Electrotunable lubricity with ionic liquids: the influence of nanoscale roughness

2017 ◽  
Vol 199 ◽  
pp. 279-297 ◽  
Author(s):  
Alessio David ◽  
Oscar Y. Fajardo ◽  
Alexei A. Kornyshev ◽  
Michael Urbakh ◽  
Fernando Bresme
Keyword(s):  
Ionic Liquids ◽  
Electrostatic Potential ◽  
Lower Surface ◽  
Coarse Grained ◽  
Dispersion Interactions ◽  
Surface Charges ◽  
Flat Surfaces ◽  
Non Equilibrium Molecular Dynamics ◽  
The One ◽  
Dynamics Simulations

The properties of ionic liquids can be modified by applying an external electrostatic potential, providing a route to control their performance in nanolubrication applications. Most computational studies to date have focused on the investigation of smooth surfaces. Real surfaces are generally inhomogeneous and feature roughness of different length scales. We report here a study of the possible effects that surface roughness may have on electrotunable lubricity with ionic liquids, performed here by means of non-equilibrium molecular dynamics simulations. In order to advance our understanding of the interplay of friction and substrate structure we investigate coarse grained models of ionic liquids confined in model surfaces with nanometer roughness. The friction is shown to depend on the roughness of the substrate and the direction of shear. For the investigated systems, the friction coefficient is found to increase with roughness. These results are in contrast with previous studies, where roughness induced reduction of friction was reported, and they highlight the strong sensitivity of the friction process to the structure of the surfaces. The friction force features a maximum at a specific surface charge density. This behaviour is reminiscent of the one reported in ionic liquids confined by flat surfaces, showing the generality of this physical effect in confined ionic liquids. We find that an increase of the substrate–liquid dispersion interactions shifts the maximum to lower surface charges. This effect opens a route to control electrotunable friction phenomena by tuning both the electrostatic potential and the composition of the confining surfaces.

scholarly journals Atomistic Nodal Approach: a General Molecular Dynamics Method For Computing Local Thermal Conductance at Atomistic Resolution

2021 ◽  
Author(s):  
Mingxuan Jiang ◽  
Juan D. Olarte-Plata ◽  
Fernando Bresme
Keyword(s):  
Molecular Dynamics ◽  
Thermal Transport ◽  
Temperature Drop ◽  
Thermal Conductance ◽  
Fundamental Property ◽  
Lower Surface ◽  
Coordination Numbers ◽  
Interfacial Thermal Conductance ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations

The Interfacial Thermal Conductance (ITC) is a fundamental property of mate- rials and has particular relevance at the nanoscale. The ITC quanti�es the thermal resistance between materials of dierent compositions or between uids in contact with materials. Furthermore, the ITC determines the rate of cooling/heating of the materi- als and the temperature drop across the interface. Here we propose a method to com- pute local ITCs and temperature drops of nanoparticle- uid interfaces. Our approach resolves the ITC at the atomic level using the atomic coordinates of the nanomaterial as nodes to compute local thermal transport properties. We obtain high-resolution descriptions of the interfacial thermal transport by combining the atomistic nodal ap- proach, computational geometry techniques and \computational farming" using Non- Equilibrium Molecular Dynamics simulations. We illustrate our method by analyzing various nanoparticles as a function of their size and geometry, targeting experimentally relevant structures like capped octagonal rods, cuboctahedrons, decahedrons, rhombic dodecahedrons, cubes, icosahedrons, truncated octahedrons, octahedrons and spheres. We show that the ITC of these very dierent geometries can be accurately described in terms of the local coordination number of the atoms in the nanoparticle surface. Nanoparticle geometries with lower surface coordination numbers feature higher ITCs, and the ITC generally increases with decreasing particle size.

scholarly journals Onsager Transport Coefficients and Transference Numbers in Polyelectrolyte Solutions and Polymerized Ionic Liquids

2020 ◽  
Author(s):  
Kara D. Fong ◽  
Julian Self ◽  
Bryan D. McCloskey ◽  
Kristin Persson
Keyword(s):  
Ionic Liquids ◽  
Transport Coefficients ◽  
Lithium Ion ◽  
Transference Number ◽  
Einstein Equations ◽  
Coarse Grained ◽  
Transference Numbers ◽  
Polyelectrolyte Solutions ◽  
Dynamics Simulations ◽  
The Ideal

