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Updated: 16 hours 4 min ago

Re: some additional works

Mon, 2018-04-02 23:12

In reply to some additional works

Dear Yanfei,

Thanks for kind words and sharing the two inspiring papers. Structural lubricity and thermal lubricity are two interesting and important phenomena in friction, which might lead to revolutionary applications if properly controlled.

I fully agree with you that the collective behavior exhibited by the flexibile graphene interface shares the same physical origin as the structural lubricity you mentioned. Interface commensurability is an important variable of the contact state. Recently, we have experimentally demonstrated that it indeeds can be actively tuned by straining the system. We will keep posted for this.

Regarding the thermal effect, most availabe theories adopt the approach of thermal activation. This usually leads to a logarithmic dependence on sliding velocity. However, our recent experiments show an unsual power law dependence for friction on moire superlattice structure. Do you happen to have any thoughts on this abnormal behavior? 

 

Qunyang

Dear Mike,

Mon, 2018-04-02 22:40

In reply to applications

Dear Mike,

Thanks for your kind comments. Here are my personal thoughts to the questions you raised.

1) Energetic argument has been widely used to help describe the atomic-scale friction process [See the book by Mate as Ref-29]. For example, one can use the one-dimensional Prandtl-Tomlinson model (i.e. a particle sliding along a peridoc energy landscape) to predict the atomic stick-slip friction behavior, including the shape and magnitue of the of energy corrugation[36-37], the thermal and velocity dependence[38-42]. In most cases, the energy corrugation was regarded as an effective energy landscape experienced by the sliding tip due to tip-substrate surface interaction. However, in some cases involving 2D materials, we showed that this energy landscape was not only determined by the tip-surface sliding interface alone, but also the deformation configuration inside the bulk materials [e.g. Refs-27, 28].

2) This is a good question. How to use 2D materials in real applications needs to be answered by the society not by myself :)  However, from tribological applications, the frictional properties of these 2D materials are quite attractive. Even though they may exhibit the thickness-dependent friction and monolayer samples are more frictional than thick samples, overall friction of these 2D layer structured materials are very low. For exmaple, covering a SiO2 surface (or Cu surface) with a monolayer graphene, friction can be reduced more than 50 times compared to the bare SiO2 or Cu surfaces [43-44]. More potential applications in tirbology can be found in the review article [4]. But I have to pointed out that, depsite the high potential of these atomically thin lubricants, there are some challenging issues to be addressed before they can be fully implemented.  

 

Refs:

36. Socoliuc, A., Bennewitz, R., Gnecco, E. & Meyer, E. Transition from stick-slip to continuous sliding in atomic friction: Entering a new regime of ultralow friction. Phys. Rev. Lett. 92, 134301 (2004).
37. Perez, D., Dong, Y. L., Martini, A. & Voter, A. F. Rate theory description of atomic stick-slip friction. Phys. Rev. B 81, 245415 (2010).
38. Gnecco, E. et al. Velocity dependence of atomic friction. Phys. Rev. Lett. 84, 1172-1175 (2000).
39. Li, Q. Y., Dong, Y. L., Perez, D., Martini, A. & Carpick, R. W. Speed Dependence of Atomic Stick-Slip Friction in Optimally Matched Experiments and Molecular Dynamics Simulations. Phys. Rev. Lett. 106, 4, (2011).
40. Sang, Y., Dube, M. & Grant, M. Thermal effects on atomic friction. Phys. Rev. Lett. 87, 174301 (2001).
41. Riedo, E., Gnecco, E., Bennewitz, R., Meyer, E. & Brune, H. Interaction potential and hopping dynamics governing sliding friction. Phys. Rev. Lett. 91 084502  (2003).
42. Krylov, S. Y., Jinesh, K. B., Valk, H., Dienwiebel, M. & Frenken, J. W. M. Thermally induced suppression of friction at the atomic scale. Phys. Rev. E 71, 4 (2005).
43. Huang, Y., Yao, Q., Qi. Y., Cheng, Y., Wang, H., Li, Q. and Meng, Y. Wear evolution of monolayer graphene at the macroscale, Carbon, 115, 600 (2017).
44. Qi, Y., Liu, J., Zhang, J., Dong, Y. and Li, Q. Wear resistance limited by step edge failure: The rise and fall of graphene as an atomically-thin lubricating material, ACS Applied Materials and Interfaces, 9(1), 1099-1106 (2017).

