Molecular Dynamics Simulation of Droplet Impact on a Hydrophobic 3D Elastic Surface.

The phenomenon of droplets impacting elastic surfaces is common in nature and in many engineering applications. It has been shown that droplet impact on an elastic surface drastically reduces droplet contact time and hinders droplet spreading. However, most of the current studies are based on experiments, and the analysis of the influence mechanism of the elastic substrate on the dynamic behavior of droplets is not complete. In addition, the simulations of droplet impact on elastic substrates are mainly focused on 2D elastic films or vibrating rigid substrates, ignoring the effect of 3D elastic substrate deformation on the droplet dynamic behavior. Therefore, in this paper, we propose to model the droplet impact on a 3D hydrophobic elastic substrate using the molecular dynamics method. We find that droplet pancake rebound can substantially reduce the droplet contact time. Moreover, we record the conditions required for the pancake rebound of the droplet. Furthermore, we investigated the effects of the elastic modulus of the substrate and the initial velocity of the droplet on the droplet contact time, contact area, and spreading factor. This study further elucidates the influence mechanism of the elastic substrate on the dynamic behavior of the droplet and provides theoretical guidance for regulating the dynamic behavior of the droplet in related fields.

Molecular Dynamics Simulation of Nanodroplets Impacting Stripe-Textured Surfaces.

The dynamic behavior of droplets impacting on textured surfaces has an important influence on many engineering applications, such as anti-icing and self-cleaning. However, the mechanism and law of the effect of textured surfaces on the impact behavior of nanodroplets has not been fully revealed yet. In this paper, the molecular dynamics (MD) method is used to model the dynamic behavior of nanodroplets after impacting the solid surface with a striped texture. The influences of texture gap and texture angle on the real contact area, spreading factor, contact time, and bounce velocity of the droplet after impact are also quantitatively analyzed. It is shown that the striped texture produces significant anisotropy in the spreading and contraction behavior of nanodroplets after impact, and the anisotropy is more pronounced on the ridged texture surface than on the grooved texture surface. In addition, we find that the texture gap has little effect on the dynamic behavior of nanodroplets impacting the textured surface. However, as the bottom angle of the texture increases, the real contact area and bounce velocity of the nanodroplet increase significantly, while the contact time and spreading factor decrease. This work further elucidates the characteristics and mechanisms of nanodroplets impacting on stripe-textured surfaces and provides a theoretical basis for the design of nanostructured surfaces in relevant applications.

Study on Lubricant Release-Recycle Performance of Porous Polyimide Retainer Materials.

Oil-impregnated porous polyimide (PI) materials can provide continuous lubricant supply, which is widely used to manufacture space rolling bearing retainers. The lubrication performance of porous polyimide materials mainly depends on their ability to release and recycle lubricants, which is closely related to pore size. In this paper, to investigate the effect of pore size, porous polyimide materials with different pore sizes were prepared by preheating the retainer tube billet during the limit pressing process. The lubricant content rates at each stage were measured by the lubricant immersion and centrifugal release experiment to show the variation of the lubricant content rate in the porous PI sample during a working cycle. At first, the lubricant can be absorbed into the pore. It is found that the absorption rate is faster for lubricants with a smaller viscosity. Moreover, lubricant thinning caused by temperature rise also improves the absorption rate. After lubricant absorption to saturation, the lubricant is released under the centrifugal effect to provide the lubricant. Increasing pore size and using low-viscosity lubricants are the main ways to improve lubrication. Finally, the lubricant on the surface can be recycled into the pore by capillary effect. The smaller the pore size, the faster the lubricant recycles to saturation. These insights gained in this study can provide guidance for the choice of an oil-impregnated porous retainer in different working conditions.

Molecular Dynamics Simulations of Lubricant Outflow in Porous Polyimide Retainers of Bearings.

