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Okay, so as already indicated my name is Mehmet Baikara I'm an associate professor of mechanical engineering here in California to see Merced. Today I will be talking about new avenues in structural superlubricity you can also call this new new questions in structural superlubricity. Specifically we will be talking about contact aging and friction switches. And this is work that has been performed by my PhD student way who's a who's an NSF fellow working on this project with me. And we thank the organizers area and Andrea and Quan Chi Zheng for giving us a chance to present our work here. I really wish I could be in three years for this with two kids at home and and and call it it was unfortunately not possible but I really enjoyed my last time there in person so hopefully next time I will be able to come in person as well. And maybe if you are able to turn your video on we can have your smiling face. I tried but it was not allowed either and I didn't want to take too much of your time so. Yeah, it says host has stopped my video so I cannot turn it on. Go ahead then sorry about that. That's okay. So yeah, I'll just, I'll just go on and start my talk then thank you very much and yeah. Today we will be talking about the concept of structural superlubricity. Usually my first light is about the importance of friction but I guess for this audience this is this is not relevant so we'll just start with the structural superlubricity business. So, if you take a look at this or think about this from a very macroscopic point of view, and what it involves is essentially two surfaces that are structurally commensurate. A good way to visualize this can be found on Wikipedia. Let's imagine you have these two egg cartons or egg crates if you will on top of each other. If you place them such that the hills of one crate fit exactly into the wells of the other. You have what they call interlocking. So the two objects are interlocked to each other and if you try to slide one with respect to each other you will face a lot of resistance. So yeah, that's that's probably very easy to imagine. Now take one of these crates specifically the one on the top let's call that the slider, and then let's rotate it a little bit. When we do this little bit of rotation well the heels of the first crate cannot fit anymore into the wells of the of the second crate, and you have what we call a structurally incommensurate alignment. In this case, the slider essentially floats on the substrate, and it is very easy to to slide it laterally, and I think I was just able to turn my on my video so that's good. So okay, this is the macroscopic sort of demonstration or maybe a visualization of what we mean by structural superlubricity. Now let's go on to a much smaller length scale, let's go to the length scale of atoms and see how it applies there. So when we talk about structural superlubricity on a on a very small length scale, and what we can do is the following thought experiments as as proposed by her church Hermione Schwartz here in their 2008 review. So let's imagine we have a one dimensional substrate so to make things easy let's do this a one D linear substrate. Let's make it crystalline such that we have a fixed distance between the atoms of the of the substrate. And then let's imagine we have the smallest slider we can think of on the substrate so the smallest thing we can think of if you're an engineer or or physicist is usually an atom. So let's put an atom on this on the substrate. So if this was a microscopic scenario, what happened would be what would happen is that this this ball would try to fall into a well to minimize its gravitational potential energy. Well, we're in a very small length scale so gravitation doesn't matter to these items. But what matters is that they have interactions between them so what we see here is a very regular interaction potential that this slider at some experiences on this on the substrate. So what it does is it falls into a potential energy well. And then in order to move this slider on the substrate to move it, let's say one spot to the right to the next while we have to overcome this energy barrier here. That's what that's what causes resistance to motion, which manifests as friction. Now, if we go ahead and make the slider larger, instead of one atom, let's have two items, but also impose the condition that the distance between these items is a fixed one. So crystalline slider that is different from the distance between the items of the substrate we have again structural incommensurability. We cannot have the two items of the slider in the wells so what they have to do is they have to rise up in the potential energy landscape. So although the number of atoms sliding has increased, the energy barrier per atom has decreased. And this is the whole idea behind structural superlubricity as you have an increase in contact area, potential energy barrier per atom decreases. So you end up with a friction force that doesn't scale linearly with number of atoms or contact area, but sublinearly. And this power factor that defines the sublinear relationship is between zero and 0.5. You can read papers by by us with the wine by under a suremise and where they nicely describe this is told depends on the on the shape and the relative orientation of the slider with respect to the substrate. So that's the whole idea behind structural superlubricity. As you get larger and larger contacts, you get a very modest increase in friction, which results in nearly frictionless sliding. Well this sounds very good and it's all it's all fantastic you have low friction that's all what we all want in many of the applications, but it's also when you read about it a bit unrealistic, because there are multiple conditions that need to be met for this for this situation to occur. You need essentially atomically flat surfaces before both the slider and the substrate, because edge effects tend to destroy this relationship. And then you need to have a molecularly clean interface, because if you have mobile molecules at the interface as nicely described by Martin Muser in one of his papers, or maybe a couple of his papers, what these molecules do is they wonder around the interface find themselves nice potential minima to sit in and essentially lock the two surfaces together, such that this state of structural incommensurability breaks down. So therefore it's rather unrealistic when you first think about it well this is a nice theoretical exercise but how do we make this work. A lot of this work experimental work targeting structural superlubricity has been performed under three conditions, but you may remember this paper that we published in our group a few years ago, where we have shown that it is indeed possible to achieve quantitatively verified structural superlubricity by for friction force versus contact area spectroscopy essentially if you utilize a material system that consists of gold nano islands on graphite. By thermal deposition you push them around with a with the tip of your atomic force microscope, you end up with very low friction