Frictional properties of MoS2 on a multi-level rough wall under starved lubrication.

Owing to nano-MoS2's excellent anti-friction and anti-wear properties, nano-MoS2, which can act as a nano-additive in lubricating oil or solid lubricants, is believed to have great potential in the lubrication of power machinery and moving parts of a spacecraft. The molecular dynamics method was used to construct a rough surface and a multi-level asperity structure to simulate starved lubrication before oil film breakdown, and the lubrication mechanism of MoS2 as a nano-additive or directly coated on the textured surface could reduce the friction coefficient and wear was explained from the atomic perspective. Simulations showed that the multilayer MoS2 played a role of load-bearing at light load or low velocity, and slipped into the grooves to repair the surface under heavy load or high velocity. Even if local asperity contact occurs, MoS2 nanoparticles could accelerate the detachment of the initial asperity contact to prevent large-scale adhesion. The MoS2 nanoparticles transformed the pure liquid oil film into a liquid-solid composite oil film, which was more suitable for lubrication under heavy load and high velocity because it increased the contact area, protected the friction surface and prevented asperity contact. The proposed lubrication mechanism contributes to understanding the frictional properties of layered nanomaterials under extreme conditions and provides a reference for further application of MoS2 materials in the field of lubrication.

A study of how solid-liquid interactions affect flow resistance and heat transfer at different temperatures based on molecular dynamics simulations.

Non-equilibrium molecular dynamics simulations of liquid flow through the surface were performed to investigate the flow resistance and thermal resistance under conditions of different solid-liquid interactions and surface temperatures. A novel phenomenon was observed in the simulation, namely the rise of surface temperature increases the flow resistance when solid-liquid interaction is weak, but decreases the flow resistance when solid-liquid interaction is strong. A higher density of the boundary layer brings a larger friction force to increase the flow resistance. For heat transfer, it is innovative to calculate the heat conduction and convection of the boundary region discretely. The results showed that the heat transfer performance of the interface is not positively correlated with the boundary liquid density, and the structure of the boundary liquid is also crucial. We believe that this research can improve the existing theory of flow heat transfer and provide a more effective method for analyzing the flow heat transfer of the solid-liquid interface.

Hydrophobic surface-assisted SiO2/DI-water nanofluids for enhancing heat transfer and reducing flow resistance.

Nanofluids for heat transfer application suffer from inevitable pump power consumption and adhesion effect with interface during flow. The hydrophobic treatment for heat transfer surface may be one of the most prospective strategies to achieve heat transfer enhancement and flow resistance reduction. However, the limitations of hydrophobic treatment technique and process make it difficult to fabricate desirable large size and high curvature hydrophobic surface. Herein, a facile displacement reaction method is applied to prepare the lath-like silver crystals and micro-nano gaps in the inner surface of copper tube with assistance of benzoic acid dispersant. The result shows that the convective heat transfer coefficient increases to 18.1% and the Darcy friction factor decreases to 4.9% at the volume concentration of 2.0% when SiO2/DI-water (deionized water) nanofluids flow through the hydrophobic surface. The hydrophobic surface-assisted strategy may provide an effective scheme for wide applications of nanofluids in heat exchange equipment.

FDTD method study on the effects of geometric parameters on the W- Al2O3 nano-structure thermal emitter.

In this paper, a 2D periodic structure made of tungsten (W) grating patch on a thin Al2O3 spacer and an opaque W film is proposed, as a broadband selective thermal emitter. We numerically investigated the spectral emissivity of the structure from the deep by finite-difference time-domain method. Geometric parameters effects on the emissivity are discussed and the mechanisms of magnetic polaritons in the structure are further analyzed in detail. In addition, by adding another metal-dielectric stack upon the pre-existing grating structure, the emissivity of the composite structure can be further enhanced, and the normal emissivity exceeds 0.95 in the wavelength range of 0.65-1.95 µm, some even close to unity. Furthermore, the composite structures are found to exhibit insensitivity to polar angle and polarization.

Investigating control of convective heat transfer and flow resistance of Fe3O4/deionized water nanofluid in magnetic field in laminar flow.

This paper studies the convective heat transfer and flow resistance of Fe3O4/deionized water nanofluids in laminar flow under the control of an external magnetic field. The basic thermophysical parameters including viscosity, specific heat capacity and thermal conductivity are investigated to describe the fundamental performance of heat transfer and flow resistance. In the absence of the magnetic field, the heat transfer coefficients and flow friction could not change significantly at nanoparticle volume concentration of 0.05%. In the presence of the magnetic field, it can enhance heat transfer and flow resistance by 6% and 3.5% when the magnets interlace on both sides of the tube. The dynamic magnetic experiments discussed the heat transfer increase process in detail. The heat transfer and the flow resistance increase by 11.7% and 5.4% when magnetic field strength is 600 Gs, nanoparticle volume concentration is 2% and Reynolds number is 2000. The radial shuttle movement of magnetic nanoparticles in the cross-section, micro convection in base fluid and the slip velocity between the nanoparticles and the base fluid are considered the main reasons for heat transfer enhancement.

