Polymer brushes lead to small friction and wear and thus hold great potential for industrial applications. However, interdigitation of opposing brushes makes them prone to damage. Here we report ...molecular dynamics simulations revealing that immiscible brush systems can form slick interfaces, in which interdigitation is eliminated and dissipation strongly reduced. We test our findings with friction force microscopy experiments on hydrophilic and hydrophobic brush systems in both symmetric and asymmetric setups. In the symmetric setup both brushes are chemically alike, while the asymmetric system consists of two different brushes that each prefer their own solvent. The trends observed in the experimentally measured force traces and the friction reduction are similar to the simulations and extend to fully immersed contacts. These results reveal that two immiscible brush systems in mechanical contact slide at a fluid-fluid interface while having load-bearing ability. This makes them ideal candidates for tribological applications.
The Green's function molecular dynamics method, which enables one to study the elastic response of a three-dimensional solid to an external stress field by taking into consideration only the surface ...atoms, was implemented as an extension to an open source classical molecular dynamics simulation code LAMMPS. This was done in the style of fixes. The first fix, FixGFC, measures the elastic stiffness coefficients for a (small) solid block of a given material by making use of the fluctuation–dissipation theorem. With the help of the second fix, FixGFMD, the coefficients obtained from FixGFC can then be used to compute the elastic forces for a (large) block of the same material. Both fixes are designed to be run in parallel and to exploit the functions provided by LAMMPS.
Program title: FixGFC/FixGFMD
Catalogue identifier: AECW_v1_0
Program summary URL:
http://cpc.cs.qub.ac.uk/summaries/AECW_v1_0.html
Program obtainable from: CPC Program Library, Queen's University, Belfast, N. Ireland
Licensing provisions: yes
No. of lines in distributed program, including test data, etc.: 33 469
No. of bytes in distributed program, including test data, etc.: 1 383 631
Distribution format: tar.gz
Programming language: C++
Computer: All
Operating system: Linux
Has the code been vectorized or parallelized?: Parallelized via MPI
RAM: Depends on the problem
Classification: 7.7
External routines: MPI, FFTW 2.1.5 (
http://www.fftw.org/), LAMMPS version May 21, 2008 (
http://lammps.sandia.gov/)
Nature of problem: Using molecular dynamics to study elastically deforming solids imposes very high computational costs because portions of the solid far away from the interface or contact points need to be included in the simulation to reproduce the effects of long-range elastic deformations. Green's function molecular dynamics (GFMD) incorporates the full elastic response of semi-infinite solids so that only surface atoms have to be considered in molecular dynamics simulations, thus reducing the problem from three dimensions to two dimensions without compromising the physical essence of the problem.
Solution method: See “Nature of problem”.
Restrictions: The mean equilibrium positions of the GFMD surface atoms must be in a plane and be periodic in the plane, so that the Born–von Karman boundary condition can be used. In addition, only deformation within the harmonic regime is expected in the surface layer during Green's function molecular dynamics.
Running time: FixGFC varies from minutes to days, depending on the system size, the numbers of processors used, and the complexity of the force field. FixGFMD varies from seconds to days depending on the system size and numbers of processors used.
References: 1 C. Campañá, M.H. Müser, Phys. Rev. B 74 (2006) 075420.
A generic model for frictional forces between two monoatomic crystals is investigated by molecular dynamics simulations. Two solids, each composed of several atomic layers, are brought into contact ...and moved against each other. The mechanisms that lead to finite pinning (static friction) forces are analyzed by varying the geometry, the interfacial interaction, and the externally applied force. Material transfer leading to welded junctions is seen to be responsible for friction between strongly adhering surfaces. Chemically passivated surfaces pin if they deform plastically. In no region of the model's parameter range can finite frictional forces be attributed to multistable elasticity. Such wearless pinning mechanisms play the predominant role in Frenkel–Kontorova and Tomlinson models. In the parameter range where pinning is observed, externally driven sliding induces wear at the interface.
Previous molecular dynamics simulations of friction between polymer brushes in relative sliding motion Kreer, T.; Müser, M. H.; Binder, K.; Klein, J. Langmuir 2001, 17, 7804 are extended beyond ...steady-state conditions. We study two different protocols: (i) stop and return and (ii) stop and go. In protocol (i), the relative, lateral motion between the two surfaces is stopped abruptly and reimposed opposite to the initial direction after the system could relax for some time. Protocol (ii) is similar except that the sliding direction is maintained. In the constant-velocity steady state, the average lateral extension l c of the polymers is found to be a power law of the sliding velocity v, namely, l c ∝ v 0.3. When the sliding direction is inverted, a shear stress maximum is observed after the two walls have slid a relative distance of 2l c. This maximum occurs when the average inclination of the polymers is 90°, and it is accompanied by brush swelling. In protocol (ii), no brush swelling is found and shear stress maxima are absent in the itinerant stages of the go phase, with the exception of large v. We conclude that dissipation mechanisms for oscillatory shear are similar to those for constant-velocity sliding if the driving amplitude 𝒜 distinctly exceeds 2l c. Moreover, enhanced loss at 𝒜 ≈ 2l c is not necessarily related to stick−slip motion.