 

Qunyang

I'm thinking about applying

Mon, 2018-04-02 20:13

In reply to Three PhD positions and two postdoc positions in computational mechanics of materials

I'm thinking about applying for this opening. Need a few years to learn Italian.

some additional works

Mon, 2018-04-02 19:55

In reply to Journal club for April 2018: Friction of atomically-thin sheets

Qunyang:

Very fantastic works! I always enjoy reading your nice works that combine experiments/modeling/simulation.

I want to add two additional phenomena:

(1) structural lubricity, which looks like your Fig. 6. The collective result of the interface incommensurability leads to a reduced friction force. See Fig. 5 in https://www.sciencedirect.com/science/article/pii/S0022509610001900?via%...

(2) thermal lubricity, which shows reduced friction at elevated temperature due to thermal activation. See results in https://www.sciencedirect.com/science/article/pii/S2352431615000267?via%...

Yanfei

applications

Mon, 2018-04-02 13:54

In reply to Journal club for April 2018: Friction of atomically-thin sheets

Very interesting review, I wasn't aware of some of this work.  I am not particularly surprised that a loosely adhered single layer of material can behave differently than a strongly adhered one. Some random thoughts are

1) you mention a type of energy balance for a tip sliding on a single layer -- has this energetic approach been written precisely, or is it just a qualitative balance?

2) what are the applications you think are worth pursuing?  With such increased friction, this is detrimental to applications? I understand most of these papers explore the "fundamentals", but what about applications?  How to solve high stickiness and high friction if this is required?

Thanks

Mike

Hurry up!

Tue, 2018-03-27 06:35

Fracture energy and mode independent failure mode

Sat, 2018-03-24 08:09

In reply to Bridge

Dear All, 

I have to specify different values of fracture Energy in each mode, for example Mode I, Mode II and Mode III particularly by using the principle of mode indepedent situtation, in the available commercial Finite Element Code ABAQUS. But I noticed that in the available commercial ABAQUS finite element code, where it is possible to define different values of fracture energy only if one adopt  mixed mode condition. It is because, the available interaction criteria such as  power law and BK which are allow to specify individually fracture energy for each mode but in such situations are mode are depedent.  Therfore, is there alternative method to specify the different values of  fracture energy for each mode in ABAQUS for the case mode indepednet condition?  

 

Post-Doctoral Position: National University of Ireland Galway

Thu, 2018-03-22 16:21

In reply to Post-Doctoral Position:Finite element analysis and experimental testing of blood clots and design of thrombectomy devices

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Project title: Finite element analysis and experimental testing of blood clot fracture and design of thrombectomy devices to remove clots from vessels in the brain

Post-Doctoral Position: National University of Ireland Galway

Thu, 2018-03-22 16:20

In reply to Post-Doctoral Position:Finite element analysis and experimental testing of blood clots and design of thrombectomy devices

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<p class="MsoNormal"><strong style="mso-bidi-font-weight: normal;"><span style="text-decoration: underline;">Project title: </span>Finite element analysis and experimental testing of blood clot fracture and design of thrombectomy devices to remove clots from vessels in the brain</strong></p>

Courant–Friedrichs–Lewy | CFL number

Wed, 2018-03-21 21:49

In reply to CFL number

CFL which is also known as Courant–Friedrichs–Lewy is a condition involved in the important role of measuring the stability of unstable computational numerical methods involved in modeling of convection or wave phenomena.  Effectively, the CFL condition helps to clearly express the net distance that any information travels during the timestep length within the mesh must be lower than the distance between mesh elements. This means that information from a given cell or mesh element must propagate only to its immediately adjacent neighbors.

