 Okay, hello everybody I'm glad to be here in particular in person and I'd like to thank you that to change the Organizers especially are you forgiving the opportunity to be here? Okay, I would talk to you about trajectory and isotropy observed in the experiment of AFM team over a perfect crystalline Surface it was an interesting observation in laboratory and then I Did some simulation to to to help to explain what's happening there Perhaps it's too late To go to some Introducted things like this in the last day of this four-day Conference, but let me mention that We deal with friction in every day life in some cases we need to Increase friction in some cases we need to decrease or remove friction completely In other words, we would like to control Friction and to control the friction we need to have a better understanding of friction and better understanding perhaps means Understanding the friction in the most fundamental lengths of scale that is in atomic scale So there is some hope to scale up what we have in atomic scale for friction superlipid city We are and tribology in our all to macroscopic scales okay, and There is a very important Number actual here almost one set of the energy in a vehicle is wasted as heat just because of Not good control on the friction right now, so it's very important to Have a Complete control and friction if it is possible anyways a friction has A new actuality family As soon as atomic force microscopy was invented in 1986 then We understood friction also at atomic scale you have Heard so many things about atomic friction and almost everybody here. I think is familiar with atomic first microscopy and For friction you need to measure at the lateral force in atomic force microscopy So we have a nanotribulogy Just next to the conventional tribology in macroscopic scale I would like to Mention that there are so many similarities not similarities Analogy actually between macroscopic and microscopic Friction for example when you draw a line with a pen on a piece of paper or With a piece of chalk here on the blackboard It's very Similar to what happens when the tip of an atomic force microscope is a sliding our an atomic surface It's almost clear when you push a box on the table There is some analogy in atomic scale. You can push Absorbates on atomic surfaces by atomic force Microscopy tip and so many other analogies can be fine But it does not mean that Microscopic and microscopic frictions are the same. There are some Appearance similarities, but they are actually Completely different things I I would say for example You have here During the last year and last days here that the friction in Microscopic scale depends on velocity, but in macroscopic scale for a rather large wide of wide range of velocities The velocity does not affect the friction. So I mean microscopic and Microscopic friction are not actually the same thing But anyways In both cases or when one slider is giving or is moving On the surface there is some opposite force which is called friction in both cases What I'm focusing in this talk is the Dependence of the slide of the friction on the slide in direction and if I am fast enough I would also mention something else which Actually extends an empirical law in microscopic to microscopic part. I'm not sure that I will be fast enough to go to that anyways the main topic of This talk is related to an experimental observation Which was reported by Fessler Fessler was a PhD student in Basel when one when I was also a PhD student there and He got some very nice experiment a pity silicon tip our AFM and an NACL 001 surface and measure the friction force aligned direction very precisely directions actually and found that there is some Asymmetry actually between Equivalent directions One knows that for example Mowing line this direction should be different from a moving line this direction But a line B and C one with expect perhaps identical forces What Fessler showed was that actually this crystallographic Lee identical Directions may show different frictions and it was very interesting the Data's here are the experimental data that show that With respect to this 45 degrees there is some asymmetry and it was the first questions that we were thinking how to answer it the second question which was also interesting for us was The asymmetry observed in dissipation This is also an experimental result. You can see the friction maps here over the same surface that is NACL surface along one over direction the left panel shows and forward scanning The panel on the right is Friction force aligned the same scanning line, but in backward direction So the only difference between left and right panel is the direction of the motion of the tip So perhaps one expect that in both directions the dissipation should be the same but Gregor showed that actual as is seen in this profile in this 2d profile the black one is for Forward the red one is for the backward and this panel is for line number one The direction of the sliding is important and the dissipation differs in forward and backward and it is not by Accidents there are so many scanning lines that show this asymmetry. It's the second interesting question Okay, and also a third one which is perhaps related to the various two ones Was the asymmetry of tip trajectory? Gregor was able. I don't know how he was able to detect the trajectory of the apex of the Friction force microscopy tip and Detected that once the tip is moving along one scanning line It may happen that it's most aligned strides Line actually from one side to the other side or It can do a zigzag motion But the most interesting thing is that the combination of forward and backward Profiles may be way different allowing different scanning lines. There are five different situations here Line number zero Corresponds to this panel here to this panel a I would say you can see that a line number zero forward and backward motion lead to Strikes motion of the tip apex, but a line number one Align line number one the forward Scanning happens along a straight line, but in the backward motion it goes to a zigzag one and Number two shows a zigzag zigzag motion line number Four shells zigzag is right one and line number five shows zigzag zigzag, but in two different actually Side Directions of the scanning line and it was somehow mysterious is what's happening here and why one should see this asymmetry in the Tip trajectory so We started to think about how to explain it of course PT model is the basic model for friction explanation and You have here so much about it. So I'm not going to go to the details here a single Particle is moving at some constant Velocity our corrugated surface which is presented