A computationally-efficient method for evaluating friction in molecular rotary bearings is presented. This method estimates drag from fluctuations in molecular dynamics simulations via the fluctuation–dissipation theorem. This is effective even for simulation times short compared to a bearing's energy damping time and for rotation speeds comparable to or below typical thermal values. We apply this method to two molecular rotary bearings of similar size at 300 K: previously studied nested (9,9) (14,14) double-walled carbon nanotubes and a hypothetical rotary joint consisting of single acetylenic bonds in a rigid diamondoid housing. The acetylenic joint has a rotational frictional drag coefficient of 2 × 10−35 kg m2 s−1. The friction for the nested nanotubes is 120 times larger, comparable to values reported by previous studies. This fluctuation-based method could evaluate dissipation in a variety of molecular systems with similarly rigid and symmetric bearings.
In @cite_66 , four different MD simulation configurations using the LAMMPS code and AIREBO force field were used to obtain consistent friction estimates for a rotating nested nanotube bearing. Other dissipation estimates for rotating nested nanotube bearings have been determined using intralayer interactions based on the Brenner potential @cite_47 with interlayer interactions based on the Kolmogorov-Crespi registry-dependent potential @cite_3 , and with the COMPASS force field @cite_24 . Dissipation estimates for sliding nested nanotube bearings have been determined using a custom force field and custom numerical simulator in @cite_27 @cite_69 , and a custom force field based on @cite_60 and the r-RESPA integrator in @cite_56 .
@cite_69 @cite_60 @cite_3 @cite_56 @cite_24 @cite_27 @cite_47 @cite_66
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We use a molecular dynamics simulation to investigate the properties and design space of molecular gears fashioned from carbon nanotubes with teeth added via a benzyne reaction known to occur with C60. Brenner's reactive hydrocarbon potential is used to model interatomic forces within each molecular gear. A Lennard-Jones 6-12 potential or the Buckingham (exp +6) potential plus electrostatic interaction terms are used for intermolecular interactions between gears. A number of gear and gear shaft configurations are simulated on parallel computers. One gear is powered by forcing the atoms near the end of the nanotube to rotate, and a second gear is allowed to rotate by keeping the atoms near the end of its nanotube constrained to a cylinder. The meshing aromatic gear teeth transfer angular momentum from the powered gear to the driven gear. Results suggest that these gears can operate at up to 50-100 GHz in a vacuum at room temperature. The failure mode involves tooth slip, not bond breaking, so failed gears can be returned to operation by lowering the temperature and or rotation rate. M This article features multimedia enhancements available from the abstract page in the online journal; see http: www.iop.org. Atomistic simulations of double-walled carbon nanotubes (DWCNTs) as rotational bearings were performed. Molecular mechanics (MM) calculations show that the interlayer energy surface of the bearings is nearly flat. Thermal effects on the bearings were studied with molecular dynamics (MD) simulations at finite temperature. These simulations show that the interlayer corrugation against rotation, and hence the interlayer friction coefficient, is extremely small, suggesting the possible application of DWCNTs as wearless bearings. Extreme operational conditions of the bearings for which the bearings disintegrate are also reported. Abstract Simulation of friction in carbon nanotube (CNT) bearing systems has been a popular topic, yet many questions remain open. For example, quantitative estimates of friction reported to date range by as much as eight orders of magnitude, and simulation techniques employ a variety of disparate simulation paradigms and parameters. This paper presents a new suite of consistently implemented but complementary and independent molecular simulations of rotary CNT bearing systems, which span the approaches reported to date, yet agree quantitatively within the error margin, or about one order of magnitude. A comparison between these new results, existing simulation results, and one experimental friction datum is presented. Furthermore, quantitative relationships between friction and the system operating parameters are determined. Friction is found to vary linearly with rotating speed and system temperature, while system length and mean diameter do not have a strong effect. Finally, based on phonon energy spectrum computations, we show that friction is a broadband phenomenon, which does not depend strongly on a specific vibrational mode, but rather appears to occur from the aggregate interactions of many modes at different frequencies. This work reports an in-depth analysis of the mechanics of CNT friction and shows that it can be simulated using complementary approaches, while still obtaining the same result. The analysis reveals the underlying mechanism causing friction and how the governing parameters affect its behavior. We report on a study of the translational sliding motion and dynamic friction in systems of double-walled carbon nanotubes using molecular dynamics simulations combined with theoretical analysis. The sliding motion is described by a one-dimensional analytical model which includes the van der Waals force between the nanotubes, a dynamic friction force, and a small Langevin-type fluctuating force. The dynamic friction force is shown to be linear in the velocity over a large domain of initial conditions in armchair-armchair, zigzag-armchair, and zigzag-zigzag double-walled nanotubes. Beyond this domain, evidence is obtained for nonlinear effects which increase friction. In armchair-armchair systems, the dynamic friction is observed to be nonlinearly enhanced by the excitation of internal modes. In the linear domain, the coefficient of proportionality between the dynamic friction force and the velocity is shown to be given by Kirkwood's formula in terms of the force autocorrelation function.