Meeting the Contact-Mechanics Challenge Müser, Martin H.; Dapp, Wolf B.; Bugnicourt, Romain ...
Tribology letters,
12/2017, Volume:
65, Issue:
4
Journal Article
Peer reviewed
Open access
This paper summarizes the submissions to a recently announced contact-mechanics modeling challenge. The task was to solve a typical, albeit mathematically fully defined problem on the adhesion ...between nominally flat surfaces. The surface topography of the rough, rigid substrate, the elastic properties of the indenter, as well as the short-range adhesion between indenter and substrate, were specified so that diverse quantities of interest, e.g., the distribution of interfacial stresses at a given load or the mean gap as a function of load, could be computed and compared to a reference solution. Many different solution strategies were pursued, ranging from traditional asperity-based models via Persson theory and brute-force computational approaches, to real-laboratory experiments and all-atom molecular dynamics simulations of a model, in which the original assignment was scaled down to the atomistic scale. While each submission contained satisfying answers for at least a subset of the posed questions, efficiency, versatility, and accuracy differed between methods, the more precise methods being, in general, computationally more complex. The aim of this paper is to provide both theorists and experimentalists with benchmarks to decide which method is the most appropriate for a particular application and to gauge the errors associated with each one.
Some fundamental features of friction between two polymer bearing surfaces in relative sliding motion are investigated by molecular dynamics simulations. End-tethered and adsorbed polymers are ...considered under good and poor solvent conditions. The shear stress is measured while varying the solvent's viscosity, surface separation, degree of polymerization and grafting density. For all systems we observe shear thinning that is attributed to the orientation of the chains along the shear direction. This effect is particularly strong for brushes, for which the shear stress during the steady sliding state is mainly determined by the degree of overlap between the brushes.
Friction between two solid bodies in sliding motion takes place on a large spectrum of length and time scales: From the nanometer/second scale in an atomic force microscope up to the extremely ...macroscopic scales of tectonic motion. Despite our familiarity with friction, fundamental questions about its atomistic origins remain unanswered. Phenomenological laws that describe the friction in many systems were published more than 300 years ago by Amontons: The frictional force is proportional to the applied load and independent of the apparent area of contact. The atomistic origins of this simple law is still controversial. Many explanations, which seemed to be well-established until recently, have been called into question by new experimental results. Computer simulations have also revealed flaws in previous theoretical approaches and led to new insights into the atomistic processes responsible for friction. In this paper, selected computer simulation studies of friction will be discussed. Special attention will be given to how it is possible to gain insight into tribological processes that take place on macroscopic time scales with the help of atomistic computer simulations which are typically constrained to the nanometer and nanosecond regime.
A lubricant layer solidifies when it is confined between two walls at large normal pressures. The atomic scale motion that occurs when the two confining surfaces slide past each other induces flow in ...the lubricant layer that is akin of plastic flow. This results in friction‐velocity relationships similar to Coulomb’s law of friction. Moreover, the lubricant layer does not necessarily melt, even when the two solids are in stick slip motion. In this paper, atomic‐scale details of the plastic flow mechanism are investigated by means of molecular dynamics simulations.
Schmierstoffe unter hohem lokalem Druck: Flüssigkeiten verhalten sich wie Feststoffe
Eine molekular dünne Schicht Schmierstoff verfestigt sich, wenn sie zwischen zwei Festkörpern großen Drücken ausgesetzt ist. Die atomaren Bewegungsmechanismen, die in dem Schmierstofffilm auftreten, wenn die Festkörper gegeneinander verschoben werden, können als plastisches Fliessen verstanden werden. Dieser Mechanismus führt zu einer Kraft‐Geschwindigkeitsrelation, die dem Coulomb’schen Reibungsgesetz ähnlich ist. Selbst wenn die beiden Wände in Stick‐Slip Bewegung sind, verflüssigt sich der Schmiermittelfilm nicht automatisch. In dieser Arbeit werden die Details des angesprochenen plastischen Fliessens anhand von Molekular‐Dynamik Simulationen im Detail untersucht.
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