeed, it rose around on the surfaces. And the reason why is that is because on the lotus leaf it has this two levels of micro nanostructures which helps them to have a thin layer of air. It's kind of like a playing air hockey. If you have that air layer everything on top of it is very slippery but without that air layer everything becomes sticky. So that superhydrophobic lotus leaf is working on this mechanism. So we thought this would be the very first example to compare with our slippery rough surface for water harvesting performance. And if you look at movie number seven which we saw a side by side comparison of the water harvesting between the superhydrophobic surface and the slippery rough surface. The left is the superhydrophobic surface and on the right is your slippery rough surface which is gathering bigger drops. They're running off more quickly. You can see it's going to be much more efficient at pulling water, condensing water and turning it into usable water. That's right and even if you look at the microscopic version of the process you will see that for superhydrophobic surface there's a lot of droplets that are sticking at the micro nanostructures. But for the slippery rough surface once droplets of form on the surface they just transport away immediately. So in this example we saw that slippery rough surface performs much better than just the wacky superhydrophobic surface. So that is a good news. And then the second surface that we compared slippery rough surface with a slips as I mentioned earlier was really a product that is inspired by the LaPrentis picture plant. So we compare the water harvesting performance of slips and the slippery rough surface. It is on the left hand side coding if the movies comes up in a second. We're looking at here and I can see on the left side these tiny little drops on your slip surface and on the right again the slippery rough surface is pulling bigger droplets. They're running down, they're condensing together, forming large droplets at the bottom. Again it's going to be a much more usable system for harvesting water. That's exactly right, yeah. Wonderful, amazing to watch that, amazing. Yeah, and now if we put slippery rough surface together with superhydrophobic surface and slips then you can see all three surfaces in the same condition for water harvesting if we go to movie number 10. Yeah, so we got that on now, yeah. Yeah, so you will see that on the very left hand side is the slippery rough surface in the midnight is a slips and on the right hand side, the far way hand side is the superhydrophobic surface. Under the same exact experimental condition where water is suiting onto the older surfaces at the same time and you can see the big differences between the water harvesting on slippery rough surfaces. Yeah, at the left you can see all the big droplets collecting at the bottom really quickly now and all the medium sized droplets running off very quickly into them. It's gathering very efficiently, very neat. Yeah, indeed like if this material is scaled to a large scale we estimate that our material can actually collect about 120 liters per meter square per day. So under the, and like just give an idea, typical for like for example, fog harvesting material that it can collect about one to 20 liters per meter square per day. So that means slippery rough surface actually have at least an older manatee of higher performance than current fog harvesting material. Yeah, that will have tremendous impact in places, mountainous desert regions where they had nice cool nights and even with very small amounts of moisture in the air if it has something to condense against it, it will do it. You've got a system there that will now make it much more efficient. Grab that water, turn into little droplets and move those droplets down into reservoir very quickly. That's exactly right, yeah. Now just I ask you to speculate here for a moment. What about in a warm humid tropical environment on a tropical island where you never really get any fog or anything, but the air is just constantly humid, not very much difference in temperature between day and night. What do you think your surface is going to do? Yeah, so with that right now we are developing surfaces that can condense water in those conditions. The concept of slippery rough surface will still apply in that case, but then we need to cool the surface down to the dew point where the water droplets start to condense on the surfaces. So we will need additional energy input in those scenarios. What we need is actually what my guest last week, Oswath Raman from Sky Cool Systems has developed a photonic radiator system that passively dumps out heat and cools itself so the system actually stays cooler than the air around it even in bright sunlight. So we can combine your system with his, cool yours down and it will condense the water very efficiently. Yep, that will work. Like any system like what you just described combined with the surfaces will work to condense water in a human environment. Yeah, that could be something. We could be onto something here. Definitely. I mean we laugh, but the tropical islands here, particularly the low-lying ones have very, very, very limited freshwater supplies and must await rain to gather rain water to drink. But they're air is human all the time. I keep thinking if there were ways to gather the humidity out of the air very efficiently and effectively as you've apparently developed and gather that then they would not have to wait for rain. Even in drought conditions on these islands, air is humid. They're right by the ocean. There's no place in these islands more than a few hundred yards from the ocean. So there's plenty of moisture in the air. Yeah, that's really the goal. One of our important goals for this research is ultimately develop a system that you can decentralize the water supply regardless where you go. You can get fresh water anywhere from clean air. That is really our ultimate goal. Excellent, excellent. This sounds like something that will be tremendously useful out here. I'd love to stay in touch with you and test this on out and see if we could work a good system out because the people in Marshall Islands and Kiribati and all these places are facing the increased climate variability. They get these longer and longer droughts and they just can't keep up big enough rainwater catchment systems in some cases to hold enough water well enough long enough to survive during droughts and they need a lot of water. But if you're gathering a direct the out of the air all the time that should really be that could be game changing technology for them. I thank you for your work. It's truly amazing. I think we have one more figure to look at, right? Figure four I believe. That's right. One last figure was really summarizing the performance of slippery rough surface versus other state-of-the-art technology. At the right is your slippery rough surface, right? That's right. On the very right hand side, the tallest bar is slippery rough surface. The right hand side, the two bars on the right hand side. And the west is a subitrophobic surface that is on the left hand side and the middle two are the slippery surface. So as you can see quantitatively we measure how much water that can be collected by slippery rough surface. And as I mentioned earlier, slippery rough surface at our left scale it can scale to about 120 liters of water per meter square per day. And that is about an order of magnitude higher than the typical harvesting material that can collect water daily at this point. That's stunning. It's amazing that by manipulating the materials, manipulating the surface, the surface chemistry, the surface morphology, you've managed to get that level of improvement in this particular dimension that you wanted to do with the water harvesting. This is the nature inspiration and nanotechnology work at its best. Exactly. Nature is endlessly inventive and it's very nice to see people like you working so hard to take advantage of nature's inventiveness. Add your own inventiveness on top of it and build more and more useful surfaces, more and more useful products for us that can help people in all kinds of situations. That's about going to wrap us up. Taksing Wong from Pennsylvania State, excuse me. I thank you very much for being here with me on Likeable Science. You've been a great guest. I learned just a huge amount from you and as I'm sure I have our audience, thank you so much. Even thank you for helping me. You're most welcome. I look forward to talking to you again in the future. Sounds good. And for our audience here, I hope you'll come back and join us next week for another episode of Likeable Science here on Think Tech Hawaii.