Electrolytes featuring negatively-charged polymers such as nonaqueous polyelectrolyte solutions and polymerized ionic liquids are currently under investigation as potential high cation transference number (t<sub>+</sub>) electrolytes for lithium ion batteries. Herein, we use coarse-grained molecular dynamics simulations to characterize the Onsager transport coefficients of polyelectrolyte solutions as a function of chain length and concentration. For all systems studied, we find that the rigorously computed transference number is substantially lower than that approximated by the ideal solution (Nernst-Einstein) equations typically used to characterize these systems due to the presence of strong anion-anion and cation-anion correlations. None of the polyelectrolyte solutions achieve t<sub>+</sub> greater than that of the conventional binary salt electrolyte, with some solutions having negative t<sub>+</sub>. This work demonstrates that the Nernst-Einstein assumption does not provide a physically meaningful estimate of the transference number in these solutions and calls into question the expectation of polyelectrolytes to exhibit high cation transference number.

scholarly journals Onsager Transport Coefficients and Transference Numbers in Polyelectrolyte Solutions and Polymerized Ionic Liquids

2020 ◽  
Author(s):  
Kara D. Fong ◽  
Julian Self ◽  
Bryan D. McCloskey ◽  
Kristin Persson
Keyword(s):  
Ionic Liquids ◽  
Transport Coefficients ◽  
Lithium Ion ◽  
Transference Number ◽  
Einstein Equations ◽  
Coarse Grained ◽  
Transference Numbers ◽  
Polyelectrolyte Solutions ◽  
Dynamics Simulations ◽  
The Ideal

Electrolytes featuring negatively-charged polymers such as nonaqueous polyelectrolyte solutions and polymerized ionic liquids are currently under investigation as potential high cation transference number (t<sub>+</sub>) electrolytes for lithium ion batteries. Herein, we use coarse-grained molecular dynamics simulations to characterize the Onsager transport coefficients of polyelectrolyte solutions as a function of chain length and concentration. For all systems studied, we find that the rigorously computed transference number is substantially lower than that approximated by the ideal solution (Nernst-Einstein) equations typically used to characterize these systems due to the presence of strong anion-anion and cation-anion correlations. None of the polyelectrolyte solutions achieve t<sub>+</sub> greater than that of the conventional binary salt electrolyte, with some solutions having negative t<sub>+</sub>. This work demonstrates that the Nernst-Einstein assumption does not provide a physically meaningful estimate of the transference number in these solutions and calls into question the expectation of polyelectrolytes to exhibit high cation transference number.

Relating structure and dynamics of ionic liquids under shear by means of reverse non-equilibrium molecular dynamics simulations

2021 ◽  
Author(s):  
Kalil Bernardino ◽  
Mauro Ribeiro
Keyword(s):  
Molecular Dynamics ◽  
Ionic Liquids ◽  
Shear Rate ◽  
Molecular Dynamics Simulations ◽  
Structure And Dynamics ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations ◽  
Non Equilibrium

The effect of shear rate on the viscosity and the structure of 1-ethyl-3-methylimidazolium based ionic liquids with three different anions (tetrafluoroborate, dicyanamide, and bis(trifluoromethylsulfonyl)imide) was studied by means of reverse...

scholarly journals Atomistic Nodal Approach: a General Molecular Dynamics Method For Computing Local Thermal Conductance at Atomistic Resolution

2021 ◽  
Author(s):  
Mingxuan Jiang ◽  
Juan D. Olarte-Plata ◽  
Fernando Bresme
Keyword(s):  
Molecular Dynamics ◽  
Thermal Transport ◽  
Temperature Drop ◽  
Thermal Conductance ◽  
Fundamental Property ◽  
Lower Surface ◽  
Fluid Interfaces ◽  
Coordination Numbers ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations

