Lubricant has been widely applied to reduce wear and friction between the contact surfaces when they are in relative motion. In the current study, a nonequilibrium molecular dynamics (NEMD) simulation was specifically established to conduct a comprehensive investigation on the dynamic contact between two iron surfaces in a boundary friction system considering the mixed C4-alkane and nanoparticles as lubricant. The main research objective was to explore the effects of fluid and nanoparticles addition on the surface contact and friction force. It was found that nanoparticles acted like ball bearings between the contact surfaces, leading to a change of sliding friction mode to rolling friction mode. Under normal loads, plastic deformation occurred at the top surface because nanoparticles were mainly supporting the normal load. By increasing the number of C4-alkane molecules between two contact surfaces, the contact condition has been changed from partial to full lubrication. In addition, an attractive force from the solid–liquid LJ interaction between C4-alkane and surfaces was observed at the early stage of sliding, due to the large space formed by wall surfaces and nanoparticles. The findings in this paper would be beneficial for understanding the frictional behavior of a simple lubricant with or without nanoparticles addition in a small confinement.

In this work, large-scale molecular dynamics (MD) computational simulations were performed in order to explore the sliding contact responses of rough surfaces with hexadecane lubricant and added nanoparticles. Simulation results revealed that the frictional state was dependent on the fluid, nanoparticle, and surface roughness. Three lubricating conditions were compared based on considerations of different amounts of fluid molecules. The lubricant was not able to separate the frictional contact surfaces if the quantity of lubricant molecules was insufficient. Particularly, there were no lubricating contributions when the amount of lubricant was too low, and the lubricant therefore only filled the pits in the surface roughness. Thus, the normal load was primarily supported by the contact between the two surfaces and nanoparticles, leading to significant surface morphology changes. In contrast, the frictional contact surfaces were able to be completely separated by the lubricant when there was a sufficient amount of fluid, and a very good lubricating effect could thus be achieved, resulting in a smaller friction force. In addition, the changes in surface morphology, contact area, and RMS are discussed in this paper, in order to reveal the dynamic frictional process.

Human-robot collaborative digital twin (HRC-DT) systems often suffer dynamic mismatches between virtual models and their physical counterparts due to data noise, mapping latency, system instability, and external sensor disturbances. To mitigate these issues, we propose a spatiotemporal alignment framework that fuses multi-source data resampling with adaptive feedback control. First, all sampling subsystems and the digital twin platform are time-synchronized via the Network Time Protocol (NTP). We then construct a motion-confidence-driven adaptive filter (MCDAF) to suppress abrupt distortions in human skeletal data, followed by a spatial-correlation-based predictive resampling (SCPR) module that aligns sensor streams with DT update timestamps. To reduce computational and communication load while preserving the necessary human body tracking accuracy, we introduce a dynamic sampling rate proportional-derivative (PD) controller based on human joint dynamics and human-robot distance. Finally, to promptly detect any physical disturbances that the depth camera may encounter during in operation, we developed an image-based self-check module for the depth-camera offset, along with a dynamic interference guard (DIG) mechanism, and a 6D pose detection-based rapid recalibration method. Experimental results demonstrate that the proposed approach enhances dynamic consistency compared with conventional methods, and provides a robust and efficient solution for reliable HRC-DT deployment.