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Lubrication is used for reduction of friction and wear of contacting surfaces. It is of importance for both natural joints as knee or hip-joint and engineering applications such as gearboxes or bearings. Depending on the interrelation of lubricant thickness and surface roughness and on the normal load, several lubrication regimes are distinguished:
(1) Hydrodynamic lubrication is applicable if the gap is larger than the roughness and elastic deformation of bodies can be neglected. Theory of hydrodynamic lubrication was developed by Petrov and Reynolds already in 1880th(see V.L. Popov, Contact Mechanics and Friction. Physical Principles and Applications, 2ndEd., Springer, 2017.)
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(2) Elastohydrodynamic lubrication (EHL) is of importance for heavy loaded machine parts as gears or bearing. The dramatic history of EHL can be found in the following paper:
E. Popova, V.L. Popov, On the history of elastohydrodynamics: The dramatic destiny of Alexander Mohrenstein‐Ertel and his contribution to the theory and practice of lubrication, ZAMM, 95(7):652-663, 2015. https://onlinelibrary.wiley.com/doi/pdf/10.1002/zamm.201400050 
or in a lecture at the Youtube:
(3) Mixed lubrication occurs if the lubricating gap becomes comparable to the roughness.
(4) Boundary lubrication determines the behavior in direct contact of surfaces and is due to only several molecules thick layers of additives.
At the Department, numerical packages for simulation of hydrodynamic and elastohydrodynamic lubrication have been developed in the last years.