forces, essentially for more than 30 islands here you can see results summarized with friction forces below one nano Newton. When you look at the contact areas that are being measured here, this is indeed a very low value for comparison one can take a look at antimony island measurements on graphite performed nicely by Andre Schirmeisen group. And they saw friction forces for antimony islands that are at least or not at least but about two orders of magnitude larger than this. So we have ultra low friction for this material system under ambient conditions, which is rather nice because you can think about all sorts of applications potentially for small scale mechanical systems that nearly that slide nearly frictionless. However, there are a lot of remaining questions to realize these sort of holy grail goals. How does structural superlubricity behave with respect to sliding speed with respect to changes in temperature. Is there a contact size limit for this there are theoretical predictions that there are. And another question is, how does structural superlubricity stand the test of time so to say, or how, how do we have contact aging essentially for structurally superlubric contacts. So when you think about contact aging one of the first papers that come to mind is is this one by Rob Carpick's group in nature from 2011. Essentially what they have shown us using a silicon oxide tip and a silicon oxide wafer. They're doing slides stop slide experiments. Eventually they wait certain amounts of time before they initiate sliding between these two objects. And what they find is that the static friction friction value that needs to be overcome to bring this material system to motion is linearly or sorry logarithmically increasing with time, as you can see right here, the more time the slider spends on the substrate, the higher the static friction goes. Well in this paper this was explained through a formation of chemical bonds over time. So it's structurally superlubric contact. It's an interesting question whether a similar effect would be observed. And this is sort of what we tried to address here. So what we do is essentially again the same material system gold islands on graphite. Most of the gold islands are accumulated on graphite step edges as expected as you can see here, but we occasionally have nice ones that are on terraces like this. This is the tip on top manipulation method. So what we do is we go and image our material system, a particular region of the surface in tapping mode AFM in a light fashion, in order not to initiate any sort of motion. We locate an island we are interested in we land on the island with our tip and do contact mode scanning essentially in an area that is obviously smaller than the upper surface of the island. So we do this because of the superlubric character of the material system, instead of the tip sliding on the island, what ends up happening is that the tip drags the island, as it goes to the right and left, as it's doing these scans, and the measured friction forces are essentially the friction forces that are at the interface between the island and the graphite, and you get fridge, sorry topography maps like this that look rather flat. If you look here it's rather tight, a color bar, so you get flat topography maps that prove that the tip is not moving up or down during this motion, and it's just dragging the island laterally. Okay, so performing these tip on top measurements on our gold graphite material system, what we observe is essentially three different effects that are interesting. So the first one is called the rejuvenation effect, we very often observe that the friction value measured for the first manipulation we perform on a particular island is significantly higher than subsequent manipulations. So we go ahead and do a scan slide the island left and right left and right and stop, and then do it again, we usually do this five times for a given island, and the first value the first friction value which we infer from friction force maps that are rather homogeneous like is typically much higher than the other ones. So the second effect that we see is similar in character but a little bit different. This is called the aging effect or we just coined this the aging effect. What we observe is the following so you have your first island, I mean the only island you're looking at in this experiment, you're sliding it, you're doing five different experiments, then you go and have an extended lunch for four hours, come back to the lab, measure the same island again. And now you again see that the initial friction value that you measure is much higher than the subsequent ones. So this, this value that we had is actually back in the first experiment is back in the second experiment after waiting for a certain amount of time. So we call this the aging effect. And then finally something that is a little bit different is the switching or the friction switches that we occasionally or sometimes pretty often see on some of the islands. What we see is essentially during these manipulations that we perform on a single island. We observe a switch between high and friction low brand high and low friction branches. So as you can see right here in this one scan, this is a friction map belonging to a single nano nano island that we have. We have a high friction region this is that bright part, and then at some point is a top down scan so at some point there's an abrupt change. Something happens and then we switch to a low friction region. We don't only have high to low switches we also have low to high switches, depending on the experiment. So again, we have this switching happening quite often, depending on the type of island, we look at, and we often observe a combination of all these three trends. You can have the, this rejuvenation effect, you can have the aging effect. This is the same island on the left and right but measured in a with 18 hours apart between the measurements, and then you can have the switches in this case from low to high friction branches. So we can see a combination of all three trends. Obviously experimentally these are all very interesting and the fact we can observe them is due, due to the, due to the reason that they're doing the tip on top manipulations where we have control over the speed over the distance over the direction in which we push the islands, which we didn't have in that 2016 paper I showed earlier where we were just pushing the islands from the side, recording some friction values and they at some point they just flew away and they could never find them again. So with this method we could do the, we can do the experiments much more controllably and have much more data. Okay, so how much time do I have left. How much for minutes. Okay, good, good, so this should be enough. So these, these experiments that I showed you were performed on a sample set that we coined big islands because the average island size here, I mean, what we mean here is the contact size between the islands and the substrate was about 34,000 nanometer square. In order to probe if we have potential size effects we repeated the