The retainer of a space rolling bearing widely made of porous polyimide (PI) materials is oil-impregnated and can continuously release lubricants for lubrication. Understanding the lubricant supply mechanism in porous polyimide retainers is important to improve the lubrication performance of space bearings and therefore extends the bearing life. In this work, molecular dynamics simulations are adopted to model the lubricant outflow process from the pore of the PI material. Coarse-grained models are constructed to investigate the lubricant migration behaviors with different pore sizes and radii of rotation. At rest, a lubricant within the pore fails to outflow due to the capillary effect, which decreases with the increase of the pore size. However, for the rotating pores, if the inertial forces generated by the rotational motion exceed the capillary forces, the lubricants will begin to accumulate and some of the lubricants will flow out. Furthermore, the lubricant in the larger pore is easier to outflow due to the smaller capillary forces. This study quantifies the inertial effect and reveals that the centrifugal force is the main mechanism of lubricant outflow from the pores.

Molecular Dynamics Simulations of Lubricant Recycling in Porous Polyimide Retainers of Bearing.

Porous polyimide (PI) materials are one of important bearing retainer materials in space applications due to the storage and continuous supply of a lubricant through the porous structure. Understanding the lubricant recycling process in porous polyimide retainers is of vital importance to improve lubricant supply performance of bearing. In this work, through molecular dynamic simulations, coarse-grained models are built to study lubricant recycling processes on porous and solid surfaces. A spontaneous imbibition behavior is observed when the lubricant is present on the porous surface. The dynamic change in the contact angle in this process and the deviation of the effective radius from the volumetric radius because of the molecular structure of polyimide causes the classical Lucas-Washburn (L-W) equation fail to describe the process. By fitting dynamic contact angle and effective radius, a modified L-W equation is developed, which well predicts the process of imbibition. Furthermore, it is found that the lubricants between the porous polyimide surface and the solid surface are recycled by extrusion, and spontaneous imbibition does not occur. In this case, the accumulation of lubricant pressure and weak interfacial interaction between the lubricant and the solid surface are also the main factors that promote lubricant recycling.

Grain size and hydroxyl-coverage dependent tribology of polycrystalline graphene.

Functional groups and grain boundaries of polycrystalline graphenes play important roles in their tribological behaviors but the mechanism is still elusive. Here, we have investigated the influences of hydroxyl groups, coverage, and grain size on the surface corrugation, friction, and motion behavior of polycrystalline graphene using molecular dynamics simulations. The results show that the corrugation of polycrystalline graphene increases with respect to an increase in grain size. The introduction of hydroxyl groups suppresses the corrugation. The friction between carbon nanotube (CNT) and polycrystalline graphene increases the formation of hydrogen bonds when the interfaces are grafted with hydroxyl groups. The highest amount of friction appears when the ratio of hydroxyl groups on CNT, and polycrystalline graphene, is about 15%-5%. This is due to the balance between the interface space and the formed hydrogen bonds. Furthermore, polycrystalline slides following the movement of CNT owing to high friction. In addition, the energy dissipation as a result of the vibration of the hydroxyl groups plays a more important role as the ratio of hydroxyl groups increases.

Study of Nanoscale Friction Behaviors of Graphene on Gold Substrates Using Molecular Dynamics.

In this paper, we investigate the friction behaviors of graphene flakes sliding on a gold substrate using molecular dynamics simulations. The effects of flake size, flake shape, relative rotation angle between flake and substrate, and crystal orientation of substrate on the friction process are thoroughly studied. It is found that under the same load, the average friction forces per atom are smaller for a bigger graphene flake, which exhibits an obvious size effect. It is also shown that flake shape is critical in determining the friction in the sliding process. The average friction forces per atom for the square flake are much bigger than those for the triangular and round flakes. Moreover, the average friction forces per atom for the triangular flake are the smallest. We also find that the orientation of graphene flake relative to gold substrate plays a vital role in the friction process. The friction forces for the graphene flake sliding along the armchair direction are much bigger than those for the flakes with rotation. In addition, it is also found that single crystalline gold substrate exhibits a significant anisotropic effect of friction, which is attributed to the anisotropic effect of potential energy corrugation. These understandings not only shed light on the underlying mechanisms of graphene flake sliding on the gold substrates but also may guide the design and fabrication of nanoscale graphene-based devices.