Multi-scale design of new lubricants featuring inhomogeneous viscosity

Friction accounts for approximately 15% of fuel energy losses in internal combustion engine vehicles. To reduce it, new lubricants should be designed. Particle-based simulations can provide insight into the lubricant behaviour under extreme contact conditions, at a high computational cost. On the other hand, continuum methods, while capable of efficiently solving macroscopic problems, cannot resolve features at the nano-scale, due to the breakdown of the continuum assumption. This paper presents a multi-scale approach combining continuum and particle-based descriptions for simulating hydrodynamic lubrication systems to design new lubricants minimizing specific friction. Inspired by studies on ionic liquids as lubricants, their layering behaviour is emulated in the continuum domain by introducing inhomogeneous viscosity in the Navier–Stokes equations. Using an evolutionary algorithm, an optimized viscosity profile, leading to a potential improvement in friction performance up to 65%, is identified for a converging hydrodynamic slider. The study is then extended to nano-hydrodynamic lubrication. Specific particle typologies, featuring the aforementioned viscosity variations, are selected using coarse grain molecular dynamics simulations. Through the appropriate tuning of the particles’ properties, viscosity inhomogeneity is achieved and friction is reduced compared to the homogeneous case. Graphical abstract