Molecular dynamics study on the mechanical properties of multilayer MoS2 under different potentials.

Experiments and simulations have shown that molybdenum disulfide (MoS2) has unique mechanical and electrical properties that make it promising for application as a flexible material in microscopic and nanoscopic electronic devices. In this paper, the molecular dynamics method is used to study the mechanical properties of multilayer MoS2 during compression and stretching under different intra-layer and inter-layer potentials to choose the most suitable ones. The results show that the increase in the inter-layer repulsive force during compression was all provided by sulfur atoms in the adjacent layer. The two intra-layer potentials represented two forms of tensile fracture: plastic fracture with structural holes or lattice distortions, and brittle fracture with instantaneous destruction. The chosen inter-layer potential had a significant influence on the structure of the multilayer MoS2 but the effect of inter-layer potential during stretching was not prominent. By comparing these results with reference values, the most suitable intra-layer and inter-layer potentials for the multilayer MoS2 were selected, and can serve as a reliable reference for subsequent simulations.

Molecular dynamics study of the frictional properties of multilayer MoS2.

To reveal the friction mechanism of molybdenum disulfide (MoS2), the frictional properties of multilayer MoS2 lubrication film were studied under variable loads and shearing velocities by the molecular dynamics (MD) method. The results showed irreversible deformation of MoS2 was caused by heavy load or high shear velocity during the friction process and the interlayer velocity changed from a linear to a ladder-like distribution; thus, the number of shear surfaces and the friction coefficient decreased. The low friction coefficient caused by heavy load or high velocity could be maintained with a decrease in load or velocity. For a solid MoS2 lubrication film, the number of shearing surfaces should be reduced as much as possible and the friction pair should be run under heavy load or high shear velocity for a period of time in advance; thus, it could exhibit excellent frictional properties under other conditions. The proposed friction mechanism provided theoretical guidance for experiments to further improve the frictional properties of MoS2.

Molecular dynamics simulation of frictional properties of Couette flow with striped superhydrophobic surfaces under different loads.

To reveal the effect of a striped superhydrophobic surface on frictional properties, molecular dynamics simulations were carried out to study the frictional properties of Couette flow. In particular, the influence of load on flow properties was considered in this work. The results showed that regions of gas in the groove and a low density region near the superhydrophobic surface were formed. Under a certain load, convex menisci appeared on top of the groove and some fluid atoms were trapped in it. Compared with the smooth hydrophobic surface, the striped superhydrophobic surface showed a reduction in friction owing to reduced liquid-solid contact area. With increasing load, the number of fluid atoms trapped in the groove increased prominently, which increased the friction force of the striped superhydrophobic surface more quickly. There was a critical load (Pcrit), such that the friction-reduction property of striped superhydrophobic surfaces appeared only when the load was smaller than it. By reducing the distance between adjacent stripes, the rate of increase in the number of fluid atoms trapped in grooves with load decreased significantly, which increased Pcrit. Under a large load, the friction force decreased with the distance between adjacent stripes. However, under a small load we observed the opposite trend.

The molecular dynamic simulation on impact and friction characters of nanofluids with many nanoparticles system.

Impact and friction model of nanofluid for molecular dynamics simulation was built which consists of two Cu plates and Cu-Ar nanofluid. The Cu-Ar nanofluid model consisted of eight spherical copper nanoparticles with each particle diameter of 4 nm and argon atoms as base liquid. The Lennard-Jones potential function was adopted to deal with the interactions between atoms. Thus motion states and interaction of nanoparticles at different time through impact and friction process could be obtained and friction mechanism of nanofluids could be analyzed. In the friction process, nanoparticles showed motions of rotation and translation, but effected by the interactions of nanoparticles, the rotation of nanoparticles was trapped during the compression process. In this process, agglomeration of nanoparticles was very apparent, with the pressure increasing, the phenomenon became more prominent. The reunited nanoparticles would provide supporting efforts for the whole channel, and in the meantime reduced the contact between two friction surfaces, therefore, strengthened lubrication and decreased friction. In the condition of overlarge positive pressure, the nanoparticles would be crashed and formed particles on atomic level and strayed in base liquid.