experiments on three different sets of samples that are smaller. So we had extremely small, small and a medium range islands we control the size by changing the parameters of our goal deposition on graphite. And interestingly, in all of these three samples we rarely observed any of these three effects. I'm going to summarize this more in a coming slide. But before I do that, let me also say we start observing some sort of dynamic contamination layer on these samples because remember these experiments are done under ambient conditions. Typically after a month or a month and a half. So this is a phase image that you see on the left hand side and the dark regions you see here are a dynamic contamination layer, the height is only a few angstroms. So these are probably molecules lying down on the surface in a monolayer fashion. So the experiments that we do are partially done on fresh samples where we don't observe this contamination right after synthesis. And then some of the experiments we do are after several months. After the islands are synthesized so those are contaminate those are islands that have been investigated under contaminated conditions, and we see a distinct difference between the two types. I know this is a complicated table, but let me just tell you let me just quickly go over what we see here and I will have one more slide after this. So essentially what we have is a summary of all the experiments we performed over the past couple of years. On the on the rose you see fresh islands and contaminated islands. And on the columns you see the four different groups of islands we discussed in terms of size smallest small medium range and big. And as you can see, once we perform experiments on the big islands, when they're in a fresh state, we see all three effects quite frequently, and our definition of frequency is right here on the lower hand side so more than 75% of the time. When these islands become contaminated, we see the three effects, but less frequently, as you can see over here. Perhaps even more interesting is when we perform experiments on the smaller islands, we rarely observe any of these effects on don't observe them at all. So, and the numbers are right here, what's missing here, the missing piece of the puzzle here as you can see is experiments on smaller islands that are performed when they are in a fresh state, and this is one of our goals for the next for the next year. So a couple of questions, why are these contact agent effects not observed with the small islands, and why do these spontaneous switches between the friction branches occur, and why do we not see them for the smaller islands. There are possible explanations we think about inspired from the literature, there could be a dislocation mechanism that's that's involved maybe some of the smaller islands don't have this mechanism and the larger ones do. Maybe there's a lubricant layer at the interface water has been suggested by share my sense group before. Maybe we have other surface contamination effects, we actually observe surface contamination so maybe that has an effect in terms of size and and what friction values we measure in order to answer these questions we are collaborating with the group of Martin Muser from the University of Ireland who I believe gave a talk earlier today. And yeah, we're in the process of trying to understand what's happening in these experiments in terms of a theoretical explanation. And the remaining test for the experiments I have already, I have already gone over. Well this brings me to the end of my talk. As you can see from my video I'm in a very not well lit location I'm in the garage of my house and the reason is. In a second we have. Yeah, these two guys screaming in the house right now so yeah. I have been it's Memorial Day here so Voltaire is closed and I've been banished to the garage. But yeah, still, yeah, was able to give the talk which is good. So we have our lab website right here it's frequently updated if you're interested in what we do. We just, we don't only do nano tribology work we also do other FM based experiments so please take a look there. We have our Twitter accounts also if you're on social media feel free to follow us. And this is for the lab and this is for this is for Ray my student or NSF fellow who did all the measurements here. With this I would like to thank you for your attention I apologize if I went over time, and I would be happy to answer questions thank you. We do have a couple minutes for questions, and they can come from the room they can come from online. Over here. Very interesting talk. When you mentioned surface contaminants, have you have you seen any. I mean have you taken any FM imaging after after doing your friction force measurements. Yeah, yeah, so the, the typical image we see on a, this is rather large scale, but the typical image we see on a on a contaminated sample looks like this. We're not showing it here but what's interesting is when you go ahead and do a manipulation of these these are actually islands so like this little thing over here and that that guy, these are gold islands. We actually went ahead and did manipulations on these deliberately. And when you bring the tip over there and move the island the contamination layer changes it's dynamic so the shapes that you see right now after a manipulation will change. So, there's certainly something going on in terms of interactions between the islands, between the tip moving the islands around and the contamination we see on the surface. We're thinking maybe even some of the, some of these switches or aging effects we see have to do with contaminant molecules entering and exiting the interface between the gold and graphite but we're, we're, we need theoretical support for for that idea. So we have time for one more focused question. There is something in the chat. No, it's not a question. Very quick one for the switches between the branches. Do you know what's underneath the island when these switches happen, because you're dragging it around but I guess like if you know the topology below that might be. Yeah, this is the classic problem of blurry interface in surface science. In these FM experiments we have no way of seeing what's under the islands at what's at the interface essentially as we are moving them around. Whatever we can infer is from these force measurements, but trying to address your point we did a few measurements that involved topographically investigating the locations where the islands were before they were manipulated. So we had an island, we moved it, and then we took a look at the location where it was sitting before when we did that we didn't see any, any indication of something happening on the surface so nothing that looks different from the surroundings in terms of the graphite surface that that investigation was essentially inconclusive. But yeah, I wish we could see what was happening at the interface but perhaps molecular dynamic simulations can help us understand what's going on there. Okay, with that let's thank the speaker again. Let's move on to our last talk of the session, Alisa Riedo, hopefully is online.