From my experience, the solver must be able to keep the maximum Courant number around 0.5 in order for the simulation to converge nicely. 

PhD position,Texas A&M University, College Station, TX, USA

Mon, 2018-03-19 21:34

In reply to April 10, 2018 Web Workshop - Introduction to Simpleware Software

Immediate PhD position in Energy, control, and Optimization (ECO) lab in Mechanical Engineering department at Texas A&M University. The project is related to Renewable energy and grid distribution and is Computational. Being familiar with Matlab, python and Optimization algorithms and Machine Learning methods is Plus. Please send your CV to shima.hm@tamu.edu

I have update the final

Mon, 2018-03-19 07:10

In reply to final version published with some errata corrige

I have update the final version also on imechanica (see PDF attached). 

Tanks!

Fri, 2018-03-16 22:46

In reply to Hi Xiaoyan,

Hi Wendy,

Thank you very much for your explanation which is very helpful for my understanding of superlattices self-assembled by nanoparticles.

Xiaoyan

Hi Team,

Fri, 2018-03-16 15:08

In reply to Peridynamic theory vs. classical continuum theory

Hi Team,

 

I have gone through some papers on Peridynamics which tell that in Peridyanamics the ideas of " force per unit area" and "stress tensor" are not used. Then which are the quantified values can we calculate from the Peridynamics simulation.

Help will be appreciated.

--

Fri, 2018-03-16 12:41

Hi Mohamedlamine, 

Thu, 2018-03-15 18:15

In reply to Dynamic behavior of nanomaterials

Hi Mohamedlamine, 

Thanks for the question! 

The ductility/brittleness of the self-assembled materials largely depends on the properties of the organic ligands attached to the particles. In my work, this ligand was polystyrene (brittle at room temperature) so the self-assembled structures were brittle (but with high elastic modulus). Detailed cyclic testing has not been performed (a few cycles of deformation has been carried out in Heinrich Jaeger's work). I would not expect these materials to have the properties of sandwich laminates because of the large differences in their structures. 

Wendy

Hi Xiaoyan,

Thu, 2018-03-15 18:08

In reply to Thank you very much!

Hi Xiaoyan,

Nanoparticle self-assembly follows the same principles as (atomic) crystal growth. Superlattice growth begins when a solid aggregate of nanoparticles nucleate from the liquid phase, and gets bigger when additional nanoparticles are added to the initial solid cluster. A 2D structure will form when growth is confined to the lateral directions. This can occur when the particles are placed on an immiscible fluid interface, or a solid substrate that is well wetted by the nanoparticle solution. 3D nanoparticle crystals by allowing growth in 3-dimensions, such as by gently destabilizing nanoparticles in solution by adding an anti-solvent (i.e. adding acetone to a hexane-nanoparticle solution) to induce the particles to crystallize. 

Thank you for the question!

Wendy

Dynamic behavior of nanomaterials

Wed, 2018-03-14 17:38

In reply to Journal Club for March 2018: Colloidal Self-Assembly of Architected Nanomaterials

Dear Xwgu,

Very interesting analysis.

What is the behavior of these new nanomaterials under cyclic loadings. It is shown in figures 1.A and 1.B that the materials may have a somewhat brittleness. Are they very elastic like you have cited the elastic modulus' values or do they have dynamic displacements similarly to sandwitch laminates i.e cyclic and linear stresses ?

CV

Fri, 2018-03-09 21:04

In reply to Research in Fluid Structure Interaction: Looking for PhD position

I have an open position in my lab at Texas A&M University for May 2018. If you are interested please send your CV to "shima.hm@tamu.edu".

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