The Interfacial Thermal Conductance (ITC) is a fundamental property of materials and has particular relevance at the nanoscale. The ITC quantifies the thermal resistance between materials of different compositions or between fluids in contact with materials. Furthermore, the ITC determines the rate of cooling/heating of the materials and the temperature drop across the interface. Here we propose a method to compute local ITCs and temperature drops of nanoparticle-fluid interfaces. Our approach resolves the ITC at the atomic level using the atomic coordinates of the nanomaterial as nodes to compute local thermal transport properties. We obtain high-resolution descriptions of the interfacial thermal transport by combining the atomistic nodal approach, computational geometry techniques and "computational farming'' using Non-Equilibrium Molecular Dynamics simulations. We illustrate our method by analyzing various nanoparticles as a function of their size and geometry, targeting experimentally relevant structures like capped octagonal rods, cuboctahedrons, decahedrons, rhombic dodecahedrons, cubes, icosahedrons, truncated octahedrons, octahedrons and spheres. We show that the ITC of these very different geometries can be accurately described in terms of the local coordination number of the atoms in the nanoparticle surface. Nanoparticle geometries with lower surface coordination numbers feature higher ITCs, and the ITC generally increases with decreasing particle size.

scholarly journals Non-equilibrium molecular dynamics simulations of ionic liquids under extreme shear

2020 ◽  
Author(s):  
Kalil Bernardino ◽  
Mauro Carlos Costa Ribeiro
Keyword(s):  
Molecular Dynamics ◽  
Ionic Liquids ◽  
Shear Rate ◽  
Structural Changes ◽  
Polymer Chains ◽  
Practical Applications ◽  
Shear Rates ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations ◽  
Non Equilibrium

Ionic liquids are called designer solvents because their physical properties can be tuned by the selection of different combinations cation and anion. Understanding the relation between the chemical structure and the viscosity and how the shear rate can affect the relative arrangements of the ions are important for practical applications specially as lubricants. Reverse non-equilibrium molecular dynamics (RNEMD) simulations were performed to study the effect of the shear rate over the viscosity and the structure at molecular level of four different imidazolium based ionic liquids. Since it is already well known that the absence of explicit electronic polarizability in usual classical force fields leads to artificially slow dynamics in ionic liquids, a Drude polarizable force field was employed in all simulations. Non-newtonian behavior is observed at shear rates at GHz scale, with a progressive reduction of the viscosity at the same time that the structure of second and further coordination shells are partially disrupted. The liquids that displayed the greater structural changes with increasing shear rate also displayed the strongest variation in the viscosities. At the highest shear rates studied, the imidazolium rings tends to align parallel to the induced flux, an effect similar to the well-known align of polymer chains under shear, despite becoming significant for ionic liquids only at extremely high rates.

Designing high thermal conductivity of cross-linked epoxy resin via molecular dynamics simulations

2020 ◽  
Vol 22 (35) ◽  
pp. 19735-19745
Author(s):  
Ran Huo ◽  
Zhiyu Zhang ◽  
Naveed Athir ◽  
Yanhao Fan ◽  
Jun Liu ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Molecular Dynamics Simulation ◽  
Molecular Dynamics Simulations ◽  
Epoxy Resin ◽  
High Thermal Conductivity ◽  
Dynamics Simulation ◽  
Coarse Grained ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations

Coarse-grained (CG) non-equilibrium molecular dynamics simulation was used to study the thermal conductivity of a cross-linked network composed of epoxy resin (E51) and polyether amine (PEA).

scholarly journals A Coarse-Grained Force Field for Silica–Polybutadiene Interfaces and Nanocomposites

Polymers ◽  
2020 ◽  
Vol 12 (7) ◽  
pp. 1484 ◽  
Author(s):  
Alessio David ◽  
Marta Pasquini ◽  
Ugo Tartaglino ◽  
Guido Raos
Keyword(s):  
Force Field ◽  
Large Scale ◽  
Present Model ◽  
Coarse Grained ◽  
Atomistic Simulations ◽  
Silica Nanocomposites ◽  
Silica Surfaces ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations ◽  
Large Scale Simulations

We present a coarse-grained force field for modelling silica–polybutadiene interfaces and nanocomposites. The polymer, poly(cis-1,4-butadiene), is treated with a previously published united-atom model. Silica is treated as a rigid body, using one Si-centered superatom for each SiO 2 unit. The parameters for the cross-interaction between silica and the polymer are derived by Boltzmann inversion of the density oscillations at model interfaces, obtained from atomistic simulations of silica surfaces containing both Q 4 (hydrophobic) and Q 3 (silanol-containing, hydrophilic) silicon atoms. The performance of the model is tested in both equilibrium and non-equilibrium molecular dynamics simulations. We expect the present model to be useful for future large-scale simulations of rubber–silica nanocomposites.

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