by a simple sinusoidal Potential and then so many interesting Observations and experiment can be explained by this simple PT model But what about this new observation? Of course, it cannot explain it There is a Number of extensions to this PT model for example one can extend it to two dimensional as is Schematically shown here the one-dimensional PT model can be extended to two-dimension also temperature Has not to be zero one can do the same with finite temperature and These extensions are very successful to explain so many different observation and experiments including double the slips zigzag pass, but however the asymmetric forward and backward Passes cannot be still explained with this Still simple PT model Because there is in principle no symmetric in this Model okay, so we came up with Some proposal the proposal was that we should use an more realistic tip apex in simulation and The tip here is a model tip, which is Not very simple it gets actually We got it from alias Agassemi who did so many calculations and apex of silicon tips to find the most favorable Apex configurations and from Sophisticated computations that you get the global minimum energy Of the potential energy surfaces of these clusters the top must layer is in crystallographic positions, but the Apex is relaxed to get the amorphous structure, which is actually happened Which happens in reality also, so I picked this Nanotip or the n a cell surface and move it along the crystallographic directions and sampled the potential energy or something like 400 data points and Interpolated the potential energy to get DFT actually interpolated potential energy which describes the tip sample interaction So we believe that the asymmetry which is Shown in this apex should explain somehow the asymmetry saw in the experiment Let me Escape some details of the computations only let me mention that this thin Line shows the Simple sinusoidal potential, but our DFT interpolated potential shows some deviation from that as this tiny deviation is enough to Actually describe the asymmetries which is observed in experiments So we extended the PT model to two and also to three dimensions Yeah, and use this Interpolated potential and Let me show you some results perhaps the most important results are not these ones Let me go to yeah, the three questions that we are looking for the answers of them First that the last question was what is the origin of this tip trajectory asymmetry? From this asymmetric potential, which is obtained from asymmetric tip apex We saw this Simulation results and at the bottom you can see that You can always find some trajectory some scanning lines on which the tip Trajectory on forward and backward are not identical. I would say except that the most trivial one That is astride astride in both directions all four others can be reproduced by this kind of simulation So so far we assigned this asymmetry in tip trajectory to the asymmetry of tip apex What about the other two questions? We also Was interested to know what's the mechanism of asymmetric dissipations You can see here in 2d profiles in particular that forward and backward are not identical that First row is experiment where similar results can be obtained from simulations with this new potential So again the dissipation can be perhaps assigned to this asymmetry of the apex Okay, there are some more results here for example if you look at the zoom in Via of this box here. You can see that the tip trajectory is more pronounced here It means that the tip apex Actually spends more time here than here. So the forward direction is not actually where it Symmetric and regular and the situation is different in backwards But the first question was the anisotropy in friction coefficients We actually are not able to capture this today, but There is some asymmetry this small and large spheres show Mirror reflected with respect to this line So there is some kind of anisotropy, but it's much smaller than the experimental observation We assign it perhaps to the asymmetry also in the macroscopic body of the tip which we were not able to Include in our simulation Okay, we are planning to extend it to Some other examples for example a chemist colleague of mine is doing some calculations like this and if we are Successful this way wave with replace the FK model with the PT model But with an DFT potential You know just okay, I would like to act knowledge my collaborators with whom I have Friction full and fully dissipated collaborations with Areas especially here some people in Basel and some people in terror. Okay. Thank you for attention. Thank you very much Questions Thanks, very nice talk. I was wondering maybe Detail that I miss but for the tip on the NACL What is the Rotation of the tip like did you did some calculation with a different orientation? just on a certain design is autopie in the Good question, but I think the answer is somehow trivial Yeah, because the tip a faces Amorph is not crystal and so if you change it you would see different results But I would expect that forward and backward directions would be different anyway And but if it's a morph then like you did some more realization of these amorphous tip to get some Silicon it's it would be almost anyway. Yeah, even in reality Okay, but with metallic tips it would be a crystal line Thanks Thank you for the dog. I was wondering if you could do something inversely like from the like an ST Stick-asleep real experimental image you can reconstruct the shape of the tip Actually, it's way difficult because the tip of structure is not fixed During the experiment and also simulation the tip apex of structure would change during the time and Yeah It's not a good way. I would say that to detect the tip of structure from this kind of simulation All questions Online yes Okay, yes one. Hello. Yeah, so I put my question from the chat So I'm interested how the distance is between the tip and the surface When you perform the big DFT data for fitting potential on Yeah, I Yeah, not to stop sharing but I had some figure sample out of tip sample separations it to start from something like half an anguished wrong which is Completely pushed to something like trying guess wrong. Is it your question or not or? Yes, it is rather whether the interaction so the interaction Potential should depend on the load and this load yes be in Expressed as the distance of the tip to the surface. Yeah, yeah, we did it actually in different tip heights which means different loads and I showed only a typical result for that for a range of Separations we can see such a behavior. Okay nice hearing from you and us More questions Questions, I think we are then we are done. Thank you very much